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Commit 9a95f3cf authored by Paul Cassella's avatar Paul Cassella Committed by Linus Torvalds
Browse files

mm: describe mmap_sem rules for __lock_page_or_retry() and callers 



Add a comment describing the circumstances in which
__lock_page_or_retry() will or will not release the mmap_sem when
returning 0.

Add comments to lock_page_or_retry()'s callers (filemap_fault(),
do_swap_page()) noting the impact on VM_FAULT_RETRY returns.

Add comments on up the call tree, particularly replacing the false "We
return with mmap_sem still held" comments.

Signed-off-by: default avatarPaul Cassella <cassella@cray.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 4ffeaf35

arch/x86/mm/fault.c

+2 −1
Original line number Diff line number Diff line
@@ -1218,7 +1218,8 @@ __do_page_fault(struct pt_regs *regs, unsigned long error_code, 
	/*

	 * If for any reason at all we couldn't handle the fault,

	 * make sure we exit gracefully rather than endlessly redo

	 * the fault:

	 * the fault.  Since we never set FAULT_FLAG_RETRY_NOWAIT, if

	 * we get VM_FAULT_RETRY back, the mmap_sem has been unlocked.

	 */

	fault = handle_mm_fault(mm, vma, address, flags);

include/linux/pagemap.h

+3 −0
Original line number Diff line number Diff line
@@ -484,6 +484,9 @@ static inline int lock_page_killable(struct page *page) 
/*

 * lock_page_or_retry - Lock the page, unless this would block and the

 * caller indicated that it can handle a retry.

 *

 * Return value and mmap_sem implications depend on flags; see

 * __lock_page_or_retry().

 */

static inline int lock_page_or_retry(struct page *page, struct mm_struct *mm,

				     unsigned int flags)

mm/filemap.c

+23 −0
Original line number Diff line number Diff line
@@ -808,6 +808,17 @@ int __lock_page_killable(struct page *page) 
}

EXPORT_SYMBOL_GPL(__lock_page_killable);



/*

 * Return values:

 * 1 - page is locked; mmap_sem is still held.

 * 0 - page is not locked.

 *     mmap_sem has been released (up_read()), unless flags had both

 *     FAULT_FLAG_ALLOW_RETRY and FAULT_FLAG_RETRY_NOWAIT set, in

 *     which case mmap_sem is still held.

 *

 * If neither ALLOW_RETRY nor KILLABLE are set, will always return 1

 * with the page locked and the mmap_sem unperturbed.

 */

int __lock_page_or_retry(struct page *page, struct mm_struct *mm,

			 unsigned int flags)

{
@@ -1827,6 +1838,18 @@ static void do_async_mmap_readahead(struct vm_area_struct *vma, 
 * The goto's are kind of ugly, but this streamlines the normal case of having

 * it in the page cache, and handles the special cases reasonably without

 * having a lot of duplicated code.

 *

 * vma->vm_mm->mmap_sem must be held on entry.

 *

 * If our return value has VM_FAULT_RETRY set, it's because

 * lock_page_or_retry() returned 0.

 * The mmap_sem has usually been released in this case.

 * See __lock_page_or_retry() for the exception.

 *

 * If our return value does not have VM_FAULT_RETRY set, the mmap_sem

 * has not been released.

 *

 * We never return with VM_FAULT_RETRY and a bit from VM_FAULT_ERROR set.

 */

int filemap_fault(struct vm_area_struct *vma, struct vm_fault *vmf)

{

mm/gup.c

+15 −3
Original line number Diff line number Diff line
@@ -258,6 +258,11 @@ static int get_gate_page(struct mm_struct *mm, unsigned long address, 
	return ret;

}



/*

 * mmap_sem must be held on entry.  If @nonblocking != NULL and

 * *@flags does not include FOLL_NOWAIT, the mmap_sem may be released.

 * If it is, *@nonblocking will be set to 0 and -EBUSY returned.

 */

static int faultin_page(struct task_struct *tsk, struct vm_area_struct *vma,

		unsigned long address, unsigned int *flags, int *nonblocking)

{
@@ -373,7 +378,7 @@ static int check_vma_flags(struct vm_area_struct *vma, unsigned long gup_flags) 
 * with a put_page() call when it is finished with. vmas will only

 * remain valid while mmap_sem is held.

 *

 * Must be called with mmap_sem held for read or write.

 * Must be called with mmap_sem held.  It may be released.  See below.

 *

 * __get_user_pages walks a process's page tables and takes a reference to

 * each struct page that each user address corresponds to at a given
@@ -396,7 +401,14 @@ static int check_vma_flags(struct vm_area_struct *vma, unsigned long gup_flags) 
 *

 * If @nonblocking != NULL, __get_user_pages will not wait for disk IO

 * or mmap_sem contention, and if waiting is needed to pin all pages,

 * *@nonblocking will be set to 0.

 * *@nonblocking will be set to 0.  Further, if @gup_flags does not

 * include FOLL_NOWAIT, the mmap_sem will be released via up_read() in

 * this case.

