block: bio: introduce helpers to get the 1st and last bvec

The bio passed to bio_will_gap() may be fast cloned from upper
layer(dm, md, bcache, fs, ...), or from bio splitting in block
core.

Unfortunately bio_will_gap() just figures out the last bvec via
'bi_io_vec[prev->bi_vcnt - 1]' directly, and this way is obviously
wrong.

This patch introduces two helpers for getting the first and last
bvec of one bio for fixing the issue.

Cc: stable@vger.kernel.org
Reported-by: Sagi Grimberg <sagig@dev.mellanox.co.il>
Reviewed-by: Sagi Grimberg <sagig@mellanox.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Ming Lei <ming.lei@canonical.com>
Signed-off-by: Jens Axboe <axboe@fb.com>

diff --git a/include/linux/bio.h b/include/linux/bio.h
index 5349e6816cbb075e2b5d6ac57cac1e8fee373438..cb68888241084e33af2d7e11753e048431cf7837 100644 (file)
--- a/include/linux/bio.h
+++ b/include/linux/bio.h
@@ -310,6 +310,43 @@ static inline void bio_clear_flag(struct bio *bio, unsigned int bit)
        bio->bi_flags &= ~(1U << bit);
 }
 
+static inline void bio_get_first_bvec(struct bio *bio, struct bio_vec *bv)
+{
+       *bv = bio_iovec(bio);
+}
+
+static inline void bio_get_last_bvec(struct bio *bio, struct bio_vec *bv)
+{
+       struct bvec_iter iter = bio->bi_iter;
+       int idx;
+
+       if (!bio_flagged(bio, BIO_CLONED)) {
+               *bv = bio->bi_io_vec[bio->bi_vcnt - 1];
+               return;
+       }
+
+       if (unlikely(!bio_multiple_segments(bio))) {
+               *bv = bio_iovec(bio);
+               return;
+       }
+
+       bio_advance_iter(bio, &iter, iter.bi_size);
+
+       if (!iter.bi_bvec_done)
+               idx = iter.bi_idx - 1;
+       else    /* in the middle of bvec */
+               idx = iter.bi_idx;
+
+       *bv = bio->bi_io_vec[idx];
+
+       /*
+        * iter.bi_bvec_done records actual length of the last bvec
+        * if this bio ends in the middle of one io vector
+        */
+       if (iter.bi_bvec_done)
+               bv->bv_len = iter.bi_bvec_done;
+}
+
 enum bip_flags {
        BIP_BLOCK_INTEGRITY     = 1 << 0, /* block layer owns integrity data */
        BIP_MAPPED_INTEGRITY    = 1 << 1, /* ref tag has been remapped */