Re: [REGRESSION][BISECTED] Crash with Bad page state for FUSE/Flatpak related applications since v6.13

[Joanne Koong <joannelkoong@gmail.com>]

On Mon, Feb 10, 2025 at 12:27 AM Vlastimil Babka <vbabka@suse.cz> wrote:
>
> On 2/8/25 16:46, Joanne Koong wrote:
> > On Sat, Feb 8, 2025 at 2:11 AM Matthew Wilcox <willy@infradead.org> wrote:
> >>
> >> On Fri, Feb 07, 2025 at 04:22:56PM -0800, Joanne Koong wrote:
> >> > > Thanks, Josef. I guess we can at least try to confirm we're on the right track.
> >> > > Can anyone affected see if this (only compile tested) patch fixes the issue?
> >> > > Created on top of 6.13.1.
> >> >
> >> > This fixes the crash for me on 6.14.0-rc1. I ran the repro using
> >> > Mantas's instructions for Obfuscate. I was able to trigger the crash
> >> > on a clean build and then with this patch, I'm not seeing the crash
> >> > anymore.
> >>
> >> Since this patch fixes the bug, we're looking for one call to folio_put()
> >> too many.  Is it possibly in fuse_try_move_page()?  In particular, this
> >> one:
> >>
> >>         /* Drop ref for ap->pages[] array */
> >>         folio_put(oldfolio);
> >>
> >> I don't know fuse very well.  Maybe this isn't it.
> >
> > Yeah, this looks it to me. We don't grab a folio reference for the
> > ap->pages[] array for readahead and it tracks with Mantas's
> > fuse_dev_splice_write() dmesg. this patch fixed the crash for me when
> > I tested it yesterday:
> >
> > diff --git a/fs/fuse/file.c b/fs/fuse/file.c
> > index 7d92a5479998..172cab8e2caf 100644
> > --- a/fs/fuse/file.c
> > +++ b/fs/fuse/file.c
> > @@ -955,8 +955,10 @@ static void fuse_readpages_end(struct fuse_mount
> > *fm, struct fuse_args *args,
> >                 fuse_invalidate_atime(inode);
> >         }
> >
> > -       for (i = 0; i < ap->num_folios; i++)
> > +       for (i = 0; i < ap->num_folios; i++) {
> >                 folio_end_read(ap->folios[i], !err);
> > +               folio_put(ap->folios[i]);
> > +       }
> >         if (ia->ff)
> >                 fuse_file_put(ia->ff, false);
> >
> > @@ -1049,6 +1051,7 @@ static void fuse_readahead(struct readahead_control *rac)
> >
> >                 while (ap->num_folios < cur_pages) {
> >                         folio = readahead_folio(rac);
> > +                       folio_get(folio);
>
> This is almost the same as my patch, but balances the folio_put() in
> readahead_folio() with another folio_get(), while mine uses
> __readahead_folio() that does not do folio_put() in the first place.
>
> But I think neither patch proves the extraneous folio_put() comes from
> fuse_try_move_page().
>
> >                         ap->folios[ap->num_folios] = folio;
> >                         ap->descs[ap->num_folios].length = folio_size(folio);
> >                         ap->num_folios++;
> >
> >
> > I reran it just now with a printk by that ref drop in
> > fuse_try_move_page() and I'm indeed seeing that path get hit.
>
> It might get hit, but is it hit in the readahead paths? One way to test
> would be to instead of yours above or mine change, to stop doing the
> foio_put() in fuse_try_move_page(). But maybe it's called also from other
> contexts that do expect it, and will leak memory otherwise.

When I tested it a few days ago, I printk-ed the address of the folio
and it matched in fuse_readahead() and try_move_page(). I think that
proves that the extra folio_put() came from fuse_try_move_page()
through the readahead path.

>
> > Not sure why fstests didn't pick this up though since splice is
> > enabled by default in passthrough_hp, i'll look into this next week.
> >
>