[PATCH] mm: tracking shared dirty pages
Tracking of dirty pages in shared writeable mmap()s.
The idea is simple: write protect clean shared writeable pages, catch the
write-fault, make writeable and set dirty. On page write-back clean all the
PTE dirty bits and write protect them once again.
The implementation is a tad harder, mainly because the default
backing_dev_info capabilities were too loosely maintained. Hence it is not
enough to test the backing_dev_info for cap_account_dirty.
The current heuristic is as follows, a VMA is eligible when:
- its shared writeable
(vm_flags & (VM_WRITE|VM_SHARED)) == (VM_WRITE|VM_SHARED)
- it is not a 'special' mapping
(vm_flags & (VM_PFNMAP|VM_INSERTPAGE)) == 0
- the backing_dev_info is cap_account_dirty
mapping_cap_account_dirty(vma->vm_file->f_mapping)
- f_op->mmap() didn't change the default page protection
Page from remap_pfn_range() are explicitly excluded because their COW
semantics are already horrid enough (see vm_normal_page() in do_wp_page()) and
because they don't have a backing store anyway.
mprotect() is taught about the new behaviour as well. However it overrides
the last condition.
Cleaning the pages on write-back is done with page_mkclean() a new rmap call.
It can be called on any page, but is currently only implemented for mapped
pages, if the page is found the be of a VMA that accounts dirty pages it will
also wrprotect the PTE.
Finally, in fs/buffers.c:try_to_free_buffers(); remove clear_page_dirty() from
under ->private_lock. This seems to be safe, since ->private_lock is used to
serialize access to the buffers, not the page itself. This is needed because
clear_page_dirty() will call into page_mkclean() and would thereby violate
locking order.
[dhowells@redhat.com: Provide a page_mkclean() implementation for NOMMU]
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Hugh Dickins <hugh@veritas.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
diff --git a/mm/mprotect.c b/mm/mprotect.c
index 638edab..367b7f6 100644
--- a/mm/mprotect.c
+++ b/mm/mprotect.c
@@ -123,8 +123,6 @@
unsigned long oldflags = vma->vm_flags;
long nrpages = (end - start) >> PAGE_SHIFT;
unsigned long charged = 0;
- unsigned int mask;
- pgprot_t newprot;
pgoff_t pgoff;
int error;
@@ -176,24 +174,21 @@
}
success:
- /* Don't make the VMA automatically writable if it's shared, but the
- * backer wishes to know when pages are first written to */
- mask = VM_READ|VM_WRITE|VM_EXEC|VM_SHARED;
- if (vma->vm_ops && vma->vm_ops->page_mkwrite)
- mask &= ~VM_SHARED;
-
- newprot = protection_map[newflags & mask];
-
/*
* vm_flags and vm_page_prot are protected by the mmap_sem
* held in write mode.
*/
vma->vm_flags = newflags;
- vma->vm_page_prot = newprot;
+ vma->vm_page_prot = protection_map[newflags &
+ (VM_READ|VM_WRITE|VM_EXEC|VM_SHARED)];
+ if (vma_wants_writenotify(vma))
+ vma->vm_page_prot = protection_map[newflags &
+ (VM_READ|VM_WRITE|VM_EXEC)];
+
if (is_vm_hugetlb_page(vma))
- hugetlb_change_protection(vma, start, end, newprot);
+ hugetlb_change_protection(vma, start, end, vma->vm_page_prot);
else
- change_protection(vma, start, end, newprot);
+ change_protection(vma, start, end, vma->vm_page_prot);
vm_stat_account(mm, oldflags, vma->vm_file, -nrpages);
vm_stat_account(mm, newflags, vma->vm_file, nrpages);
return 0;