block: make sure a big bio is split into at most 256 bvecs

[karo-tx-linux.git] / block / blk-merge.c

diff --git a/block/blk-merge.c b/block/blk-merge.c
index 72627e3cf91e1db7a5e17b3722a366b79b85cdb3..2642e5fc8b69a03494b62638d4eca98ee07b7edc 100644 (file)
--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -94,8 +94,30 @@ static struct bio *blk_bio_segment_split(struct request_queue *q,
        bool do_split = true;
        struct bio *new = NULL;
        const unsigned max_sectors = get_max_io_size(q, bio);
+       unsigned bvecs = 0;
 
        bio_for_each_segment(bv, bio, iter) {
+               /*
+                * With arbitrary bio size, the incoming bio may be very
+                * big. We have to split the bio into small bios so that
+                * 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.
+                *
+                * 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.
+                */
+               if (bvecs++ >= BIO_MAX_PAGES)
+                       goto split;
+
                /*
                 * If the queue doesn't support SG gaps and adding this
                 * offset would create a gap, disallow it.