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From: Hugh Dickins <hughd@google.com>
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>,
	Andre Almeida <andrealmeid@collabora.com>
Subject: Re: [fuse-devel] fuse: trying to steal weird page
Date: Sun, 3 May 2020 14:34:39 -0700 (PDT)	[thread overview]
Message-ID: <alpine.LSU.2.11.2005031421230.4028@eggly.anvils> (raw)
In-Reply-To: <20200503102742.GF29705@bombadil.infradead.org>

On Sun, 3 May 2020, Matthew Wilcox wrote:
> On Sun, May 03, 2020 at 09:43:41AM +0100, Nikolaus Rath wrote:
> > Here's what I got:
> > 
> > [  221.277260] page:ffffec4bbd639880 refcount:1 mapcount:0 mapping:0000000000000000 index:0xd9
> > [  221.277265] flags: 0x17ffffc0000097(locked|waiters|referenced|uptodate|lru)
> > [  221.277269] raw: 0017ffffc0000097 ffffec4bbd62f048 ffffec4bbd619308 0000000000000000
> > [  221.277271] raw: 00000000000000d9 0000000000000000 00000001ffffffff ffff9aec11beb000
> > [  221.277272] page dumped because: fuse: trying to steal weird page
> > [  221.277273] page->mem_cgroup:ffff9aec11beb000
> 
> Great!  Here's the condition:
> 
>         if (page_mapcount(page) ||
>             page->mapping != NULL ||
>             page_count(page) != 1 ||
>             (page->flags & PAGE_FLAGS_CHECK_AT_PREP &
>              ~(1 << PG_locked |
>                1 << PG_referenced |
>                1 << PG_uptodate |
>                1 << PG_lru |
>                1 << PG_active |
>                1 << PG_reclaim))) {
> 
> mapcount is 0, mapping is NULL, refcount is 1, so that's all fine.
> flags has 'waiters' set, which is not in the allowed list.  I don't
> know the internals of FUSE, so I don't know why that is.

That list of PG_flags dates back to 2010: which 2016's 62906027091f
("mm: add PageWaiters indicating tasks are waiting for a page bit")
ought to have updated.  Though it's understandable that it did not:
surprising to find a list of PG_flags outside of mm/ and fs/proc/.
Just add PG_waiters to the list and the issue should go away.

> 
> Also, page_count() is unstable.

Agreed: fine to back out if page_count() is high,
but not good to issue a worrying warning about it.

> Unless there has been an RCU grace period
> between when the page was freed and now, a speculative reference may exist
> from the page cache.  So I would say this is a bad thing to check for.
> 
> Thanks for the swift provision of the debugging data!

  parent reply	other threads:[~2020-05-03 21:34 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 [this message]
2020-05-18 12:45         ` Miklos Szeredi
2020-05-18 14:48           ` Matthew Wilcox
2020-05-18 14:58             ` Miklos Szeredi
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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