Documentation/cachetlb.txt - kernel/msm-4.9 - Gitiles

 		Cache and TLB Flushing
 		     Under Linux

 	    David S. Miller <davem@redhat.com>

 This document describes the cache/tlb flushing interfaces called
 by the Linux VM subsystem.  It enumerates over each interface,
 describes it's intended purpose, and what side effect is expected
 after the interface is invoked.

 The side effects described below are stated for a uniprocessor
 implementation, and what is to happen on that single processor.  The
 SMP cases are a simple extension, in that you just extend the
 definition such that the side effect for a particular interface occurs
 on all processors in the system.  Don't let this scare you into
 thinking SMP cache/tlb flushing must be so inefficient, this is in
 fact an area where many optimizations are possible.  For example,
 if it can be proven that a user address space has never executed
 on a cpu (see vma->cpu_vm_mask), one need not perform a flush
 for this address space on that cpu.

 First, the TLB flushing interfaces, since they are the simplest.  The
 "TLB" is abstracted under Linux as something the cpu uses to cache
 virtual-->physical address translations obtained from the software
 page tables.  Meaning that if the software page tables change, it is
 possible for stale translations to exist in this "TLB" cache.
 Therefore when software page table changes occur, the kernel will
 invoke one of the following flush methods _after_ the page table
 changes occur:

 1) void flush_tlb_all(void)

 	The most severe flush of all.  After this interface runs,
 	any previous page table modification whatsoever will be
 	visible to the cpu.

 	This is usually invoked when the kernel page tables are
 	changed, since such translations are "global" in nature.

 2) void flush_tlb_mm(struct mm_struct *mm)

 	This interface flushes an entire user address space from
 	the TLB.  After running, this interface must make sure that
 	any previous page table modifications for the address space
 	'mm' will be visible to the cpu.  That is, after running,
 	there will be no entries in the TLB for 'mm'.

 	This interface is used to handle whole address space
 	page table operations such as what happens during
 	fork, and exec.

 3) void flush_tlb_range(struct vm_area_struct *vma,
 			unsigned long start, unsigned long end)

 	Here we are flushing a specific range of (user) virtual
 	address translations from the TLB.  After running, this
 	interface must make sure that any previous page table
 	modifications for the address space 'vma->vm_mm' in the range
 	'start' to 'end-1' will be visible to the cpu.  That is, after
 	running, here will be no entries in the TLB for 'mm' for
 	virtual addresses in the range 'start' to 'end-1'.

 	The "vma" is the backing store being used for the region.
 	Primarily, this is used for munmap() type operations.

 	The interface is provided in hopes that the port can find
 	a suitably efficient method for removing multiple page
 	sized translations from the TLB, instead of having the kernel
 	call flush_tlb_page (see below) for each entry which may be
 	modified.

 4) void flush_tlb_page(struct vm_area_struct *vma, unsigned long addr)

 	This time we need to remove the PAGE_SIZE sized translation
 	from the TLB.  The 'vma' is the backing structure used by
 	Linux to keep track of mmap'd regions for a process, the
 	address space is available via vma->vm_mm.  Also, one may
 	test (vma->vm_flags & VM_EXEC) to see if this region is
 	executable (and thus could be in the 'instruction TLB' in
 	split-tlb type setups).

 	After running, this interface must make sure that any previous
 	page table modification for address space 'vma->vm_mm' for
 	user virtual address 'addr' will be visible to the cpu.  That
 	is, after running, there will be no entries in the TLB for
 	'vma->vm_mm' for virtual address 'addr'.

 	This is used primarily during fault processing.

 5) void update_mmu_cache(struct vm_area_struct *vma,
 			 unsigned long address, pte_t pte)

 	At the end of every page fault, this routine is invoked to
 	tell the architecture specific code that a translation
 	described by "pte" now exists at virtual address "address"
 	for address space "vma->vm_mm", in the software page tables.

 	A port may use this information in any way it so chooses.
 	For example, it could use this event to pre-load TLB
 	translations for software managed TLB configurations.
 	The sparc64 port currently does this.

 6) void tlb_migrate_finish(struct mm_struct *mm)

 	This interface is called at the end of an explicit
 	process migration. This interface provides a hook
 	to allow a platform to update TLB or context-specific
 	information for the address space.

