Re: [PATCH 08/29] fsverity: add per-sb workqueue for post read processing

Andrey Albershteyn
2024-03-20T10:37:34Z

On 2024-03-19 16:30:10, Darrick J. Wong wrote:
> On Wed, Mar 13, 2024 at 10:54:39AM -0700, Darrick J. Wong wrote:
> > From: Andrey Albershteyn <aalbersh@redhat.com>
> >
> > For XFS, fsverity's global workqueue is not really suitable due to:
> >
> > 1. High priority workqueues are used within XFS to ensure that data
> > IO completion cannot stall processing of journal IO completions.
> > Hence using a WQ_HIGHPRI workqueue directly in the user data IO
> > path is a potential filesystem livelock/deadlock vector.
> >
> > 2. The fsverity workqueue is global - it creates a cross-filesystem
> > contention point.
> >
> > This patch adds per-filesystem, per-cpu workqueue for fsverity
> > work. This allows iomap to add verification work in the read path on
> > BIO completion.
> >
> > Signed-off-by: Andrey Albershteyn <aalbersh@redhat.com>
> > Reviewed-by: Darrick J. Wong <djwong@kernel.org>
> > Signed-off-by: Darrick J. Wong <djwong@kernel.org>
> > ---
> > fs/super.c | 7 +++++++
> > include/linux/fs.h | 2 ++
> > include/linux/fsverity.h | 22 ++++++++++++++++++++++
> > 3 files changed, 31 insertions(+)
> >
> >
> > diff --git a/fs/super.c b/fs/super.c
> > index d35e85295489..338d86864200 100644
> > --- a/fs/super.c
> > +++ b/fs/super.c
> > @@ -642,6 +642,13 @@ void generic_shutdown_super(struct super_block *sb)
> > sb->s_dio_done_wq = NULL;
> > }
> >
> > +#ifdef CONFIG_FS_VERITY
> > + if (sb->s_read_done_wq) {
> > + destroy_workqueue(sb->s_read_done_wq);
> > + sb->s_read_done_wq = NULL;
> > + }
> > +#endif
> > +
> > if (sop->put_super)
> > sop->put_super(sb);
> >
> > diff --git a/include/linux/fs.h b/include/linux/fs.h
> > index ed5966a70495..9db24a825d94 100644
> > --- a/include/linux/fs.h
> > +++ b/include/linux/fs.h
> > @@ -1221,6 +1221,8 @@ struct super_block {
> > #endif
> > #ifdef CONFIG_FS_VERITY
> > const struct fsverity_operations *s_vop;
> > + /* Completion queue for post read verification */
> > + struct workqueue_struct *s_read_done_wq;
> > #endif
> > #if IS_ENABLED(CONFIG_UNICODE)
> > struct unicode_map *s_encoding;
> > diff --git a/include/linux/fsverity.h b/include/linux/fsverity.h
> > index 0973b521ac5a..45b7c613148a 100644
> > --- a/include/linux/fsverity.h
> > +++ b/include/linux/fsverity.h
> > @@ -241,6 +241,22 @@ void fsverity_enqueue_verify_work(struct work_struct *work);
> > void fsverity_invalidate_block(struct inode *inode,
> > struct fsverity_blockbuf *block);
> >
> > +static inline int fsverity_set_ops(struct super_block *sb,
> > + const struct fsverity_operations *ops)
> > +{
> > + sb->s_vop = ops;
> > +
> > + /* Create per-sb workqueue for post read bio verification */
> > + struct workqueue_struct *wq = alloc_workqueue(
> > + "pread/%s", (WQ_FREEZABLE | WQ_MEM_RECLAIM), 0, sb->s_id);
>
> Looking at this more closely, why is it that the fsverity_read_queue
> is unbound and tagged WQ_HIGHPRI, whereas this one is instead FREEZEABLE
> and MEM_RECLAIM and bound?
>
> If it's really feasible to use /one/ workqueue for all the read
> post-processing then this ought to be a fs/super.c helper ala
> sb_init_dio_done_wq. That said, from Eric's comments on the v5 thread
> about fsverity and fscrypt locking horns over workqueue stalls I'm not
> convinced that's true.

There's good explanation by Dave why WQ_HIGHPRI is not a good fit
for XFS (potential livelock/deadlock):

https://lore.kernel.org/linux-xfs/20221214054357.GI3600936@dread.disaster.area/

Based on his feedback I changed it to per-filesystem.

--
- Andrey