Re: [PATCH 2] dm: Use bioset's front_pad for dm_target_io

From: Kent Overstreet
Date: Tue Sep 11 2012 - 15:50:40 EST

On Tue, Sep 11, 2012 at 03:28:57PM -0400, Mikulas Patocka wrote:
>
>
> On Tue, 4 Sep 2012, Kent Overstreet wrote:
>
> > On Tue, Sep 04, 2012 at 03:26:19PM -0400, Mikulas Patocka wrote:
> > >
> > >
> > > On Mon, 3 Sep 2012, Kent Overstreet wrote:
> > >
> > > > On Mon, Sep 03, 2012 at 04:41:37PM -0400, Mikulas Patocka wrote:
> > > > > ... or another possibility - start a timer when something is put to
> > > > > current->bio_list and use that timer to pop entries off current->bio_list
> > > > > and submit them to a workqueue. The timer can be cpu-local so only
> > > > > interrupt masking is required to synchronize against the timer.
> > > > >
> > > > > This would normally run just like the current kernel and in case of
> > > > > deadlock, the timer would kick in and resolve the deadlock.
> > > >
> > > > Ugh. That's a _terrible_ idea.
> > > >
> > > > Remember the old plugging code? You ever have to debug performance
> > > > issues caused by it?
> > >
> > > Yes, I do remember it (and I fixed one bug that resulted in missed unplug
> > > and degraded performance).
> > >
> > > But currently, deadlocks due to exhausted mempools and bios being stalled
> > > in current->bio_list don't happen (or do happen below so rarely that they
> > > aren't reported).
> > >
> > > If we add a timer, it will turn a deadlock into an i/o delay, but it can't
> > > make things any worse.
> >
> > This is all true. I'm not arguing your solution wouldn't _work_... I'd
> > try and give some real reasoning for my objections but it'll take me
> > awhile to figure out how to coherently explain it and I'm very sleep
> > deprived.
> >
> > > Currently, dm targets allocate request-specific data from target-specific
> > > mempool. mempools are in dm-crypt, dm-delay, dm-mirror, dm-snapshot,
> > > dm-thin, dm-verity. You can change it to allocate request-specific data
> > > with the bio.
> >
> > I wrote a patch for dm_target_io last night. I think I know an easy way
> > to go about converting the rest but it'll probably have to wait until
> > I'm further along with my immutable bvec stuff.
> >
> > Completely untested patch below:
>
> The patch didn't work (it has random allocation failures and crashes when
> the device is closed). The patch also contains some unrelated changes.
>
> I created this patch that does the same thing.

Cool! Thanks for finishing this.

I like your approach with rolling the bio allocation into alloc_tio() -
it solves a problem I was having with the front_pad not being zeroed -
but it does prevent the use of bio_clone_bioset(), which is going to
cause minor issues with my immutable bvec work.

Perhaps bio_alloc_bioset() should just be changed to zero the front_pad,
that'd probably be useful elsewhere anyways.

You might want to rebase onto Jens' tree, it has my patches that get rid
of bi_destructor and the front_pad conversion for request based dm:

