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    Subject[PATCH 04/11] block: Improvements to bounce-buffer handling
    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()

    Signed-off-by: NeilBrown <neilb@suse.com>
    ---
    block/blk-merge.c | 14 ++++----------
    block/bounce.c | 27 ++++++++++++++++++++++++++-
    2 files changed, 30 insertions(+), 11 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..51fb538b504d 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);
    + BUG_ON(!bounce_bio_set);
    + if (bioset_integrity_create(bounce_bio_set, BIO_POOL_SIZE))
    + BUG_ON(1);
    +
    + bounce_bio_split = bioset_create_nobvec(BIO_POOL_SIZE, 0);
    + BUG_ON(!bounce_bio_split);
    +
    return 0;
    }

    @@ -194,7 +203,23 @@ static void __blk_queue_bounce(struct request_queue *q, struct bio **bio_orig,

    return;
    bounce:
    - bio = bio_clone_bioset(*bio_orig, GFP_NOIO, fs_bio_set);
    + if (bio_segments(*bio_orig) > BIO_MAX_PAGES) {
    + int cnt = 0;
    + int sectors = 0;
    + struct bio_vec bv;
    + struct bvec_iter iter;
    + bio_for_each_segment(bv, *bio_orig, iter) {
    + if (cnt++ < BIO_MAX_PAGES)
    + sectors += bv.bv_len >> 9;
    + else
    + break;
    + }
    + 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;

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