[PATCH 1/2] Unique path for locking page in filemap_fault()

[Michel Lespinasse <walken@google.com>]

Note that if the file got truncated before we locked the page, we retry
at find_get_page(). This is similar to what find_lock_page() would do.
Linus's commit ef00e08e introduced a goto no_cached_page in that simuation,
which seems correct as well but possibly less efficient ?

----------------------- >8 ------------------------

This change introduces a single location where filemap_fault() locks
the desired page. There used to be two such places, depending if the
initial find_get_page() was successful or not.

Signed-off-by: Michel Lespinasse <walken@google.com>
---
 mm/filemap.c |   20 +++++++++++---------
 1 files changed, 11 insertions(+), 9 deletions(-)

diff --git a/mm/filemap.c b/mm/filemap.c
index 3d4df44..8ed709a 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -1539,25 +1539,27 @@ int filemap_fault(struct vm_area_struct *vma, struct vm_fault *vmf)
 		 * waiting for the lock.
 		 */
 		do_async_mmap_readahead(vma, ra, file, page, offset);
-		lock_page(page);
-
-		/* Did it get truncated? */
-		if (unlikely(page->mapping != mapping)) {
-			unlock_page(page);
-			put_page(page);
-			goto no_cached_page;
-		}
 	} else {
 		/* No page in the page cache at all */
 		do_sync_mmap_readahead(vma, ra, file, offset);
 		count_vm_event(PGMAJFAULT);
 		ret = VM_FAULT_MAJOR;
 retry_find:
-		page = find_lock_page(mapping, offset);
+		page = find_get_page(mapping, offset);
 		if (!page)
 			goto no_cached_page;
 	}
 
+	lock_page(page);
+
+	/* Did it get truncated? */
+	if (unlikely(page->mapping != mapping)) {
+		unlock_page(page);
+		put_page(page);
+		goto retry_find;
+	}
+	VM_BUG_ON(page->index != offset);
+
 	/*
 	 * We have a locked page in the page cache, now we need to check
 	 * that it's up-to-date. If not, it is going to be due to an error.
-- 
1.7.1

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