block-throttle: avoid double charge

If a bio is throttled and split after throttling, the bio could be
resubmited and enters the throttling again. This will cause part of the
bio to be charged multiple times. If the cgroup has an IO limit, the
double charge will significantly harm the performance. The bio split
becomes quite common after arbitrary bio size change.

To fix this, we always set the BIO_THROTTLED flag if a bio is throttled.
If the bio is cloned/split, we copy the flag to new bio too to avoid a
double charge. However, cloned bio could be directed to a new disk,
keeping the flag be a problem. The observation is we always set new disk
for the bio in this case, so we can clear the flag in bio_set_dev().

This issue exists for a long time, arbitrary bio size change just makes
it worse, so this should go into stable at least since v4.2.

V1-> V2: Not add extra field in bio based on discussion with Tejun

Cc: Vivek Goyal <vgoyal@redhat.com>
Cc: stable@vger.kernel.org
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Shaohua Li <shli@fb.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/block/bio.c b/block/bio.c
index 8bfdea5..9ef6cf3 100644 (file)
--- a/block/bio.c
+++ b/block/bio.c
@@ -599,6 +599,8 @@ void __bio_clone_fast(struct bio *bio, struct bio *bio_src)
        bio->bi_disk = bio_src->bi_disk;
        bio->bi_partno = bio_src->bi_partno;
        bio_set_flag(bio, BIO_CLONED);
+       if (bio_flagged(bio_src, BIO_THROTTLED))
+               bio_set_flag(bio, BIO_THROTTLED);
        bio->bi_opf = bio_src->bi_opf;
        bio->bi_write_hint = bio_src->bi_write_hint;
        bio->bi_iter = bio_src->bi_iter;

diff --git a/block/blk-throttle.c b/block/blk-throttle.c
index 825bc29..d19f416 100644 (file)
--- a/block/blk-throttle.c
+++ b/block/blk-throttle.c
@@ -2226,13 +2226,7 @@ again:
 out_unlock:
        spin_unlock_irq(q->queue_lock);
 out:
-       /*
-        * As multiple blk-throtls may stack in the same issue path, we
-        * don't want bios to leave with the flag set.  Clear the flag if
-        * being issued.
-        */
-       if (!throttled)
-               bio_clear_flag(bio, BIO_THROTTLED);
+       bio_set_flag(bio, BIO_THROTTLED);
 
 #ifdef CONFIG_BLK_DEV_THROTTLING_LOW
        if (throttled || !td->track_bio_latency)

diff --git a/include/linux/bio.h b/include/linux/bio.h
index 82f0c8f..23d29b3 100644 (file)
--- a/include/linux/bio.h
+++ b/include/linux/bio.h
@@ -492,6 +492,8 @@ extern unsigned int bvec_nr_vecs(unsigned short idx);
 
 #define bio_set_dev(bio, bdev)                         \
 do {                                           \
+       if ((bio)->bi_disk != (bdev)->bd_disk)  \
+               bio_clear_flag(bio, BIO_THROTTLED);\
        (bio)->bi_disk = (bdev)->bd_disk;       \
        (bio)->bi_partno = (bdev)->bd_partno;   \
 } while (0)

diff --git a/include/linux/blk_types.h b/include/linux/blk_types.h
index a1e628e..9e7d8bd 100644 (file)
--- a/include/linux/blk_types.h
+++ b/include/linux/blk_types.h
@@ -50,8 +50,6 @@ struct blk_issue_stat {
 struct bio {
        struct bio              *bi_next;       /* request queue link */
        struct gendisk          *bi_disk;
-       u8                      bi_partno;
-       blk_status_t            bi_status;
        unsigned int            bi_opf;         /* bottom bits req flags,
                                                 * top bits REQ_OP. Use
                                                 * accessors.
@@ -59,8 +57,8 @@ struct bio {
        unsigned short          bi_flags;       /* status, etc and bvec pool number */
        unsigned short          bi_ioprio;
        unsigned short          bi_write_hint;
-
-       struct bvec_iter        bi_iter;
+       blk_status_t            bi_status;
+       u8                      bi_partno;
 
        /* Number of segments in this BIO after
         * physical address coalescing is performed.
@@ -74,8 +72,9 @@ struct bio {
        unsigned int            bi_seg_front_size;
        unsigned int            bi_seg_back_size;
 
-       atomic_t                __bi_remaining;
+       struct bvec_iter        bi_iter;
 
+       atomic_t                __bi_remaining;
        bio_end_io_t            *bi_end_io;
 
        void                    *bi_private;