[PATCH 05/13] block: Improvements to bounce-buffer handling

NeilBrown
2017-05-02T03:42:24Z

Since commit 23688bf4f830 ("block: ensure to split after potentially
bouncing a bio") blk_queue_bounce() is called *before*
blk_queue_split().
This means that:
1/ the comments blk_queue_split() about bounce buffers are
irrelevant, and
2/ a very large bio (more than BIO_MAX_PAGES) will no longer be
split before it arrives at blk_queue_bounce(), leading to the
possibility that bio_clone_bioset() will fail and a NULL
will be dereferenced.

Separately, blk_queue_bounce() shouldn't use fs_bio_set as the bio
being copied could be from the same set, and this could lead to a
deadlock.

So:
- allocate 2 private biosets for blk_queue_bounce, one for
splitting enormous bios and one for cloning bios.
- add code to split a bio that exceeds BIO_MAX_PAGES.
- Fix up the comments in blk_queue_split()

Credit-to: Ming Lei <tom.leiming@gmail.com> (suggested using single bio_for_each_segment loop)
Signed-off-by: NeilBrown <neilb@suse.com>
---
block/blk-merge.c | 14 ++++----------
block/bounce.c | 32 ++++++++++++++++++++++++++------
2 files changed, 30 insertions(+), 16 deletions(-)

diff --git a/block/blk-merge.c b/block/blk-merge.c
index d59074556703..51c84540d3bb 100644
--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -117,17 +117,11 @@ static struct bio *blk_bio_segment_split(struct request_queue *q,
* each holds at most BIO_MAX_PAGES bvecs because
* bio_clone() can fail to allocate big bvecs.
*
- * It should have been better to apply the limit per
- * request queue in which bio_clone() is involved,
- * instead of globally. The biggest blocker is the
- * bio_clone() in bio bounce.
+ * Those drivers which will need to use bio_clone()
+ * should tell us in some way. For now, impose the
+ * BIO_MAX_PAGES limit on all queues.
*
- * If bio is splitted by this reason, we should have
- * allowed to continue bios merging, but don't do
- * that now for making the change simple.
- *
- * TODO: deal with bio bounce's bio_clone() gracefully
- * and convert the global limit into per-queue limit.
+ * TODO: handle users of bio_clone() differently.
*/
if (bvecs++ >= BIO_MAX_PAGES)
goto split;
diff --git a/block/bounce.c b/block/bounce.c
index 1cb5dd3a5da1..087ecc2dc66c 100644
--- a/block/bounce.c
+++ b/block/bounce.c
@@ -26,6 +26,7 @@
#define POOL_SIZE 64
#define ISA_POOL_SIZE 16

+struct bio_set *bounce_bio_set, *bounce_bio_split;
static mempool_t *page_pool, *isa_page_pool;

#if defined(CONFIG_HIGHMEM) || defined(CONFIG_NEED_BOUNCE_POOL)
@@ -40,6 +41,14 @@ static __init int init_emergency_pool(void)
BUG_ON(!page_pool);
pr_info("pool size: %d pages\n", POOL_SIZE);

+ bounce_bio_set = bioset_create(BIO_POOL_SIZE, 0, BIOSET_NEED_BVECS);
+ BUG_ON(!bounce_bio_set);
+ if (bioset_integrity_create(bounce_bio_set, BIO_POOL_SIZE))
+ BUG_ON(1);
+
+ bounce_bio_split = bioset_create(BIO_POOL_SIZE, 0, 0);
+ BUG_ON(!bounce_bio_split);
+
return 0;
}

@@ -186,15 +195,26 @@ static void __blk_queue_bounce(struct request_queue *q, struct bio **bio_orig,
int rw = bio_data_dir(*bio_orig);
struct bio_vec *to, from;
struct bvec_iter iter;
- unsigned i;
+ unsigned i = 0;
+ bool bounce = false;
+ int sectors = 0;

- bio_for_each_segment(from, *bio_orig, iter)
+ bio_for_each_segment(from, *bio_orig, iter) {
+ if (i++ < BIO_MAX_PAGES)
+ sectors += from.bv_len >> 9;
if (page_to_pfn(from.bv_page) > queue_bounce_pfn(q))
- goto bounce;
+ bounce = true;
+ }
+ if (!bounce)
+ return;

- return;
-bounce:
- bio = bio_clone_bioset(*bio_orig, GFP_NOIO, fs_bio_set);
+ if (sectors < bio_sectors(*bio_orig)) {
+ bio = bio_split(*bio_orig, sectors, GFP_NOIO, bounce_bio_split);
+ bio_chain(bio, *bio_orig);
+ generic_make_request(*bio_orig);
+ *bio_orig = bio;
+ }
+ bio = bio_clone_bioset(*bio_orig, GFP_NOIO, bounce_bio_set);

bio_for_each_segment_all(to, bio, i) {
struct page *page = to->bv_page;