Commit 26178ec1 authored by Linus Torvalds's avatar Linus Torvalds
Browse files

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: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 7fb08eca

arch/x86/mm/fault.c

+30 −28
Original line number Diff line number Diff line
@@ -1055,7 +1055,7 @@ __do_page_fault(struct pt_regs *regs, unsigned long error_code, 
	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 @@ good_area: 
	 * 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)))

	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;



	if (unlikely(fault & VM_FAULT_ERROR)) {

		/* 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)) {

		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) {

	if (major) {

		tsk->maj_flt++;

			perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS_MAJ, 1,

				      regs, address);

		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;

		}

		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);

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