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From: Miklos Szeredi <miklos@szeredi.hu>
To: Matthew Wilcox <willy@infradead.org>
Cc: linux-fsdevel@vger.kernel.org,
	fuse-devel <fuse-devel@lists.sourceforge.net>,
	linux-mm <linux-mm@kvack.org>, miklos <mszeredi@redhat.com>,
	"Gabriel Krisman Bertazi" <krisman@collabora.com>,
	"André Almeida" <andrealmeid@collabora.com>
Subject: Re: [fuse-devel] fuse: trying to steal weird page
Date: Mon, 18 May 2020 16:58:05 +0200	[thread overview]
Message-ID: <CAJfpegtKQ85K2iJUvm-A+cTD1TKsa1AVTDnwbeky4hyf+SJfgQ@mail.gmail.com> (raw)
In-Reply-To: <20200518144853.GT16070@bombadil.infradead.org>

On Mon, May 18, 2020 at 4:48 PM Matthew Wilcox <willy@infradead.org> wrote:
>
> On Mon, May 18, 2020 at 02:45:02PM +0200, Miklos Szeredi wrote:

> > page_cache_pipe_buf_steal() calls remove_mapping() which calls
> > page_ref_unfreeze(page, 1).  That sets the refcount to 1, right?
> >
> > What am I missing?
>
> find_get_entry() calling page_cache_get_speculative().
>
> In a previous allocation, this page belonged to the page cache.  Then it
> was freed, but another thread is in the middle of a page cache lookup and
> has already loaded the pointer.  It is now delayed by a few clock ticks.
>
> Now the page is allocated to FUSE, which calls page_ref_unfreeze().
> And then the refcount gets bumped to 2 by page_cache_get_speculative().
> find_get_entry() calls xas_reload() and discovers this page is no longer
> at that index, so it calls put_page(), but in that narrow window, FUSE
> checks the refcount and finds it's not 1.

What if that page_cache_get_speculative() happens just before
page_ref_unfreeze()?  The speculative reference would be lost and on
put_page() the page would be freed, even though fuse is still holding
the pointer.

Thanks,
Miklos

  reply	other threads:[~2020-05-18 14:58 UTC|newest]

Thread overview: 26+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-05-02 19:09 Nikolaus Rath
2020-05-02 19:52 ` Nikolaus Rath
2020-05-03  3:26   ` Matthew Wilcox
2020-05-03  8:43     ` [fuse-devel] " Nikolaus Rath
2020-05-03 10:27       ` Matthew Wilcox
2020-05-03 18:28         ` Gabriel Krisman Bertazi
2020-05-03 20:06           ` Matthew Wilcox
2020-05-03 20:25           ` Nikolaus Rath
2020-05-06 13:57             ` Vlastimil Babka
2020-05-03 21:34         ` Hugh Dickins
2020-05-18 12:45         ` Miklos Szeredi
2020-05-18 14:48           ` Matthew Wilcox
2020-05-18 14:58             ` Miklos Szeredi [this message]
2020-05-18 15:26               ` Matthew Wilcox
  -- strict thread matches above, loose matches on Subject: below --
2018-12-26 21:43 Nikolaus Rath
2019-01-07  8:28 ` [fuse-devel] " Miklos Szeredi
2019-01-07 21:05   ` Nikolaus Rath
2019-01-08  8:27     ` Miklos Szeredi
2019-01-08 10:35       ` Nikolaus Rath
2019-01-09  8:07         ` Miklos Szeredi
2019-01-11 15:39           ` Nikolaus Rath
2019-01-11 15:39             ` [fuse-devel] " Nikolaus Rath
2019-02-10 22:05             ` Nikolaus Rath
2019-02-12 14:57               ` Miklos Szeredi
2019-02-12 21:28                 ` Nikolaus Rath
2019-02-25 21:41                   ` Nikolaus Rath
2019-02-26 12:57                     ` Miklos Szeredi
2019-02-26 13:30                       ` Miklos Szeredi

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