 	The ia64 sn2 platform is one example of a platform
 	that uses this interface.

 Next, we have the cache flushing interfaces.  In general, when Linux
 is changing an existing virtual-->physical mapping to a new value,
 the sequence will be in one of the following forms:

 	1) flush_cache_mm(mm);
 	   change_all_page_tables_of(mm);
 	   flush_tlb_mm(mm);

 	2) flush_cache_range(vma, start, end);
 	   change_range_of_page_tables(mm, start, end);
 	   flush_tlb_range(vma, start, end);

 	3) flush_cache_page(vma, addr, pfn);
 	   set_pte(pte_pointer, new_pte_val);
 	   flush_tlb_page(vma, addr);

 The cache level flush will always be first, because this allows
 us to properly handle systems whose caches are strict and require
 a virtual-->physical translation to exist for a virtual address
 when that virtual address is flushed from the cache.  The HyperSparc
 cpu is one such cpu with this attribute.

 The cache flushing routines below need only deal with cache flushing
 to the extent that it is necessary for a particular cpu.  Mostly,
 these routines must be implemented for cpus which have virtually
 indexed caches which must be flushed when virtual-->physical
 translations are changed or removed.  So, for example, the physically
 indexed physically tagged caches of IA32 processors have no need to
 implement these interfaces since the caches are fully synchronized
 and have no dependency on translation information.

 Here are the routines, one by one:

 1) void flush_cache_mm(struct mm_struct *mm)

 	This interface flushes an entire user address space from
 	the caches.  That is, after running, there will be no cache
 	lines associated with 'mm'.

 	This interface is used to handle whole address space
 	page table operations such as what happens during exit and exec.

 2) void flush_cache_dup_mm(struct mm_struct *mm)

 	This interface flushes an entire user address space from
 	the caches.  That is, after running, there will be no cache
 	lines associated with 'mm'.

 	This interface is used to handle whole address space
 	page table operations such as what happens during fork.

 	This option is separate from flush_cache_mm to allow some
 	optimizations for VIPT caches.

 3) void flush_cache_range(struct vm_area_struct *vma,
 			  unsigned long start, unsigned long end)

 	Here we are flushing a specific range of (user) virtual
 	addresses from the cache.  After running, there will be no
 	entries in the cache for 'vma->vm_mm' for virtual addresses in
 	the range 'start' to 'end-1'.

 	The "vma" is the backing store being used for the region.
 	Primarily, this is used for munmap() type operations.

 	The interface is provided in hopes that the port can find
 	a suitably efficient method for removing multiple page
 	sized regions from the cache, instead of having the kernel
 	call flush_cache_page (see below) for each entry which may be
 	modified.

 4) void flush_cache_page(struct vm_area_struct *vma, unsigned long addr, unsigned long pfn)

 	This time we need to remove a PAGE_SIZE sized range
 	from the cache.  The 'vma' is the backing structure used by
 	Linux to keep track of mmap'd regions for a process, the
 	address space is available via vma->vm_mm.  Also, one may
 	test (vma->vm_flags & VM_EXEC) to see if this region is
 	executable (and thus could be in the 'instruction cache' in
 	"Harvard" type cache layouts).

 	The 'pfn' indicates the physical page frame (shift this value
 	left by PAGE_SHIFT to get the physical address) that 'addr'
 	translates to.  It is this mapping which should be removed from
 	the cache.

 	After running, there will be no entries in the cache for
 	'vma->vm_mm' for virtual address 'addr' which translates
 	to 'pfn'.

 	This is used primarily during fault processing.

 5) void flush_cache_kmaps(void)

 	This routine need only be implemented if the platform utilizes
 	highmem.  It will be called right before all of the kmaps
 	are invalidated.

 	After running, there will be no entries in the cache for
 	the kernel virtual address range PKMAP_ADDR(0) to
 	PKMAP_ADDR(LAST_PKMAP).

 	This routing should be implemented in asm/highmem.h

 6) void flush_cache_vmap(unsigned long start, unsigned long end)
    void flush_cache_vunmap(unsigned long start, unsigned long end)

 	Here in these two interfaces we are flushing a specific range
 	of (kernel) virtual addresses from the cache.  After running,
 	there will be no entries in the cache for the kernel address
 	space for virtual addresses in the range 'start' to 'end-1'.

