Re: fuse: trying to steal weird page

[Nikolaus Rath <Nikolaus-BTH8mxji4b0@public.gmane.org>]

On Jan 09 2019, Miklos Szeredi <miklos@szeredi.hu> wrote:
> On Tue, Jan 8, 2019 at 11:35 AM Nikolaus Rath <Nikolaus@rath.org> wrote:
>>
>> On Jan 08 2019, Miklos Szeredi <miklos@szeredi.hu> wrote:
>> > On Mon, Jan 7, 2019 at 10:05 PM Nikolaus Rath <Nikolaus@rath.org> wrote:
>> >>
>> >> On Jan 07 2019, Miklos Szeredi <miklos@szeredi.hu> wrote:
>> >> > On Wed, Dec 26, 2018 at 10:44 PM Nikolaus Rath <Nikolaus@rath.org> wrote:
>> >> >>
>> >> >> Hi,
>> >> >>
>> >> >> I am seeing relatively regular occurences of
>> >> >>
>> >> >> $ sudo dmesg | tail
>> >> >> [21929.138815] fuse: trying to steal weird page
>> >> >> [21929.138821] page=00000000a7dd2617 index=64 flags=17fffc0000000ad,
>> >> >> count=1, mapcount=0, mapping= (null)
>> >> >> [21930.647338] fuse: trying to steal weird page
>> >> >> [21930.647345] page=00000000a07f32af index=2848
>> >> >> flags=17fffc0000000ad, count=1, mapcount=0, mapping= (null)
>> >> >> [21932.338873] fuse: trying to steal weird page
>> >> >> [21932.338879] page=0000000067e3a012 index=64 flags=17fffc0000000ad,
>> >> >> count=1, mapcount=0, mapping= (null)
>> >> >> [21933.930703] fuse: trying to steal weird page
>> >> >> [21933.930710] page=00000000046feb25 index=845
>> >> >> flags=17fffc0000000ad, count=1, mapcount=0, mapping= (null)
>> >> >> [21936.163174] fuse: trying to steal weird page
>> >> >> [21936.163180] page=00000000fb80fe27 index=0 flags=17fffc0000000ad,
>> >> >> count=1, mapcount=0, mapping= (null)
>> >> >
>> >> > The page has the PG_dity and PG_waiters flags set which are
>> >> > incompatible with stealing.  page_cache_pipe_buf_steal() does
>> >> > apparently filter out dirty ones, so it's not a regular file that we
>> >> > are trying to streal the page from.  So the question is: what is the
>> >> > source of the splice()?
>> >>
>> >> Hmm. I think it has to be a regular file. But as I mentioned in my other
>> >> email, I did have a race condition where fd's were closed
>> >> incorrectly. Is it possible that this also triggered the above,
>> >> i.e. that the fd was closed sometime during splice?
>> >
>> > Close during a syscall that uses the fd is not an issue, because a ref
>> > to the file is acquired.  So the race is between the close() and the
>> > internal fget(); if the close() wins then fget() will fail and the
>> > syscall will return EBADF.  If the fget() wins, then the syscall can
>> > run normally despite the fact that the fd was closed.
>> >
>> > Can you tell me what filesystem is the regular file (the one being
>> > spliced into fuse) is on?
>>
>> It's ext4.
>
> Next question: is file opened with O_DIRECT or is filesystem mounted
> with DAX, or anything fancy?

Neither. But thinking about this, I guess that (because of the race) the
fd could have been closed and re-opened before the ref was acquired. So
it may have turned into a directory fd.

To be honest, I don't think it's worth investigating this unless I see
it happen again now that the race in my code is fixed.


Best,
-Nikolaus

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