On Wed, Mar 16, 2016 at 05:26:28PM +0100, Vitaly Kuznetsov wrote:
> Ming Lei <tom.leiming@xxxxxxxxx> writes:
> > We do have the above merge in bio_add_page(), so the two bios in
> > your above example shouldn't have been observed if the two buffers
> > are added to bio via the bio_add_page().
> >
> > If you see short bios in above example, maybe you need to check ntfs code:
> >
> > - if bio_add_page() is used to add buffer
> > - if using one standalone bio to transfer each 512byte, even they
> > are in same page and the sector is continuous
>
> I'm not using ntfs, mkfs.ntfs is a userspace application which shows the
> regression when virt_boundary is in place. I should have avoided
> mentioning bio_add_pc_page() here as it is unrelated to the issue.
>
> In particular, I'm concearned about the following call sites:
> blk_bio_segment_split()
> ll_back_merge_fn()
> ll_front_merge_fn()
I don't think blk_bio_segment_split would have seen such a bio vector
if it pages were added with bio_add_page. Those should already have
been combined. In any case, I think you can get what you're after just
by moving the gap check after BIOVEC_PHYS_MERGABLE. Does the following
look ok to you?
---
diff --git a/block/blk-merge.c b/block/blk-merge.c
index 2613531..4aa8e44 100644
--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -96,13 +96,6 @@ static struct bio *blk_bio_segment_split(struct request_queue *q,
const unsigned max_sectors = get_max_io_size(q, bio);
bio_for_each_segment(bv, bio, iter) {
- /*
- * If the queue doesn't support SG gaps and adding this
- * offset would create a gap, disallow it.
- */
- if (bvprvp && bvec_gap_to_prev(q, bvprvp, bv.bv_offset))
- goto split;
-
if (sectors + (bv.bv_len >> 9) > max_sectors) {
/*
* Consider this a new segment if we're splitting in
@@ -139,6 +132,13 @@ new_segment:
if (nsegs == queue_max_segments(q))
goto split;
+ /*
+ * If the queue doesn't support SG gaps and adding this
+ * offset would create a gap, disallow it.
+ */
+ if (bvprvp && bvec_gap_to_prev(q, bvprvp, bv.bv_offset))
+ goto split;
+
nsegs++;
bvprv = bv;
bvprvp = &bvprv;
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 413c84f..69cffbe 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -1400,7 +1400,8 @@ static inline bool bio_will_gap(struct request_queue *q, struct bio *prev,
bio_get_last_bvec(prev, &pb);
bio_get_first_bvec(next, &nb);
- return __bvec_gap_to_prev(q, &pb, nb.bv_offset);
+ if (!BIOVEC_PHYS_MERGEABLE(&pb, &nb))
+ return __bvec_gap_to_prev(q, &pb, nb.bv_offset);
}
return false;
--
--
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