Commit 1d616585 authored Jan 27, 2010 by Dmitry Monakhov Committed by Jens Axboe Jan 28, 2010

block: fix bio_add_page for non trivial merge_bvec_fn case



We have to properly decrease bi_size in order to merge_bvec_fn return
right result.  Otherwise this result in false merge rejects for two
absolutely valid bio_vecs.  This may cause significant performance
penalty for example fs_block_size == 1k and block device is raid0 with
small chunk_size = 8k. Then it is impossible to merge 7-th fs-block in
to bio which already has 6 fs-blocks.

Cc: <stable@kernel.org>
Signed-off-by: Dmitry Monakhov <dmonakhov@openvz.org>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

parent c84a301d

fs/bio.c

+6 −1

Original line number	Diff line number	Diff line
		@@ -542,13 +542,18 @@ static int __bio_add_page(struct request_queue q, struct bio bio, struct page

		if (page == prev->bv_page &&
		offset == prev->bv_offset + prev->bv_len) {
		unsigned int prev_bv_len = prev->bv_len;
		prev->bv_len += len;

		if (q->merge_bvec_fn) {
		struct bvec_merge_data bvm = {
		/* prev_bvec is already charged in
		bi_size, discharge it in order to
		simulate merging updated prev_bvec
		as new bvec. */
		.bi_bdev = bio->bi_bdev,
		.bi_sector = bio->bi_sector,
		.bi_size = bio->bi_size,
		.bi_size = bio->bi_size - prev_bv_len,
		.bi_rw = bio->bi_rw,
		};

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