Re: [PATCH v2 3/3] NFS: Fix a memory leak in nfs_readdir

[Trond Myklebust <Trond.Myklebust@netapp.com>]

On Wed, 2010-12-01 at 08:17 -0800, Linus Torvalds wrote:
> On Wed, Dec 1, 2010 at 7:36 AM, Trond Myklebust
> <Trond.Myklebust@netapp.com> wrote:
> >
> > We need to ensure that the entries in the nfs_cache_array get cleared
> > when the page is removed from the page cache. To do so, we use the
> > releasepage address_space operation (which also requires us to set
> > the Pg_private flag).
> 
> So I really think that the whole "releasepage" use in NFS is simply
> overly complicated and was obviously too subtle.
> 
> The whole need for odd return values, for the page lock, and for the
> addition of clearing the up-to-date bit comes from the fact that this
> wasn't really what releasepage was designed for.
> 
> 'releasepage' was really designed for the filesystem having its own
> version of 'try_to_free_buffers()', which is just an optimistic "ok,
> we may be releasing this page, so try to get rid of any IO structures
> you have cached". It wasn't really a memory management thing.
> 
> And the thing is, it looks trivial to do the memory management
> approach by adding a new callback that gets called after the page is
> actually removed from the page cache. If we do that, then there are no
> races with any other users, since we remove things from the page cache
> atomically wrt page cache lookup. So the need for playing games with
> page locking and 'uptodate' simply goes away. As does the PG_private
> thing or the interaction with invalidatepage() etc.
> 
> So this is a TOTALLY UNTESTED trivial patch that just adds another
> callback. Does this work? I dunno. But I get the feeling that instead
> of having NFS work around the odd semantics that don't actually match
> what NFS wants, introducing a new callback with much simpler semantics
> would be simpler for everybody, and avoid the need for subtle code.
> 
> Hmm?
> 
>                    Linus


> include/linux/fs.h |    1 +
>  mm/vmscan.c        |    3 +++
>  2 files changed, 4 insertions(+), 0 deletions(-)
> 
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index c9e06cc..090f0ea 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -602,6 +602,7 @@ struct address_space_operations {
>         sector_t (*bmap)(struct address_space *, sector_t);
>         void (*invalidatepage) (struct page *, unsigned long);
>         int (*releasepage) (struct page *, gfp_t);
> +       void (*freepage)(struct page *);
>         ssize_t (*direct_IO)(int, struct kiocb *, const struct iovec
> *iov,
>                         loff_t offset, unsigned long nr_segs);
>         int (*get_xip_mem)(struct address_space *, pgoff_t, int,
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index d31d7ce..1accb01 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -499,6 +499,9 @@ static int __remove_mapping(struct address_space
> *mapping, struct page *page)
>                 mem_cgroup_uncharge_cache_page(page);
>         }
>  
> +       if (mapping->a_ops->freepage)
> +               mapping->a_ops->freepage(page);

Hmm... Looking again at the problem, it appears that the same callback
needs to be added to truncate_complete_page() and
invalidate_complete_page2(). Otherwise we end up in a situation where
the page can sometimes be removed from the page cache without calling
freepage().

> +
>         return 1;
>  
>  cannot_free:

Cheers
  Trond

-- 
Trond Myklebust
Linux NFS client maintainer

NetApp
Trond.Myklebust@netapp.com
www.netapp.com