Commit 2d759a46 authored by Joe Thornber's avatar Joe Thornber Committed by Mike Snitzer
Browse files

dm thin: remap the bios in a cell immediately 



This use of direct submission in process_prepared_mapping() reduces
latency for submitting bios in a cell by avoiding adding those bios to
the deferred list and waiting for the next iteration of the worker.

But this direct submission exposes the potential for a race between
releasing a cell and incrementing deferred set.  Fix this by introducing
dm_cell_visit_release() and refactoring inc_remap_and_issue_cell()
accordingly.

Signed-off-by: default avatarJoe Thornber <ejt@redhat.com>
Signed-off-by: default avatarMike Snitzer <snitzer@redhat.com>
parent a374bb21

drivers/md/dm-bio-prison.c

+14 −0
Original line number Diff line number Diff line
@@ -241,6 +241,20 @@ void dm_cell_error(struct dm_bio_prison *prison, 
}

EXPORT_SYMBOL_GPL(dm_cell_error);



void dm_cell_visit_release(struct dm_bio_prison *prison,

			   void (*visit_fn)(void *, struct dm_bio_prison_cell *),

			   void *context,

			   struct dm_bio_prison_cell *cell)

{

	unsigned long flags;



	spin_lock_irqsave(&prison->lock, flags);

	visit_fn(context, cell);

	rb_erase(&cell->node, &prison->cells);

	spin_unlock_irqrestore(&prison->lock, flags);

}

EXPORT_SYMBOL_GPL(dm_cell_visit_release);



/*----------------------------------------------------------------*/



#define DEFERRED_SET_SIZE 64

drivers/md/dm-bio-prison.h

+8 −0
Original line number Diff line number Diff line
@@ -89,6 +89,14 @@ void dm_cell_release_no_holder(struct dm_bio_prison *prison, 
void dm_cell_error(struct dm_bio_prison *prison,

		   struct dm_bio_prison_cell *cell, int error);



/*

 * Visits the cell and then releases.  Guarantees no new inmates are

 * inserted between the visit and release.

 */

void dm_cell_visit_release(struct dm_bio_prison *prison,

			   void (*visit_fn)(void *, struct dm_bio_prison_cell *),

			   void *context, struct dm_bio_prison_cell *cell);



/*----------------------------------------------------------------*/



/*

drivers/md/dm-thin.c

+61 −29
Original line number Diff line number Diff line
@@ -343,6 +343,15 @@ static void cell_release(struct pool *pool, 
	dm_bio_prison_free_cell(pool->prison, cell);

}



static void cell_visit_release(struct pool *pool,

			       void (*fn)(void *, struct dm_bio_prison_cell *),

			       void *context,

			       struct dm_bio_prison_cell *cell)

{

	dm_cell_visit_release(pool->prison, fn, context, cell);

	dm_bio_prison_free_cell(pool->prison, cell);

}



static void cell_release_no_holder(struct pool *pool,

				   struct dm_bio_prison_cell *cell,

				   struct bio_list *bios)
@@ -697,55 +706,75 @@ static void overwrite_endio(struct bio *bio, int err) 
 */



/*

 * This sends the bios in the cell back to the deferred_bios list.

 * This sends the bios in the cell, except the original holder, back

 * to the deferred_bios list.

 */

static void cell_defer(struct thin_c *tc, struct dm_bio_prison_cell *cell)

static void cell_defer_no_holder(struct thin_c *tc, struct dm_bio_prison_cell *cell)

{

	struct pool *pool = tc->pool;

	unsigned long flags;



	spin_lock_irqsave(&tc->lock, flags);

	cell_release(pool, cell, &tc->deferred_bio_list);

	cell_release_no_holder(pool, cell, &tc->deferred_bio_list);

	spin_unlock_irqrestore(&tc->lock, flags);



	wake_worker(pool);

}



/*

 * Same as cell_defer above, except it omits the original holder of the cell.

 */

static void cell_defer_no_holder(struct thin_c *tc, struct dm_bio_prison_cell *cell)

static void thin_defer_bio(struct thin_c *tc, struct bio *bio);



struct remap_info {

	struct thin_c *tc;

	struct bio_list defer_bios;

	struct bio_list issue_bios;

};



static void __inc_remap_and_issue_cell(void *context,

				       struct dm_bio_prison_cell *cell)

{

	struct pool *pool = tc->pool;

	unsigned long flags;

	struct remap_info *info = context;

	struct bio *bio;



	spin_lock_irqsave(&tc->lock, flags);

	cell_release_no_holder(pool, cell, &tc->deferred_bio_list);

	spin_unlock_irqrestore(&tc->lock, flags);

	while ((bio = bio_list_pop(&cell->bios))) {

		if (bio->bi_rw & (REQ_DISCARD | REQ_FLUSH | REQ_FUA))

			bio_list_add(&info->defer_bios, bio);

		else {

			inc_all_io_entry(info->tc->pool, bio);



	wake_worker(pool);

			/*

			 * We can't issue the bios with the bio prison lock

			 * held, so we add them to a list to issue on

			 * return from this function.

			 */

			bio_list_add(&info->issue_bios, bio);

		}

	}

}



static void thin_defer_bio(struct thin_c *tc, struct bio *bio);



static void inc_remap_and_issue_cell(struct thin_c *tc,

				     struct dm_bio_prison_cell *cell,

				     dm_block_t block)

{

	struct bio *bio;

	struct bio_list bios;

	struct remap_info info;



	bio_list_init(&bios);

	cell_release_no_holder(tc->pool, cell, &bios);

	info.tc = tc;

	bio_list_init(&info.defer_bios);

	bio_list_init(&info.issue_bios);



	while ((bio = bio_list_pop(&bios))) {

		if (bio->bi_rw & (REQ_DISCARD | REQ_FLUSH | REQ_FUA))

	/*

	 * We have to be careful to inc any bios we're about to issue

	 * before the cell is released, and avoid a race with new bios

	 * being added to the cell.

	 */

	cell_visit_release(tc->pool, __inc_remap_and_issue_cell,

			   &info, cell);



	while ((bio = bio_list_pop(&info.defer_bios)))

		thin_defer_bio(tc, bio);

		else {

			inc_all_io_entry(tc->pool, bio);

			remap_and_issue(tc, bio, block);

		}

	}



	while ((bio = bio_list_pop(&info.issue_bios)))

		remap_and_issue(info.tc, bio, block);

}



static void process_prepared_mapping_fail(struct dm_thin_new_mapping *m)
@@ -796,10 +825,13 @@ static void process_prepared_mapping(struct dm_thin_new_mapping *m) 
	 * the bios in the cell.

	 */

	if (bio) {

		cell_defer_no_holder(tc, m->cell);

		inc_remap_and_issue_cell(tc, m->cell, m->data_block);

		bio_endio(bio, 0);

	} else

		cell_defer(tc, m->cell);

	} else {

		inc_all_io_entry(tc->pool, m->cell->holder);

		remap_and_issue(tc, m->cell->holder, m->data_block);

		inc_remap_and_issue_cell(tc, m->cell, m->data_block);

	}



out:

	list_del(&m->list);

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