Re: fuse: trying to steal weird page

[Nikolaus Rath <Nikolaus@rath.org>]

On May 02 2020, Nikolaus Rath <Nikolaus@rath.org> wrote:
> Hello,
>
> I have recently noticed that a FUSE filesystem regularly produces many
> kernel messages like this:
>
> [ 2333.009931] fuse: trying to steal weird page
> [ 2333.009937] fuse: page=00000000dd1750e3 index=2022240 flags=17ffffc0000097, count=1,
> mapcount=0, mapping=00000000125079ad
> [ 2334.595835] fuse: trying to steal weird page
> [ 2334.595841] fuse: page=000000009e8626ac index=2052288 flags=17ffffc0000097, count=1,
> mapcount=0, mapping=00000000125079ad
> [ 2334.983568] fuse: trying to steal weird page
> [ 2334.983572] fuse: page=0000000013fdd9e4 index=2059392 flags=17ffffc0000097, count=1,
> mapcount=0, mapping=00000000125079ad
> [ 2335.979905] fuse: trying to steal weird page
> [ 2335.979911] fuse: page=00000000a7057848 index=2078588 flags=17ffffc0000097, count=1,
> mapcount=0, mapping=00000000125079ad
>
> They do not seem to correlate with userspace errors, but I noticed that
> there is no significant performance difference between libfuse using
> splice, splice with SPLICE_MOVE, and not using splice. This is somewhat
> unexpected, so maybe it is related to the kernel messages?
>
> What are the implications of the above kernel message? Is there a way to
> provide more debugging information?
>
> (I have reported a similar issue before
> (https://marc.info/?l=linux-mm&m=155290847909996&w=2) and the patch may
> not be present in the kernel that I'm using. However, the previous time
> the warning had a different set of flags and a null mapping, so I think
> this one is different).
>
> $ uname -a
> Linux valve 5.3.0-46-generic #38-Ubuntu SMP Fri Mar 27 17:37:05 UTC 2020 x86_64 x86_64 x86_64 GNU/Linux


I have now reproduced this with a vanilla 5.6 kernel.

I also found that the "mapping" and "flags" parameters always have the
same values - even across different workloads and when re-mounting the
filesystem. The "page" parameter varies.


Best,
-Nikolaus

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