x86: mm: consolidate VM_FAULT_RETRY handling
The VM_FAULT_RETRY handling was confusing and incorrect for the case of
returning to kernel mode. We need to handle the exception table fixup
if we return to kernel mode due to a fatal signal - it will basically
look to the kernel user mode access like the access failed due to the VM
going away from udner it. Which is correct - the process is dying - and
avoids the whole "repeat endless kernel page faults" case.
Handling the VM_FAULT_RETRY early and in just one place also simplifies
the mmap_sem handling, since once we've taken care of VM_FAULT_RETRY we
know that we can just drop the lock. The remaining accounting and
possible error handling is thread-local and does not need the mmap_sem.
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index d7de5b8..b74a7e1 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -1055,7 +1055,7 @@
struct vm_area_struct *vma;
struct task_struct *tsk;
struct mm_struct *mm;
- int fault;
+ int fault, major = 0;
unsigned int flags = FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_KILLABLE;
tsk = current;
@@ -1230,48 +1230,50 @@
* we get VM_FAULT_RETRY back, the mmap_sem has been unlocked.
*/
fault = handle_mm_fault(mm, vma, address, flags);
+ major |= fault & VM_FAULT_MAJOR;
/*
- * If we need to retry but a fatal signal is pending, handle the
- * signal first. We do not need to release the mmap_sem because it
- * would already be released in __lock_page_or_retry in mm/filemap.c.
+ * If we need to retry the mmap_sem has already been released,
+ * and if there is a fatal signal pending there is no guarantee
+ * that we made any progress. Handle this case first.
*/
- if (unlikely((fault & VM_FAULT_RETRY) && fatal_signal_pending(current)))
- return;
+ if (unlikely(fault & VM_FAULT_RETRY)) {
+ /* Retry at most once */
+ if (flags & FAULT_FLAG_ALLOW_RETRY) {
+ flags &= ~FAULT_FLAG_ALLOW_RETRY;
+ flags |= FAULT_FLAG_TRIED;
+ if (!fatal_signal_pending(tsk))
+ goto retry;
+ }
+ /* User mode? Just return to handle the fatal exception */
+ if (fault & FAULT_FLAG_USER)
+ return;
+
+ /* Not returning to user mode? Handle exceptions or die: */
+ no_context(regs, error_code, address, SIGBUS, BUS_ADRERR);
+ return;
+ }
+
+ up_read(&mm->mmap_sem);
if (unlikely(fault & VM_FAULT_ERROR)) {
- up_read(&mm->mmap_sem);
mm_fault_error(regs, error_code, address, fault);
return;
}
/*
- * Major/minor page fault accounting is only done on the
- * initial attempt. If we go through a retry, it is extremely
- * likely that the page will be found in page cache at that point.
+ * Major/minor page fault accounting. If any of the events
+ * returned VM_FAULT_MAJOR, we account it as a major fault.
*/
- if (flags & FAULT_FLAG_ALLOW_RETRY) {
- if (fault & VM_FAULT_MAJOR) {
- tsk->maj_flt++;
- perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS_MAJ, 1,
- regs, address);
- } else {
- tsk->min_flt++;
- perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS_MIN, 1,
- regs, address);
- }
- if (fault & VM_FAULT_RETRY) {
- /* Clear FAULT_FLAG_ALLOW_RETRY to avoid any risk
- * of starvation. */
- flags &= ~FAULT_FLAG_ALLOW_RETRY;
- flags |= FAULT_FLAG_TRIED;
- goto retry;
- }
+ if (major) {
+ tsk->maj_flt++;
+ perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS_MAJ, 1, regs, address);
+ } else {
+ tsk->min_flt++;
+ perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS_MIN, 1, regs, address);
}
check_v8086_mode(regs, address, tsk);
-
- up_read(&mm->mmap_sem);
}
NOKPROBE_SYMBOL(__do_page_fault);