[dm-devel] Why queue_work unneeded for REQUEUE bio

[Mike Snitzer]

On Thu, Nov 05 2020 at 10:49pm -0500,
JeffleXu <jefflexu at linux.alibaba.com> wrote:

> Hi Mike,
> 
> 
> I have another question about dm, though it's irrelevant to this original
> 
> mail.
> 
> 
> Currently abnormal IO will call blk_queue_split() and normal IO will
> 
> be split considering @max_sectors/@chunk_sectos (in max_io_len()).
> 
> 
> Question 1: Why bio should be split considering queue_limits in dm layer?
> 
> After all the underlying device will split themselves by queue_limits if
> 
> the dm layer doesn't split by queue_limits.

Some targets have "abnormal IO" constraints in their implementation that
is reflected in the queue_limits -- discards in particular.

> Then Question 2: Currently only @max_sectors is considered when splitting
> 
> normal IO in dm layer. Should we also consider
> @max_segments/@max_segment_size
> 
> as blk_queue_split() does?

Great question, it does appear the one gap in DM's splitting for
"normal" IO. I'm less familiar with @max_segments/@max_segment_size. 

Since commit 5091cdec56fa ("dm: change max_io_len() to use
blk_max_size_offset()") DM is making use of more block core code to
calculate its splits -- the offset based splitting is much more natural 
for DM to perform given that potential for spanning multiple targets,
etc.  But DM targets really don't get involved with concern for
@max_segments/@max_segment_size

dm-crypt.c:crypt_io_hints is the only DM target code that concerns
itself with @max_segment_size -- and it is punitive by setting it to
PAGE_SIZE, please see commit 586b286b110e94e ("dm crypt: constrain crypt
device's max_segment_size to PAGE_SIZE") for more context.

Mike


> On 11/3/20 5:23 PM, Jeffle Xu wrote:
> >Hi Mike,
> >
> >Why queue_work() is unnecessary here for bio with BLK_STS_DM_REQUEUE
> >returned?
> >
> >Thanks
> >Jeffle Xu
> >
> >---
> >  drivers/md/dm.c | 4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> >
> >diff --git a/drivers/md/dm.c b/drivers/md/dm.c
> >index c18fc2548518..ae550daa99b5 100644
> >--- a/drivers/md/dm.c
> >+++ b/drivers/md/dm.c
> >@@ -908,9 +908,11 @@ static void dec_pending(struct dm_io *io, blk_status_t error)
> >  			 * Target requested pushing back the I/O.
> >  			 */
> >  			spin_lock_irqsave(&md->deferred_lock, flags);
> >-			if (__noflush_suspending(md))
> >+			if (__noflush_suspending(md)) {
> >  				/* NOTE early return due to BLK_STS_DM_REQUEUE below */
> >  				bio_list_add_head(&md->deferred, io->orig_bio);
> >+				queue_work(md->wq, &md->work);
> >+			}
> >  			else
> >  				/* noflush suspend was interrupted. */
> >  				io->status = BLK_STS_IOERR;
> 
> -- 
> Thanks,
> Jeffle
>