 	The first of these two routines is invoked after map_vm_area()
 	has installed the page table entries.  The second is invoked
 	before unmap_kernel_range() deletes the page table entries.

 There exists another whole class of cpu cache issues which currently
 require a whole different set of interfaces to handle properly.
 The biggest problem is that of virtual aliasing in the data cache
 of a processor.

 Is your port susceptible to virtual aliasing in it's D-cache?
 Well, if your D-cache is virtually indexed, is larger in size than
 PAGE_SIZE, and does not prevent multiple cache lines for the same
 physical address from existing at once, you have this problem.

 If your D-cache has this problem, first define asm/shmparam.h SHMLBA
 properly, it should essentially be the size of your virtually
 addressed D-cache (or if the size is variable, the largest possible
 size).  This setting will force the SYSv IPC layer to only allow user
 processes to mmap shared memory at address which are a multiple of
 this value.

 NOTE: This does not fix shared mmaps, check out the sparc64 port for
 one way to solve this (in particular SPARC_FLAG_MMAPSHARED).

 Next, you have to solve the D-cache aliasing issue for all
 other cases.  Please keep in mind that fact that, for a given page
 mapped into some user address space, there is always at least one more
 mapping, that of the kernel in it's linear mapping starting at
 PAGE_OFFSET.  So immediately, once the first user maps a given
 physical page into its address space, by implication the D-cache
 aliasing problem has the potential to exist since the kernel already
 maps this page at its virtual address.

   void copy_user_page(void *to, void *from, unsigned long addr, struct page *page)
   void clear_user_page(void *to, unsigned long addr, struct page *page)

 	These two routines store data in user anonymous or COW
 	pages.  It allows a port to efficiently avoid D-cache alias
 	issues between userspace and the kernel.

 	For example, a port may temporarily map 'from' and 'to' to
 	kernel virtual addresses during the copy.  The virtual address
 	for these two pages is chosen in such a way that the kernel
 	load/store instructions happen to virtual addresses which are
 	of the same "color" as the user mapping of the page.  Sparc64
 	for example, uses this technique.

 	The 'addr' parameter tells the virtual address where the
 	user will ultimately have this page mapped, and the 'page'
 	parameter gives a pointer to the struct page of the target.

 	If D-cache aliasing is not an issue, these two routines may
 	simply call memcpy/memset directly and do nothing more.

   void flush_dcache_page(struct page *page)

 	Any time the kernel writes to a page cache page, _OR_
 	the kernel is about to read from a page cache page and
 	user space shared/writable mappings of this page potentially
 	exist, this routine is called.

 	NOTE: This routine need only be called for page cache pages
 	      which can potentially ever be mapped into the address
 	      space of a user process.  So for example, VFS layer code
 	      handling vfs symlinks in the page cache need not call
 	      this interface at all.

 	The phrase "kernel writes to a page cache page" means,
 	specifically, that the kernel executes store instructions
 	that dirty data in that page at the page->virtual mapping
 	of that page.  It is important to flush here to handle
 	D-cache aliasing, to make sure these kernel stores are
 	visible to user space mappings of that page.

 	The corollary case is just as important, if there are users
 	which have shared+writable mappings of this file, we must make
 	sure that kernel reads of these pages will see the most recent
 	stores done by the user.

 	If D-cache aliasing is not an issue, this routine may
 	simply be defined as a nop on that architecture.

         There is a bit set aside in page->flags (PG_arch_1) as
 	"architecture private".  The kernel guarantees that,
 	for pagecache pages, it will clear this bit when such
 	a page first enters the pagecache.

 	This allows these interfaces to be implemented much more
 	efficiently.  It allows one to "defer" (perhaps indefinitely)
 	the actual flush if there are currently no user processes
 	mapping this page.  See sparc64's flush_dcache_page and
 	update_mmu_cache implementations for an example of how to go
 	about doing this.

 	The idea is, first at flush_dcache_page() time, if
 	page->mapping->i_mmap is an empty tree and ->i_mmap_nonlinear
 	an empty list, just mark the architecture private page flag bit.
 	Later, in update_mmu_cache(), a check is made of this flag bit,
 	and if set the flush is done and the flag bit is cleared.