git://git.kernel.org/pub/scm/linux/kernel/git/axboe/linux-block.git for-next

>
> Mikulas
>
> ---
> Use front pad to allocate dm_target_io
>
> dm_target_io was previously allocated from a mempool. For each
> dm_target_io, there is exactly one bio allocated from a bioset.
>
> This patch merges these two allocations into one allocating: we create a
> bioset with front_pad equal to the size of dm_target_io - so that every
> bio allocated from the bioset has sizeof(struct dm_target_io) bytes
> before it. We allocate a bio and use the bytes before the bio as
> dm_target_io.
>
> This idea was introdiced by Kent Overstreet <koverstreet@xxxxxxxxxx>
>
> Signed-off-by: Mikulas Patocka <mpatocka@xxxxxxxxxx>
>
> ---
> drivers/md/dm.c | 96 +++++++++++++++++++++++---------------------------------
> 1 file changed, 41 insertions(+), 55 deletions(-)
>
> Index: linux-3.5.3-fast/drivers/md/dm.c
> ===================================================================
> --- linux-3.5.3-fast.orig/drivers/md/dm.c 2012-09-10 16:06:30.000000000 +0200
> +++ linux-3.5.3-fast/drivers/md/dm.c 2012-09-11 19:32:36.000000000 +0200
> @@ -71,6 +71,7 @@ struct dm_target_io {
> struct dm_io *io;
> struct dm_target *ti;
> union map_info info;
> + struct bio clone;
> };
>
> /*
> @@ -464,7 +465,9 @@ static void free_io(struct mapped_device
>
> static void free_tio(struct mapped_device *md, struct dm_target_io *tio)
> {
> - mempool_free(tio, md->tio_pool);
> + tio->clone.bi_private = md->bs;
> + tio->clone.bi_end_io = NULL;
> + bio_put(&tio->clone);
> }
>
> static struct dm_rq_target_io *alloc_rq_tio(struct mapped_device *md,
> @@ -710,13 +713,7 @@ static void clone_endio(struct bio *bio,
> }
> }
>
> - /*
> - * Store md for cleanup instead of tio which is about to get freed.
> - */
> - bio->bi_private = md->bs;
> -
> free_tio(md, tio);
> - bio_put(bio);
> dec_pending(io, error);
> }
>
> @@ -1032,12 +1029,12 @@ int dm_set_target_max_io_len(struct dm_t
> }
> EXPORT_SYMBOL_GPL(dm_set_target_max_io_len);
>
> -static void __map_bio(struct dm_target *ti, struct bio *clone,
> - struct dm_target_io *tio)
> +static void __map_bio(struct dm_target *ti, struct dm_target_io *tio)
> {
> int r;
> sector_t sector;
> struct mapped_device *md;
> + struct bio *clone = &tio->clone;
>
> clone->bi_end_io = clone_endio;
> clone->bi_private = tio;
> @@ -1061,12 +1058,6 @@ static void __map_bio(struct dm_target *
> /* error the io and bail out, or requeue it if needed */
> md = tio->io->md;
> dec_pending(tio->io, r);
> - /*
> - * Store bio_set for cleanup.
> - */
> - clone->bi_end_io = NULL;
> - clone->bi_private = md->bs;
> - bio_put(clone);
> free_tio(md, tio);
> } else if (r) {
> DMWARN("unimplemented target map return value: %d", r);
> @@ -1094,15 +1085,13 @@ static void dm_bio_destructor(struct bio
> /*
> * Creates a little bio that just does part of a bvec.
> */
> -static struct bio *split_bvec(struct bio *bio, sector_t sector,
> - unsigned short idx, unsigned int offset,
> - unsigned int len, struct bio_set *bs)
> +static void split_bvec(struct dm_target_io *tio, struct bio *bio,
> + sector_t sector, unsigned short idx, unsigned int offset,
> + unsigned int len, struct bio_set *bs)
> {
> - struct bio *clone;
> + struct bio *clone = &tio->clone;
> struct bio_vec *bv = bio->bi_io_vec + idx;
>
> - clone = bio_alloc_bioset(GFP_NOIO, 1, bs);
> - clone->bi_destructor = dm_bio_destructor;
> *clone->bi_io_vec = *bv;
>
> clone->bi_sector = sector;
> @@ -1119,22 +1108,19 @@ static struct bio *split_bvec(struct bio
> bio_integrity_trim(clone,
> bio_sector_offset(bio, idx, offset), len);
> }
> -
> - return clone;
> }
>
> /*
> * Creates a bio that consists of range of complete bvecs.
> */
> -static struct bio *clone_bio(struct bio *bio, sector_t sector,
> - unsigned short idx, unsigned short bv_count,
> - unsigned int len, struct bio_set *bs)
> +static void clone_bio(struct dm_target_io *tio, struct bio *bio,
> + sector_t sector, unsigned short idx,
> + unsigned short bv_count, unsigned int len,
> + struct bio_set *bs)
> {
> - struct bio *clone;
> + struct bio *clone = &tio->clone;
