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

[Miklos Szeredi <miklos@szeredi.hu>]

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On Fri, Mar 1, 2019 at 9:40 PM Nikolaus Rath <Nikolaus@rath.org> wrote:
>
> On Feb 26 2019, Miklos Szeredi <miklos@szeredi.hu> wrote:
> > On Tue, Feb 26, 2019 at 9:35 PM Nikolaus Rath <Nikolaus@rath.org> wrote:
> >>
> >> [ Moving fuse-devel and linux-fsdevel to Bcc ]
> >>
> >> Hello linux-mm people,
> >>
> >> I am posting this here as advised by Miklos (see below). In short, I
> >> have a workload that reliably produces kernel messages of the form:
> >>
> >> [ 2562.773181] fuse: trying to steal weird page
> >> [ 2562.773187] page=<something> index=<something> flags=17ffffc00000ad, count=1, mapcount=0, mapping= (null)
> >>
> >> What are the implications of this message? Is something activelly going
> >> wrong (aka do I need to worry about data integrity)?
> >
> > Fuse is careful and basically just falls back on page copy, so it
> > definitely shouldn't affect data integrity.
> >
> > The more interesting question is: how can page_cache_pipe_buf_steal()
> > return a dirty page?  The logic in remove_mapping() should prevent
> > that, but something is apparently slipping through...
> >
> >>
> >> Is there something I can do to help debugging (and hopefully fixing)
> >> this?
> >>
> >> This is with kernel 4.18 (from Ubuntu cosmic).
> >
> > One thought: have you tried reproducing with a recent vanilla
> > (non-ubuntu) kernel?
>
> Yes, I can reproduce with e.g. 5.0.0-050000rc8 (from
> https://kernel.ubuntu.com/~kernel-ppa/mainline/v5.0-rc8/). However, here
> the flag value is different:
>
> [  278.183571] fuse: trying to steal weird page
> [  278.183576]   page=000000000aab208c index=14944 flags=17ffffc0000097, count=1, mapcount=0, mapping=          (null)
>
> (but still the same across all messages observed with this kernel so
> far).

Ah, so it's just the PG_waiters flag that is triggering the "weird
page" message.   And it looks like it's okay if PG_waiters remains
set, at least that's what I infer from the comments in
wake_up_page_bit().  Patch attached.

I'm not sure about the Ubuntu one, you should try filing a bug report
with them, I think.

Thanks,
Miklos

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diff --git a/fs/fuse/dev.c b/fs/fuse/dev.c
index 8a63e52785e9..769605cdf2bd 100644
--- a/fs/fuse/dev.c
+++ b/fs/fuse/dev.c
@@ -905,6 +905,7 @@ static int fuse_check_page(struct page *page)
 	       1 << PG_uptodate |
 	       1 << PG_lru |
 	       1 << PG_active |
+	       1 << PG_waiters |
 	       1 << PG_reclaim))) {
 		printk(KERN_WARNING "fuse: trying to steal weird page\n");
 		printk(KERN_WARNING "  page=%p index=%li flags=%08lx, count=%i, mapcount=%i, mapping=%p\n", page, page->index, page->flags, page_count(page), page_mapcount(page), page->mapping);