linux-kernel - Re: [PATCH 5.15] fuse: Fix race condition in writethrough path A race

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Message-ID: <CAJnrk1Y+rdH11k_n947Z2rofu39=9=C5CRK5USi7Z1CnEG7fcg@mail.gmail.com>
Date: Wed, 15 Oct 2025 13:28:39 -0700
From: Joanne Koong <joannelkoong@...il.com>
To: Brian Foster <bfoster@...hat.com>
Cc: Miklos Szeredi <miklos@...redi.hu>, lu gu <giveme.gulu@...il.com>, 
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Bernd Schubert <bernd@...ernd.com>
Subject: Re: [PATCH 5.15] fuse: Fix race condition in writethrough path A race

On Wed, Oct 15, 2025 at 12:44 PM Brian Foster <bfoster@...hat.com> wrote:
>
> On Wed, Oct 15, 2025 at 10:19:15AM -0700, Joanne Koong wrote:
> > On Wed, Oct 15, 2025 at 7:09 AM Miklos Szeredi <miklos@...redi.hu> wrote:
> > >
> > > On Wed, 15 Oct 2025 at 06:00, lu gu <giveme.gulu@...il.com> wrote:
> > > >
> > > > >  Attaching a test patch, minimally tested.
> > > > Since I only have a test environment for kernel 5.15, I ported this
> > > > patch to the FUSE module in 5.15. I ran the previous LTP test cases
> > > > more than ten times, and the data inconsistency issue did not reoccur.
> > > > However, a deadlock occur. Below is the specific stack trace.
> > >
> > > This is does not reproduce for me on 6.17 even after running the test
> > > for hours.  Without seeing your backport it is difficult to say
> > > anything about the reason for the deadlock.
> > >
> > > Attaching an updated patch that takes care of i_wb initialization on
> > > CONFIG_CGROUP_WRITEBACK=y.
> >
> > I think now we'll also need to always set
> > mapping_set_writeback_may_deadlock_on_reclaim(), eg
> >
> > @@ -3125,8 +3128,7 @@ void fuse_init_file_inode(struct inode *inode,
> > unsigned int flags)
> >
> >         inode->i_fop = &fuse_file_operations;
> >         inode->i_data.a_ops = &fuse_file_aops;
> > -       if (fc->writeback_cache)
> > -               mapping_set_writeback_may_deadlock_on_reclaim(&inode->i_data);
> > +       mapping_set_writeback_may_deadlock_on_reclaim(&inode->i_data);
> >
> >
> > Does this completely get rid of the race? There's a fair chance I'm
> > wrong here but doesn't the race still happen if the read invalidation
> > happens before the write grabs the folio lock? This is the scenario
> > I'm thinking of:
> >
> > Thread A (read):
> > read, w/ auto inval and a outdated mtime triggers invalidate_inode_pages2()
> > generic_file_read_iter() is called, which calls filemap_read() ->
> > filemap_get_pages() -> triggers read_folio/readahead
> > read_folio/readahead fetches data (stale) from the server, unlocks folios
> >
> > Thread B (writethrough write):
> > fuse_perform_write() -> fuse_fill_write_pages():
> > grabs the folio lock and copies new write data to page cache, sets
> > writeback flag and unlocks folio, sends request to server
> >
> > Thread A (read):
> > the read data that was fetched from the server gets copied to the page
> > cache in filemap_read()
> > overwrites the write data in the page cache with the stale data
> >
> > Am i misanalyzing something in this sequence?
> >
>
> Maybe I misread the description, but I think folios are locked across
> read I/O, so I don't follow how we could race with readahead in this
> way. Hm?

Ah I see where my analysis went wrong - the "copy_folio_to_iter()"
call in filemap_read() copies the data into the client's user buffer,
not the data into the page cache. The data gets copied to the page
cache in the fuse code in fuse_copy_out_args() (through
fuse_dev_do_write()), which has to be under the folio lock. Yeah
you're right, there's no race condition here then. Thanks for clearing
this up.

>
> Brian
>
> > Thanks,
> > Joanne
> > >
> > > Thanks,
> > > Miklos
> >
>