>
> - clone = bio_alloc_bioset(GFP_NOIO, bio->bi_max_vecs, bs);
> __bio_clone(clone, bio);
> - clone->bi_destructor = dm_bio_destructor;
> clone->bi_sector = sector;
> clone->bi_idx = idx;
> clone->bi_vcnt = idx + bv_count;
> @@ -1148,14 +1134,17 @@ static struct bio *clone_bio(struct bio
> bio_integrity_trim(clone,
> bio_sector_offset(bio, idx, 0), len);
> }
> -
> - return clone;
> }
>
> static struct dm_target_io *alloc_tio(struct clone_info *ci,
> - struct dm_target *ti)
> + struct dm_target *ti, int nr_iovecs)
> {
> - struct dm_target_io *tio = mempool_alloc(ci->md->tio_pool, GFP_NOIO);
> + struct dm_target_io *tio;
> + struct bio *clone;
> +
> + clone = bio_alloc_bioset(GFP_NOIO, nr_iovecs, ci->md->bs);
> + tio = container_of(clone, struct dm_target_io, clone);
> + tio->clone.bi_destructor = dm_bio_destructor;
>
> tio->io = ci->io;
> tio->ti = ti;
> @@ -1167,8 +1156,8 @@ static struct dm_target_io *alloc_tio(st
> static void __issue_target_request(struct clone_info *ci, struct dm_target *ti,
> unsigned request_nr, sector_t len)
> {
> - struct dm_target_io *tio = alloc_tio(ci, ti);
> - struct bio *clone;
> + struct dm_target_io *tio = alloc_tio(ci, ti, ci->bio->bi_max_vecs);
> + struct bio *clone = &tio->clone;
>
> tio->info.target_request_nr = request_nr;
>
> @@ -1177,15 +1166,13 @@ static void __issue_target_request(struc
> * ci->bio->bi_max_vecs is BIO_INLINE_VECS anyway, for both flush
> * and discard, so no need for concern about wasted bvec allocations.
> */
> - clone = bio_alloc_bioset(GFP_NOIO, ci->bio->bi_max_vecs, ci->md->bs);
> __bio_clone(clone, ci->bio);
> - clone->bi_destructor = dm_bio_destructor;
> if (len) {
> clone->bi_sector = ci->sector;
> clone->bi_size = to_bytes(len);
> }
>
> - __map_bio(ti, clone, tio);
> + __map_bio(ti, tio);
> }
>
> static void __issue_target_requests(struct clone_info *ci, struct dm_target *ti,
> @@ -1214,14 +1201,13 @@ static int __clone_and_map_empty_flush(s
> */
> static void __clone_and_map_simple(struct clone_info *ci, struct dm_target *ti)
> {
> - struct bio *clone, *bio = ci->bio;
> + struct bio *bio = ci->bio;
> struct dm_target_io *tio;
>
> - tio = alloc_tio(ci, ti);
> - clone = clone_bio(bio, ci->sector, ci->idx,
> - bio->bi_vcnt - ci->idx, ci->sector_count,
> - ci->md->bs);
> - __map_bio(ti, clone, tio);
> + tio = alloc_tio(ci, ti, bio->bi_max_vecs);
> + clone_bio(tio, bio, ci->sector, ci->idx, bio->bi_vcnt - ci->idx,
> + ci->sector_count, ci->md->bs);
> + __map_bio(ti, tio);
> ci->sector_count = 0;
> }
>
> @@ -1259,7 +1245,7 @@ static int __clone_and_map_discard(struc
>
> static int __clone_and_map(struct clone_info *ci)
> {
> - struct bio *clone, *bio = ci->bio;
> + struct bio *bio = ci->bio;
> struct dm_target *ti;
> sector_t len = 0, max;
> struct dm_target_io *tio;
> @@ -1299,10 +1285,10 @@ static int __clone_and_map(struct clone_
> len += bv_len;
> }
>
> - tio = alloc_tio(ci, ti);
> - clone = clone_bio(bio, ci->sector, ci->idx, i - ci->idx, len,
> - ci->md->bs);
> - __map_bio(ti, clone, tio);
> + tio = alloc_tio(ci, ti, bio->bi_max_vecs);
> + clone_bio(tio, bio, ci->sector, ci->idx, i - ci->idx, len,
> + ci->md->bs);
> + __map_bio(ti, tio);
>
> ci->sector += len;
> ci->sector_count -= len;
> @@ -1327,12 +1313,11 @@ static int __clone_and_map(struct clone_
>
> len = min(remaining, max);
>
> - tio = alloc_tio(ci, ti);
> - clone = split_bvec(bio, ci->sector, ci->idx,
> - bv->bv_offset + offset, len,
> - ci->md->bs);
> + tio = alloc_tio(ci, ti, 1);
> + split_bvec(tio, bio, ci->sector, ci->idx,
> + bv->bv_offset + offset, len, ci->md->bs);
>
> - __map_bio(ti, clone, tio);
> + __map_bio(ti, tio);
>
> ci->sector += len;
> ci->sector_count -= len;
> @@ -2765,7 +2750,8 @@ struct dm_md_mempools *dm_alloc_md_mempo
> if (!pools->tio_pool)
> goto free_io_pool_and_out;
>
> - pools->bs = bioset_create(pool_size, 0);
> + pools->bs = bioset_create(pool_size, (type == DM_TYPE_BIO_BASED) ?
> + offsetof(struct dm_target_io, clone) : 0);
> if (!pools->bs)
> goto free_tio_pool_and_out;
>
--
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