[PATCH] block: don't make BLK_DEF_MAX_SECTORS too big

[Shaohua Li <shli@fb.com>]

bio_alloc_bioset() allocates bvecs from bvec_slabs which can only
allocate maximum 256 bvec (eg, 1M for 4k pages). We can't bump
BLK_DEF_MAX_SECTORS to exceed this value otherwise bio_alloc_bioset will
fail.

In the future, we can extend the size either bvec_slabs array is
expanded or the upcoming multipage bvec is added if pages are
contiguous. This one is suitable for stable.

Fixes: d2be537c3ba (block: bump BLK_DEF_MAX_SECTORS to 2560)
Reported-by: Sebastian Roesner <sroesner-kernelorg@roesner-online.de>
Cc: stable@vger.kernel.org (4.2+)
Cc: Ming Lei <ming.lei@canonical.com>
Reviewed-by: Jeff Moyer <jmoyer@redhat.com>
Signed-off-by: Shaohua Li <shli@fb.com>
---
 include/linux/blkdev.h | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 7e5d7e0..da64325 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -1153,7 +1153,11 @@ extern int blk_verify_command(unsigned char *cmd, fmode_t has_write_perm);
 enum blk_default_limits {
 	BLK_MAX_SEGMENTS	= 128,
 	BLK_SAFE_MAX_SECTORS	= 255,
-	BLK_DEF_MAX_SECTORS	= 2560,
+	/*
+	 * if you change this, please also change bvec_alloc and BIO_MAX_PAGES.
+	 * Otherwise bio_alloc_bioset will break.
+	 */
+	BLK_DEF_MAX_SECTORS	= BIO_MAX_SECTORS,
 	BLK_MAX_SEGMENT_SIZE	= 65536,
 	BLK_SEG_BOUNDARY_MASK	= 0xFFFFFFFFUL,
 };
-- 
2.8.0.rc2