 *

 * A caller using such a combination of @nonblocking and @gup_flags

 * must therefore hold the mmap_sem for reading only, and recognize

 * when it's been released.  Otherwise, it must be held for either

 * reading or writing and will not be released.

 *

 * In most cases, get_user_pages or get_user_pages_fast should be used

 * instead of __get_user_pages. __get_user_pages should be used only if
@@ -528,7 +540,7 @@ EXPORT_SYMBOL(__get_user_pages); 
 * such architectures, gup() will not be enough to make a subsequent access

 * succeed.

 *

 * This should be called with the mm_sem held for read.

 * This has the same semantics wrt the @mm->mmap_sem as does filemap_fault().

 */

int fixup_user_fault(struct task_struct *tsk, struct mm_struct *mm,

		     unsigned long address, unsigned int fault_flags)

mm/memory.c

+31 −3
Original line number Diff line number Diff line
@@ -2399,7 +2399,10 @@ EXPORT_SYMBOL(unmap_mapping_range); 
/*

 * We enter with non-exclusive mmap_sem (to exclude vma changes,

 * but allow concurrent faults), and pte mapped but not yet locked.

 * We return with mmap_sem still held, but pte unmapped and unlocked.

 * We return with pte unmapped and unlocked.

 *

 * We return with the mmap_sem locked or unlocked in the same cases

 * as does filemap_fault().

 */

static int do_swap_page(struct mm_struct *mm, struct vm_area_struct *vma,

		unsigned long address, pte_t *page_table, pmd_t *pmd,
@@ -2688,6 +2691,11 @@ static int do_anonymous_page(struct mm_struct *mm, struct vm_area_struct *vma, 
	return VM_FAULT_OOM;

}



/*

 * The mmap_sem must have been held on entry, and may have been

 * released depending on flags and vma->vm_ops->fault() return value.

 * See filemap_fault() and __lock_page_retry().

 */

static int __do_fault(struct vm_area_struct *vma, unsigned long address,

		pgoff_t pgoff, unsigned int flags, struct page **page)

{
@@ -3016,6 +3024,12 @@ static int do_shared_fault(struct mm_struct *mm, struct vm_area_struct *vma, 
	return ret;

}



/*

 * We enter with non-exclusive mmap_sem (to exclude vma changes,

 * but allow concurrent faults).

 * The mmap_sem may have been released depending on flags and our

 * return value.  See filemap_fault() and __lock_page_or_retry().

 */

static int do_linear_fault(struct mm_struct *mm, struct vm_area_struct *vma,

		unsigned long address, pte_t *page_table, pmd_t *pmd,

		unsigned int flags, pte_t orig_pte)
@@ -3040,7 +3054,9 @@ static int do_linear_fault(struct mm_struct *mm, struct vm_area_struct *vma, 
 *

 * We enter with non-exclusive mmap_sem (to exclude vma changes,

 * but allow concurrent faults), and pte mapped but not yet locked.

 * We return with mmap_sem still held, but pte unmapped and unlocked.

 * We return with pte unmapped and unlocked.

 * The mmap_sem may have been released depending on flags and our

 * return value.  See filemap_fault() and __lock_page_or_retry().

 */

static int do_nonlinear_fault(struct mm_struct *mm, struct vm_area_struct *vma,

		unsigned long address, pte_t *page_table, pmd_t *pmd,
@@ -3172,7 +3188,10 @@ static int do_numa_page(struct mm_struct *mm, struct vm_area_struct *vma, 
 *

 * We enter with non-exclusive mmap_sem (to exclude vma changes,

 * but allow concurrent faults), and pte mapped but not yet locked.

 * We return with mmap_sem still held, but pte unmapped and unlocked.

 * We return with pte unmapped and unlocked.

 *

 * The mmap_sem may have been released depending on flags and our

 * return value.  See filemap_fault() and __lock_page_or_retry().

 */

static int handle_pte_fault(struct mm_struct *mm,

		     struct vm_area_struct *vma, unsigned long address,
@@ -3232,6 +3251,9 @@ static int handle_pte_fault(struct mm_struct *mm, 


/*

 * By the time we get here, we already hold the mm semaphore

 *

 * The mmap_sem may have been released depending on flags and our

 * return value.  See filemap_fault() and __lock_page_or_retry().

 */

static int __handle_mm_fault(struct mm_struct *mm, struct vm_area_struct *vma,

			     unsigned long address, unsigned int flags)
@@ -3313,6 +3335,12 @@ static int __handle_mm_fault(struct mm_struct *mm, struct vm_area_struct *vma, 
	return handle_pte_fault(mm, vma, address, pte, pmd, flags);

}



/*

 * By the time we get here, we already hold the mm semaphore

 *

 * The mmap_sem may have been released depending on flags and our

 * return value.  See filemap_fault() and __lock_page_or_retry().

 */

int handle_mm_fault(struct mm_struct *mm, struct vm_area_struct *vma,

		    unsigned long address, unsigned int flags)

{