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

[Matthew Wilcox <willy@infradead.org>]

On Fri, Feb 07, 2025 at 07:39:02PM +0100, Vlastimil Babka wrote:
> On 2/7/25 18:29, Josef Bacik wrote:
> > On Fri, Feb 07, 2025 at 05:49:34PM +0100, Vlastimil Babka wrote:
> >> On 2/7/25 10:34, Miklos Szeredi wrote:
> >> > [Adding Joanne, Willy and linux-mm].
> >> > 
> >> > 
> >> > On Thu, 6 Feb 2025 at 11:54, Christian Heusel <christian@heusel.eu> wrote:
> >> >>
> >> >> Hello everyone,
> >> >>
> >> >> we have recently received [a report][0] on the Arch Linux Gitlab about
> >> >> multiple users having system crashes when using Flatpak programs and
> >> >> related FUSE errors in their dmesg logs.
> >> >>
> >> >> We have subsequently bisected the issue within the mainline kernel tree
> >> >> to the following commit:
> >> >>
> >> >>     3eab9d7bc2f4 ("fuse: convert readahead to use folios")
> >> 
> >> I see that commit removes folio_put() from fuse_readpages_end(). Also it now
> >> uses readahead_folio() in fuse_readahead() which does folio_put(). So that's
> >> suspicious to me. It might be storing pointers to pages to ap->pages without
> >> pinning them with a refcount.

you don't need to pin them with a refcount.  the folio is locked, so the
page cache has a refcount until you unlock it (possibly by calling
folio_end_read()).