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

[Nikolaus Rath <Nikolaus@rath.org>]

On May 03 2020, Gabriel Krisman Bertazi <krisman@collabora.com> wrote:
> Matthew Wilcox <willy@infradead.org> writes:
>
>> 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.
>>
>
> On the first message, Nikolaus sent the following line:
>
>>> [ 2333.009937] fuse: page=00000000dd1750e3 index=2022240 flags=17ffffc0000097, count=1,
>>> mapcount=0, mapping=00000000125079ad
>
> It should be noted that on the second run, where we got the dump_page
> log, it indeed had a null mapping, which is similar to what Nikolaus
> asked on the previous thread he linked to, but looks like this wasn't
> the case on at least some of the reproductions of the issue.  On the
> line above, the condition that triggered the warning was page->mapping
> != NULL.  I don't know what to do with this information, though.

Indeed, that's curious. I've modified the patch slightly to print both
the old and the new message to confirm. And indeed:

[  260.882873] fuse: trying to steal weird page
[  260.882879] fuse:   page=00000000813e7570 index=2010048 flags=17ffffc0000097, count=1, mapcount=0, mapping=0000000094844a11
[  260.882882] page:ffffe13431bcc000 refcount:1 mapcount:0 mapping:0000000000000000 index:0x1eabc0
[  260.882885] flags: 0x17ffffc0000097(locked|waiters|referenced|uptodate|lru)
[  260.882889] raw: 0017ffffc0000097 ffffe13431ca7c48 ffffe13431bcbfc8 0000000000000000
[  260.882891] raw: 00000000001eabc0 0000000000000000 00000001ffffffff ffff91fe31318000
[  260.882892] page dumped because: fuse: trying to steal weird page
[  260.882893] page->mem_cgroup:ffff91fe31318000
[  262.608438] fuse: trying to steal weird page
[  262.608444] fuse:   page=000000000c21d0c7 index=2040704 flags=17ffffc0000097, count=1, mapcount=0, mapping=0000000094844a11
[  262.608447] page:ffffe134319ebf80 refcount:1 mapcount:0 mapping:0000000000000000 index:0x1f2380
[  262.608450] flags: 0x17ffffc0000097(locked|waiters|referenced|uptodate|lru)
[  262.608455] raw: 0017ffffc0000097 ffffe134319e9fc8 ffffe134319ebf48 0000000000000000
[  262.608457] raw: 00000000001f2380 0000000000000000 00000001ffffffff ffff91fe31318000
[  262.608458] page dumped because: fuse: trying to steal weird page
[  262.608459] page->mem_cgroup:ffff91fe31318000
[  262.770209] fuse: trying to steal weird page
[  262.770215] fuse:   page=0000000018a813ac index=2045120 flags=17ffffc0000097, count=1, mapcount=0, mapping=0000000094844a11
[  262.770218] page:ffffe134319953c0 refcount:1 mapcount:0 mapping:0000000000000000 index:0x1f34c0
[  262.770221] flags: 0x17ffffc0000097(locked|waiters|referenced|uptodate|lru)
[  262.770225] raw: 0017ffffc0000097 ffffe134319ae788 ffffe13431995388 0000000000000000
[  262.770228] raw: 00000000001f34c0 0000000000000000 00000001ffffffff ffff91fe31318000
[  262.770229] page dumped because: fuse: trying to steal weird page
[  262.770230] page->mem_cgroup:ffff91fe31318000



Best,
-Nikolaus

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