 	IMPORTANT NOTE: It is often important, if you defer the flush,
 			that the actual flush occurs on the same CPU
 			as did the cpu stores into the page to make it
 			dirty.  Again, see sparc64 for examples of how
 			to deal with this.

   void copy_to_user_page(struct vm_area_struct *vma, struct page *page,
                          unsigned long user_vaddr,
                          void *dst, void *src, int len)
   void copy_from_user_page(struct vm_area_struct *vma, struct page *page,
                            unsigned long user_vaddr,
                            void *dst, void *src, int len)
 	When the kernel needs to copy arbitrary data in and out
 	of arbitrary user pages (f.e. for ptrace()) it will use
 	these two routines.

 	Any necessary cache flushing or other coherency operations
 	that need to occur should happen here.  If the processor's
 	instruction cache does not snoop cpu stores, it is very
 	likely that you will need to flush the instruction cache
 	for copy_to_user_page().

   void flush_anon_page(struct vm_area_struct *vma, struct page *page,
                        unsigned long vmaddr)
   	When the kernel needs to access the contents of an anonymous
 	page, it calls this function (currently only
 	get_user_pages()).  Note: flush_dcache_page() deliberately
 	doesn't work for an anonymous page.  The default
 	implementation is a nop (and should remain so for all coherent
 	architectures).  For incoherent architectures, it should flush
 	the cache of the page at vmaddr.

   void flush_kernel_dcache_page(struct page *page)
 	When the kernel needs to modify a user page is has obtained
 	with kmap, it calls this function after all modifications are
 	complete (but before kunmapping it) to bring the underlying
 	page up to date.  It is assumed here that the user has no
 	incoherent cached copies (i.e. the original page was obtained
 	from a mechanism like get_user_pages()).  The default
 	implementation is a nop and should remain so on all coherent
 	architectures.  On incoherent architectures, this should flush
 	the kernel cache for page (using page_address(page)).


   void flush_icache_range(unsigned long start, unsigned long end)
   	When the kernel stores into addresses that it will execute
 	out of (eg when loading modules), this function is called.

 	If the icache does not snoop stores then this routine will need
 	to flush it.

   void flush_icache_page(struct vm_area_struct *vma, struct page *page)
 	All the functionality of flush_icache_page can be implemented in
 	flush_dcache_page and update_mmu_cache. In 2.7 the hope is to
 	remove this interface completely.
	Cache and TLB Flushing
	Under Linux

	David S. Miller <davem@redhat.com>

	This document describes the cache/tlb flushing interfaces called
	by the Linux VM subsystem. It enumerates over each interface,
	describes it's intended purpose, and what side effect is expected
	after the interface is invoked.

	The side effects described below are stated for a uniprocessor
	implementation, and what is to happen on that single processor. The
	SMP cases are a simple extension, in that you just extend the
	definition such that the side effect for a particular interface occurs
	on all processors in the system. Don't let this scare you into
	thinking SMP cache/tlb flushing must be so inefficient, this is in
	fact an area where many optimizations are possible. For example,
	if it can be proven that a user address space has never executed
	on a cpu (see vma->cpu_vm_mask), one need not perform a flush
	for this address space on that cpu.

	First, the TLB flushing interfaces, since they are the simplest. The
	"TLB" is abstracted under Linux as something the cpu uses to cache
	virtual-->physical address translations obtained from the software
	page tables. Meaning that if the software page tables change, it is
	possible for stale translations to exist in this "TLB" cache.
	Therefore when software page table changes occur, the kernel will
	invoke one of the following flush methods _after_ the page table
	changes occur:

	1) void flush_tlb_all(void)

	The most severe flush of all. After this interface runs,
	any previous page table modification whatsoever will be
	visible to the cpu.

	This is usually invoked when the kernel page tables are
	changed, since such translations are "global" in nature.

	2) void flush_tlb_mm(struct mm_struct *mm)

	This interface flushes an entire user address space from
	the TLB. After running, this interface must make sure that
	any previous page table modifications for the address space
	'mm' will be visible to the cpu. That is, after running,
	there will be no entries in the TLB for 'mm'.

