[PATCH] fs/mbcache: make count_objects more robust.

[PATCH] fs/mbcache: make count_objects more robust.

[Jiang Biao]

When running ltp stress test for 7*24 hours, the vmscan occasionally
complains the following warning continuously,

mb_cache_scan+0x0/0x3f0 negative objects to delete
nr=-9232265467809300450
...

The tracing result shows the freeable(mb_cache_count returns)
is -1, which causes the continuous accumulation and overflow of
total_scan.

This patch make sure the mb_cache_count not return negative value,
which make the mbcache shrinker more robust.

Signed-off-by: Jiang Biao <jiang.biao2@zte.com.cn>
---
 fs/mbcache.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/fs/mbcache.c b/fs/mbcache.c
index d818fd2..b8b8b9c 100644
--- a/fs/mbcache.c
+++ b/fs/mbcache.c
@@ -269,6 +269,9 @@ static unsigned long mb_cache_count(struct shrinker *shrink,
 	struct mb_cache *cache = container_of(shrink, struct mb_cache,
 					      c_shrink);
 
+	/* Unlikely, but not impossible */
+	if (unlikely(cache->c_entry_count < 0))
+		return 0;
 	return cache->c_entry_count;
 }
 
-- 
2.7.4

Re: [PATCH] fs/mbcache: make count_objects more robust.

[Jan Kara]

On Mon 27-11-17 11:30:19, Jiang Biao wrote:
> When running ltp stress test for 7*24 hours, the vmscan occasionally
> complains the following warning continuously,
> 
> mb_cache_scan+0x0/0x3f0 negative objects to delete
> nr=-9232265467809300450
> ...
> 
> The tracing result shows the freeable(mb_cache_count returns)
> is -1, which causes the continuous accumulation and overflow of
> total_scan.
> 
> This patch make sure the mb_cache_count not return negative value,
> which make the mbcache shrinker more robust.
> 
> Signed-off-by: Jiang Biao <jiang.biao2@zte.com.cn>

Going through some old email... 

a) c_entry_count is unsigned so your patch is a nop as Coverity properly
noticed.

b) c_entry_count being outside 0..2*cache->c_max_entries is a plain bug. I
went through the logic and cannot find out how that could happen though.

But in either case your patch just does not make sense.

								Honza

> ---
>  fs/mbcache.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/fs/mbcache.c b/fs/mbcache.c
> index d818fd2..b8b8b9c 100644
> --- a/fs/mbcache.c
> +++ b/fs/mbcache.c
> @@ -269,6 +269,9 @@ static unsigned long mb_cache_count(struct shrinker *shrink,
>  	struct mb_cache *cache = container_of(shrink, struct mb_cache,
>  					      c_shrink);
>  
> +	/* Unlikely, but not impossible */
> +	if (unlikely(cache->c_entry_count < 0))
> +		return 0;
>  	return cache->c_entry_count;
>  }
>  
> -- 
> 2.7.4
> 
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR

Re: [PATCH] fs/mbcache: make count_objects more robust.

[Jan Kara]

On Fri 05-01-18 08:54:56, jiang.biao2@zte.com.cn wrote:
> > On Mon 27-11-17 11:30:19, Jiang Biao wrote:
> > > When running ltp stress test for 7*24 hours, the vmscan occasionally
> > > complains the following warning continuously,
> >> 
> >>  mb_cache_scan+0x0/0x3f0 negative objects to delete
> >>  nr=-9232265467809300450
> >>  ...
> >> 
> >>  The tracing result shows the freeable(mb_cache_count returns)
> >>  is -1, which causes the continuous accumulation and overflow of
> >>  total_scan.
> >> 
> >>  This patch make sure the mb_cache_count not return negative value,
> >>  which make the mbcache shrinker more robust.
> >> 
> >>  Signed-off-by: Jiang Biao <jiang.biao2@zte.com.cn>
> > 
> > Going through some old email...
> > a) c_entry_count is unsigned so your patch is a nop as Coverity properly
> > noticed.
> Indeed, would the following casting be good?
> +    if (unlikely((int)(cache->c_entry_count) < 0))
> +        return 0;

That check would at least have a chance of hitting but still it is just
hiding the real problem.

> > b) c_entry_count being outside 0..2*cache->c_max_entries is a plain bug. I
> > went through the logic and cannot find out how that could happen though.
> Is there any possibility that decreasing c_entry_count from 0 to -1 
> in mb_cache_entry_delete?

If we think we have -1 entries in a list, we have a larger problem than
just the wrong behavior of the shrinker. This is just a plain counter of
entries protected by a spinlock so there isn't space for accounting errors
or anything like that. If you can reproduce the problem on some reasonably
recent kernel, I'd be interested in debugging this.

								Honza

-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR