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

[Matthew Wilcox <willy@infradead.org>]

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.

Also, page_count() is unstable.  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!