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: Paul Cassella <cassella@cray.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 23 insertions, 0 deletions

diff --git a/mm/filemap.c b/mm/filemap.c
index 9c99d6868a5e..d2d08bf00f7c 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -819,6 +819,17 @@ 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)
 {
@@ -1835,6 +1846,18 @@ 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)
 {