	This interface is used to handle whole address space
	page table operations such as what happens during
	fork, and exec.

	3) void flush_tlb_range(struct vm_area_struct *vma,
	unsigned long start, unsigned long end)

	Here we are flushing a specific range of (user) virtual
	address translations from the TLB. After running, this
	interface must make sure that any previous page table
	modifications for the address space 'vma->vm_mm' in the range
	'start' to 'end-1' will be visible to the cpu. That is, after
	running, here will be no entries in the TLB for 'mm' for
	virtual addresses in the range 'start' to 'end-1'.

	The "vma" is the backing store being used for the region.
	Primarily, this is used for munmap() type operations.

	The interface is provided in hopes that the port can find
	a suitably efficient method for removing multiple page
	sized translations from the TLB, instead of having the kernel
	call flush_tlb_page (see below) for each entry which may be
	modified.

	4) void flush_tlb_page(struct vm_area_struct *vma, unsigned long addr)

	This time we need to remove the PAGE_SIZE sized translation
	from the TLB. The 'vma' is the backing structure used by
	Linux to keep track of mmap'd regions for a process, the
	address space is available via vma->vm_mm. Also, one may
	test (vma->vm_flags & VM_EXEC) to see if this region is
	executable (and thus could be in the 'instruction TLB' in
	split-tlb type setups).

	After running, this interface must make sure that any previous
	page table modification for address space 'vma->vm_mm' for
	user virtual address 'addr' will be visible to the cpu. That
	is, after running, there will be no entries in the TLB for
	'vma->vm_mm' for virtual address 'addr'.

	This is used primarily during fault processing.

	5) void update_mmu_cache(struct vm_area_struct *vma,
	unsigned long address, pte_t pte)

	At the end of every page fault, this routine is invoked to
	tell the architecture specific code that a translation
	described by "pte" now exists at virtual address "address"
	for address space "vma->vm_mm", in the software page tables.

	A port may use this information in any way it so chooses.
	For example, it could use this event to pre-load TLB
	translations for software managed TLB configurations.
	The sparc64 port currently does this.

	6) void tlb_migrate_finish(struct mm_struct *mm)

	This interface is called at the end of an explicit
	process migration. This interface provides a hook
	to allow a platform to update TLB or context-specific
	information for the address space.

	The ia64 sn2 platform is one example of a platform
	that uses this interface.

	Next, we have the cache flushing interfaces. In general, when Linux
	is changing an existing virtual-->physical mapping to a new value,
	the sequence will be in one of the following forms:

	1) flush_cache_mm(mm);
	change_all_page_tables_of(mm);
	flush_tlb_mm(mm);

	2) flush_cache_range(vma, start, end);
	change_range_of_page_tables(mm, start, end);
	flush_tlb_range(vma, start, end);

	3) flush_cache_page(vma, addr, pfn);
	set_pte(pte_pointer, new_pte_val);
	flush_tlb_page(vma, addr);

	The cache level flush will always be first, because this allows
	us to properly handle systems whose caches are strict and require
	a virtual-->physical translation to exist for a virtual address
	when that virtual address is flushed from the cache. The HyperSparc
	cpu is one such cpu with this attribute.

	The cache flushing routines below need only deal with cache flushing
	to the extent that it is necessary for a particular cpu. Mostly,
	these routines must be implemented for cpus which have virtually
	indexed caches which must be flushed when virtual-->physical
	translations are changed or removed. So, for example, the physically
	indexed physically tagged caches of IA32 processors have no need to
	implement these interfaces since the caches are fully synchronized
	and have no dependency on translation information.

	Here are the routines, one by one:

	1) void flush_cache_mm(struct mm_struct *mm)

	This interface flushes an entire user address space from
	the caches. That is, after running, there will be no cache
	lines associated with 'mm'.

	This interface is used to handle whole address space
	page table operations such as what happens during exit and exec.

	2) void flush_cache_dup_mm(struct mm_struct *mm)

	This interface flushes an entire user address space from
	the caches. That is, after running, there will be no cache
	lines associated with 'mm'.

	This interface is used to handle whole address space
	page table operations such as what happens during fork.

	This option is separate from flush_cache_mm to allow some
	optimizations for VIPT caches.

