Re: [fuse-devel] fuse: trying to steal weird page

[Nikolaus Rath <Nikolaus@rath.org>]

On Jan 11 2019, Nikolaus Rath <Nikolaus@rath.org> wrote:
> 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.

Bad news. I can now reliably reproduce the issue again.

I have no clue why it disappeared for a while. This is exactly the same
filesystem code, but I can't rule out that there has been some routine upgrade
in the base system (compiler or kernel).

Any suggestions for debugging this?

As I said before, splice() source is a regular file on ext4, opened
without O_DIRECT or DAX. If I disable FUSE_CAP_SPLICE_WRITE, the error
message no longer occurs.


Best,
-Nikolaus

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