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

[Kent Overstreet <koverstreet@google.com>]

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@google.com>
> 
> Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
> 
> ---
>  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;
>