	3) void flush_cache_range(struct vm_area_struct *vma,
	unsigned long start, unsigned long end)

	Here we are flushing a specific range of (user) virtual
	addresses from the cache. After running, there will be no
	entries in the cache for 'vma->vm_mm' for virtual addresses in
	the range 'start' to 'end-1'.

	The "vma" is the backing store being used for the region.
	Primarily, this is used for munmap() type operations.

	The interface is provided in hopes that the port can find
	a suitably efficient method for removing multiple page
	sized regions from the cache, instead of having the kernel
	call flush_cache_page (see below) for each entry which may be
	modified.

	4) void flush_cache_page(struct vm_area_struct *vma, unsigned long addr, unsigned long pfn)

	This time we need to remove a PAGE_SIZE sized range
	from the cache. The 'vma' is the backing structure used by
	Linux to keep track of mmap'd regions for a process, the
	address space is available via vma->vm_mm. Also, one may
	test (vma->vm_flags & VM_EXEC) to see if this region is
	executable (and thus could be in the 'instruction cache' in
	"Harvard" type cache layouts).

	The 'pfn' indicates the physical page frame (shift this value
	left by PAGE_SHIFT to get the physical address) that 'addr'
	translates to. It is this mapping which should be removed from
	the cache.

	After running, there will be no entries in the cache for
	'vma->vm_mm' for virtual address 'addr' which translates
	to 'pfn'.

	This is used primarily during fault processing.

	5) void flush_cache_kmaps(void)

	This routine need only be implemented if the platform utilizes
	highmem. It will be called right before all of the kmaps
	are invalidated.

	After running, there will be no entries in the cache for
	the kernel virtual address range PKMAP_ADDR(0) to
	PKMAP_ADDR(LAST_PKMAP).

	This routing should be implemented in asm/highmem.h

	6) void flush_cache_vmap(unsigned long start, unsigned long end)
	void flush_cache_vunmap(unsigned long start, unsigned long end)

	Here in these two interfaces we are flushing a specific range
	of (kernel) virtual addresses from the cache. After running,
	there will be no entries in the cache for the kernel address
	space for virtual addresses in the range 'start' to 'end-1'.

	The first of these two routines is invoked after map_vm_area()
	has installed the page table entries. The second is invoked
	before unmap_kernel_range() deletes the page table entries.

	There exists another whole class of cpu cache issues which currently
	require a whole different set of interfaces to handle properly.
	The biggest problem is that of virtual aliasing in the data cache
	of a processor.

	Is your port susceptible to virtual aliasing in it's D-cache?
	Well, if your D-cache is virtually indexed, is larger in size than
	PAGE_SIZE, and does not prevent multiple cache lines for the same
	physical address from existing at once, you have this problem.

	If your D-cache has this problem, first define asm/shmparam.h SHMLBA
	properly, it should essentially be the size of your virtually
	addressed D-cache (or if the size is variable, the largest possible
	size). This setting will force the SYSv IPC layer to only allow user
	processes to mmap shared memory at address which are a multiple of
	this value.

	NOTE: This does not fix shared mmaps, check out the sparc64 port for
	one way to solve this (in particular SPARC_FLAG_MMAPSHARED).

	Next, you have to solve the D-cache aliasing issue for all
	other cases. Please keep in mind that fact that, for a given page
	mapped into some user address space, there is always at least one more
	mapping, that of the kernel in it's linear mapping starting at
	PAGE_OFFSET. So immediately, once the first user maps a given
	physical page into its address space, by implication the D-cache
	aliasing problem has the potential to exist since the kernel already
	maps this page at its virtual address.

	void copy_user_page(void to, void from, unsigned long addr, struct page *page)
	void clear_user_page(void to, unsigned long addr, struct page page)

	These two routines store data in user anonymous or COW
	pages. It allows a port to efficiently avoid D-cache alias
	issues between userspace and the kernel.

	For example, a port may temporarily map 'from' and 'to' to
	kernel virtual addresses during the copy. The virtual address
	for these two pages is chosen in such a way that the kernel
	load/store instructions happen to virtual addresses which are
	of the same "color" as the user mapping of the page. Sparc64
	for example, uses this technique.

