On Wed 24-05-17 16:22:36, Shaohua Li wrote:
> On Wed, May 24, 2017 at 01:40:13PM +0200, Jan Kara wrote:
> > Commit b685d3d65ac7 "block: treat REQ_FUA and REQ_PREFLUSH as
> > synchronous" removed REQ_SYNC flag from WRITE_{FUA|PREFLUSH|...}
> > definitions. generic_make_request_checks() however strips REQ_FUA and
> > REQ_PREFLUSH flags from a bio when the storage doesn't report volatile
> > write cache and thus write effectively becomes asynchronous which can
> > lead to performance regressions
> >
> > Fix the problem by making sure all bios which are synchronous are
> > properly marked with REQ_SYNC.
>
> DM and MD are different trees, so probably you should separate them to 2
> patches.
OK, I can do that.
> For the md part (md.c, raid5-cache.c), some placed which use REQ_FUA
> are missed, like raid5.c and raid5-ppl.c
So ops_run_io() in raid5.c only copy REQ_FUA from some internal raid5
flags. My thinking was that we want to just propagate whatever we were
instructed to do here.
The case in ppl_write_empty_header() is clearly missed, I'll fix that.
Thanks. I'm not quite sure about ppl_submit_iounit() - I don't see a place
where we are waiting for those bios to complete. If it is likely to happen
soon after bio submission, we should add REQ_SYNC there.
> Can't remember if others asked the question in your first post, sorry,
> but why we don't add REQ_SYNC in generic_make_request_checks() if we are
> going to stripe REQ_FUA, REQ_PREFLUSH. That will be less error prone.
Well, strictly speaking users of REQ_FUA do not necessarily have to use
REQ_SYNC. These are two different orthogonal things - one is a request for
bypassing disk cache, the other is a hint to the IO scheduler that there is
someone waiting for the IO to complete. Most of the time you wait for
REQ_FUA request immediately but I can see some uses in filesystems
where we might want to submit REQ_FUA request in the background (like when
doing background cleaning of the journal).
Honza
> > CC: linux-raid@xxxxxxxxxxxxxxx
> > CC: Shaohua Li <shli@xxxxxxxxxx>
> > CC: Mike Snitzer <snitzer@xxxxxxxxxx>
> > CC: dm-devel@xxxxxxxxxx
> > Fixes: b685d3d65ac791406e0dfd8779cc9b3707fea5a3
> > Signed-off-by: Jan Kara <jack@xxxxxxx>
> > ---
> > drivers/md/dm-snap-persistent.c | 3 ++-
> > drivers/md/md.c | 2 +-
> > drivers/md/raid5-cache.c | 4 ++--
> > 3 files changed, 5 insertions(+), 4 deletions(-)
> >
> > Guys, I don't know enough about DM/MD to judge whether I've identified all the
> > places that want REQ_SYNC right. Can you please have a look?
> >
> > diff --git a/drivers/md/dm-snap-persistent.c b/drivers/md/dm-snap-persistent.c
> > index b93476c3ba3f..b92ab4cb0710 100644
> > --- a/drivers/md/dm-snap-persistent.c
> > +++ b/drivers/md/dm-snap-persistent.c
> > @@ -741,7 +741,8 @@ static void persistent_commit_exception(struct dm_exception_store *store,
> > /*
> > * Commit exceptions to disk.
> > */
> > - if (ps->valid && area_io(ps, REQ_OP_WRITE, REQ_PREFLUSH | REQ_FUA))
> > + if (ps->valid && area_io(ps, REQ_OP_WRITE,
> > + REQ_SYNC | REQ_PREFLUSH | REQ_FUA))
> > ps->valid = 0;
> >
> > /*
> > diff --git a/drivers/md/md.c b/drivers/md/md.c
> > index 10367ffe92e3..212a6777ff31 100644
> > --- a/drivers/md/md.c
> > +++ b/drivers/md/md.c
> > @@ -765,7 +765,7 @@ void md_super_write(struct mddev *mddev, struct md_rdev *rdev,
> > test_bit(FailFast, &rdev->flags) &&
> > !test_bit(LastDev, &rdev->flags))
> > ff = MD_FAILFAST;
> > - bio->bi_opf = REQ_OP_WRITE | REQ_PREFLUSH | REQ_FUA | ff;
> > + bio->bi_opf = REQ_OP_WRITE | REQ_SYNC | REQ_PREFLUSH | REQ_FUA | ff;
> >
> > atomic_inc(&mddev->pending_writes);
> > submit_bio(bio);
> > diff --git a/drivers/md/raid5-cache.c b/drivers/md/raid5-cache.c
> > index 4c00bc248287..0a7af8b0a80a 100644
> > --- a/drivers/md/raid5-cache.c
> > +++ b/drivers/md/raid5-cache.c
> > @@ -1782,7 +1782,7 @@ static int r5l_log_write_empty_meta_block(struct r5l_log *log, sector_t pos,
> > mb->checksum = cpu_to_le32(crc32c_le(log->uuid_checksum,
> > mb, PAGE_SIZE));
> > if (!sync_page_io(log->rdev, pos, PAGE_SIZE, page, REQ_OP_WRITE,
> > - REQ_FUA, false)) {
> > + REQ_SYNC | REQ_FUA, false)) {
> > __free_page(page);
> > return -EIO;
> > }
> > @@ -2388,7 +2388,7 @@ r5c_recovery_rewrite_data_only_stripes(struct r5l_log *log,
> > mb->checksum = cpu_to_le32(crc32c_le(log->uuid_checksum,
> > mb, PAGE_SIZE));
> > sync_page_io(log->rdev, ctx->pos, PAGE_SIZE, page,
> > - REQ_OP_WRITE, REQ_FUA, false);
> > + REQ_OP_WRITE, REQ_SYNC | REQ_FUA, false);
> > sh->log_start = ctx->pos;
> > list_add_tail(&sh->r5c, &log->stripe_in_journal_list);
> > atomic_inc(&log->stripe_in_journal_count);
> > --
> > 2.12.0
> >
--
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR
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