Re: OOM kills when running fsstress on CIFS

On Tue, May 25, 2010 at 07:49:29AM -0400, Jeff Layton wrote:
> On Tue, 25 May 2010 21:16:39 +1000
> Nick Piggin <[email protected]> wrote:
> 
> > On Tue, May 25, 2010 at 06:57:05AM -0400, Jeff Layton wrote:
> > > Since 2.6.34, I've been able to consistently reproduce OOM kills when running fsstress (from the LTP suite) on CIFS. I spent some time yesterday and bisected it down to this patch:
> > > 
> > > ---------------------[snip]---------------------
> > > commit 315e995c63a15cb4d4efdbfd70fe2db191917f7a
> > > Author: Nick Piggin <[email protected]>
> > > Date:   Wed Apr 21 03:18:28 2010 +0000
> > > 
> > >     [CIFS] use add_to_page_cache_lru
> > >     
> > >     add_to_page_cache_lru is exported, so it should be used. Benefits over
> > >     using a private pagevec: neater code, 128 bytes fewer stack used, percpu
> > >     lru ordering is preserved, and finally don't need to flush pagevec
> > >     before returning so batching may be shared with other LRU insertions.
> > >     
> > >     Signed-off-by: Nick Piggin <[email protected]>
> > >     Reviewed-by: Dave Kleikamp <[email protected]>
> > >     Signed-off-by: Steve French <[email protected]>
> > > ---------------------[snip]---------------------
> > > 
> > > Here's how I've been reproducing it:
> > > 
> > > Mount up a samba share with -o sec=krb5i,nounix,noserverino
> > > 
> > > Run: fsstress -d /path/to/dir/on/cifs/ -n 1000 -l0 -p8
> > > 
> > > ...within an hour or two, I start getting OOM kills. After backing out
> > > the patch above, I was able to run the test overnight. I'm not sure yet
> > > what the actual problem is, but there seems to be something wrong with
> > > that patch.
> > > 
> > > Thoughts?
> > 
> > Yep, it's my fault. The problem is the refcounting. Previously the
> > code hands off the references to the LRU, wheras now the lru takes
> > a new reference. (the other filesystems converted to use this
> > function seemed to more conventionally open-code lru_cache_add).
> > 
> > Can we get rid of a refcount increment anywhere? Otherwise we'll
> > need to just drop the references after adding the pages.
> > 
> 
> The only caller of this function is cifs_readpages, and I don't see
> where it takes any references. I'm guessing that the pages come from
> the VFS with the refcount already incremented?

Yep. I think we should just page_cache_release after doing the
add_to_page_cache_lru, like the generic code does.

It's a little suboptimal tinkering with refcounts like that, but
a cleanup pass to fix it up all over the tree and allow
add_to_page_cache_lru to take over a reference would be better
place to fix that.

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