	The 'addr' parameter tells the virtual address where the
	user will ultimately have this page mapped, and the 'page'
	parameter gives a pointer to the struct page of the target.

	If D-cache aliasing is not an issue, these two routines may
	simply call memcpy/memset directly and do nothing more.

	void flush_dcache_page(struct page *page)

	Any time the kernel writes to a page cache page, _OR_
	the kernel is about to read from a page cache page and
	user space shared/writable mappings of this page potentially
	exist, this routine is called.

	NOTE: This routine need only be called for page cache pages
	which can potentially ever be mapped into the address
	space of a user process. So for example, VFS layer code
	handling vfs symlinks in the page cache need not call
	this interface at all.

	The phrase "kernel writes to a page cache page" means,
	specifically, that the kernel executes store instructions
	that dirty data in that page at the page->virtual mapping
	of that page. It is important to flush here to handle
	D-cache aliasing, to make sure these kernel stores are
	visible to user space mappings of that page.

	The corollary case is just as important, if there are users
	which have shared+writable mappings of this file, we must make
	sure that kernel reads of these pages will see the most recent
	stores done by the user.

	If D-cache aliasing is not an issue, this routine may
	simply be defined as a nop on that architecture.

	There is a bit set aside in page->flags (PG_arch_1) as
	"architecture private". The kernel guarantees that,
	for pagecache pages, it will clear this bit when such
	a page first enters the pagecache.

	This allows these interfaces to be implemented much more
	efficiently. It allows one to "defer" (perhaps indefinitely)
	the actual flush if there are currently no user processes
	mapping this page. See sparc64's flush_dcache_page and
	update_mmu_cache implementations for an example of how to go
	about doing this.

	The idea is, first at flush_dcache_page() time, if
	page->mapping->i_mmap is an empty tree and ->i_mmap_nonlinear
	an empty list, just mark the architecture private page flag bit.
	Later, in update_mmu_cache(), a check is made of this flag bit,
	and if set the flush is done and the flag bit is cleared.

	IMPORTANT NOTE: It is often important, if you defer the flush,
	that the actual flush occurs on the same CPU
	as did the cpu stores into the page to make it
	dirty. Again, see sparc64 for examples of how
	to deal with this.

	void copy_to_user_page(struct vm_area_struct vma, struct page page,
	unsigned long user_vaddr,
	void dst, void src, int len)
	void copy_from_user_page(struct vm_area_struct vma, struct page page,
	unsigned long user_vaddr,
	void dst, void src, int len)
	When the kernel needs to copy arbitrary data in and out
	of arbitrary user pages (f.e. for ptrace()) it will use
	these two routines.

	Any necessary cache flushing or other coherency operations
	that need to occur should happen here. If the processor's
	instruction cache does not snoop cpu stores, it is very
	likely that you will need to flush the instruction cache
	for copy_to_user_page().

	void flush_anon_page(struct vm_area_struct vma, struct page page,
	unsigned long vmaddr)
	When the kernel needs to access the contents of an anonymous
	page, it calls this function (currently only
	get_user_pages()). Note: flush_dcache_page() deliberately
	doesn't work for an anonymous page. The default
	implementation is a nop (and should remain so for all coherent
	architectures). For incoherent architectures, it should flush
	the cache of the page at vmaddr.

	void flush_kernel_dcache_page(struct page *page)
	When the kernel needs to modify a user page is has obtained
	with kmap, it calls this function after all modifications are
	complete (but before kunmapping it) to bring the underlying
	page up to date. It is assumed here that the user has no
	incoherent cached copies (i.e. the original page was obtained
	from a mechanism like get_user_pages()). The default
	implementation is a nop and should remain so on all coherent
	architectures. On incoherent architectures, this should flush
	the kernel cache for page (using page_address(page)).


	void flush_icache_range(unsigned long start, unsigned long end)
	When the kernel stores into addresses that it will execute
	out of (eg when loading modules), this function is called.

	If the icache does not snoop stores then this routine will need
	to flush it.

	void flush_icache_page(struct vm_area_struct vma, struct page page)
	All the functionality of flush_icache_page can be implemented in
	flush_dcache_page and update_mmu_cache. In 2.7 the hope is to
	remove this interface completely.