Re: BUG_ON() in workingset_node_shadows_dec() triggers

[Johannes Weiner <hannes@cmpxchg.org>]

On Wed, Oct 05, 2016 at 11:25:34AM +0200, Johannes Weiner wrote:
> Patch below. NOTE: I'm traveling without access to my test rig right
> now and so I have only lightly tested this on my laptop. I'm also
> jetlagged like crazy, so please triple check my thinking. The patch
> does fix the reproducer case and has otherwise been stable here.

There is an issue I spotted in the more recent fuse fix. Same caveat
applies for now regarding testing and brain function, but it should be
a relatively obvious one.

---

>From eaf606185719acaf66412413b7dbac5c683efe11 Mon Sep 17 00:00:00 2001
From: Johannes Weiner <hannes@cmpxchg.org>
Date: Tue, 4 Oct 2016 16:58:06 +0200
Subject: [PATCH] mm: filemap: fix mapping->nrpages double accounting in
 fuse

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@cmpxchg.org>
---
 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