linux-kernel - Re: [PATCH v2 3/3] NFS: Fix a memory leak in nfs

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Date:	Wed, 1 Dec 2010 08:47:34 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Rik van Riel <riel@...hat.com>
Cc:	Trond Myklebust <Trond.Myklebust@...app.com>,
	Nick Bowler <nbowler@...iptictech.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-nfs@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Hugh Dickins <hughd@...gle.com>,
	Christoph Hellwig <hch@....de>,
	Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH v2 3/3] NFS: Fix a memory leak in nfs_readdir

On Wed, Dec 1, 2010 at 8:35 AM, Rik van Riel <riel@...hat.com> wrote:
>
> Surely somebody can have just looked up the page and
> gotten a reference count, right before your ->freepage
> call is invoked?

No.

The removal from the page cache is atomic, even in the presence of the
lockless lookup.

The page cache lookup does a "get_page_unless_zero()" on the count, so
when __remove_mapping() has removed the page using
"page_freeze_refs()", it's really gone, and cannot be looked up.

And if that is broken, then we have much more serious problems (like
aliasing the same page when doing mmap/read etc), so that's more than
just an implementation detail, it's a fundamental requirement of the
whole page-cache design.

And that's the whole point of adding this callback to the
__remove_mapping() stage: that's the _only_ point where we really end
up knowing that "yes, we really removed that page, and there are no
more users".

                              Linus
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