Commit 05f1dd53152173 (block: add queue flag for disabling SG merging) uses bi_vcnt to assign bio->bi_phys_segments if sg merging is disabled. When using device mapper on top of a blk-mq device (virtio_blk in my test), we'd end up overflowing the scatterlist in __blk_bios_map_sg.
__bio_clone_fast copies bi_iter and bi_io_vec but not bi_vcnt, so blk_recount_segments would report bi_phys_segments as 0. Since rq->nr_phys_segments is 0 as well, the checks to ensure that we don't exceed the queue's segment limit end up allowing more bios (and segments) to attach the a request until we finally map it. That also means we pass the BUG_ON at the beginning of virtio_queue_rq, ultimately causing memory corruption and a crash. If we copy bi_vcnt in __bio_clone_fast, the bios and requests properly report the number of segments and everything works as expected. Originally reported at http://bugzilla.opensuse.org/show_bug.cgi?id=888259 Reported-by: Stephen Kulow <co...@suse.com> Signed-off-by: Jeff Mahoney <je...@suse.com> --- block/bio.c | 1 + 1 file changed, 1 insertion(+) --- a/block/bio.c +++ b/block/bio.c @@ -564,6 +564,7 @@ void __bio_clone_fast(struct bio *bio, s bio->bi_rw = bio_src->bi_rw; bio->bi_iter = bio_src->bi_iter; bio->bi_io_vec = bio_src->bi_io_vec; + bio->bi_vcnt = bio_src->bi_vcnt; } EXPORT_SYMBOL(__bio_clone_fast); -- Jeff Mahoney SUSE Labs -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/