[PATCH] mm: filemap: fix mapping->nrpages double accounting in fuse

[Johannes Weiner]

22f2ac51b6d6 ("mm: workingset: fix crash in shadow node shrinker
caused by replace_page_cache_page()") switched replace_page_cache()
from raw radix tree operations to page_cache_tree_insert() but didn't
take into account that the latter function, unlike the raw radix tree
op, handles mapping->nrpages. As a result, that counter is bumped for
each page replacement rather than balanced out even.

The mapping->nrpages counter is used to skip needless radix tree walks
when invalidating, truncating, syncing inodes without pages, as well
as statistics for userspace. Since the error is positive, we'll do
more page cache tree walks than necessary; we won't miss a necessary
one. And we'll report more buffer pages to userspace than there
are. The error is limited to fuse inodes.

Fixes: 22f2ac51b6d6 ("mm: workingset: fix crash in shadow node shrinker caused by replace_page_cache_page()")
Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>
---
 mm/filemap.c | 1 -
 1 file changed, 1 deletion(-)

diff --git a/mm/filemap.c b/mm/filemap.c
index 2d0986a64f1f..c17395825650 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -619,7 +619,6 @@ int replace_page_cache_page(struct page *old, struct page *new, gfp_t gfp_mask)
 		__delete_from_page_cache(old, NULL);
 		error = page_cache_tree_insert(mapping, new, NULL);
 		BUG_ON(error);
-		mapping->nrpages++;
 
 		/*
 		 * hugetlb pages do not participate in page cache accounting.
-- 
2.10.0