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

[Joanne Koong <joannelkoong@gmail.com>]

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);
                        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.

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.