linux-kernel - [PATCH 3.16 098/133] bcache: fix sequential large write IO bypass

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Message-ID: <lsq.1511315893.37532114@decadent.org.uk>
Date:   Wed, 22 Nov 2017 01:58:13 +0000
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org, "Jens Axboe" <axboe@...nel.dk>,
        "Tang Junhui" <tang.junhui@....com.cn>,
        "Kent Overstreet" <kent.overstreet@...il.com>,
        "Eric Wheeler" <bcache@...ux.ewheeler.net>
Subject: [PATCH 3.16 098/133] bcache: fix sequential large write IO bypass

3.16.51-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Tang Junhui <tang.junhui@....com.cn>

commit c81ffa32a214c84b08900fbc9d432187bd948eba upstream.

Sequential write IOs were tested with bs=1M by FIO in writeback cache
mode, these IOs were expected to be bypassed, but actually they did not.
We debug the code, and find in check_should_bypass():
    if (!congested &&
        mode == CACHE_MODE_WRITEBACK &&
        op_is_write(bio_op(bio)) &&
        (bio-＞bi_opf & REQ_SYNC))
        goto rescale
that means, If in writeback mode, a write IO with REQ_SYNC flag will not
be bypassed though it is a sequential large IO, It's not a correct thing
to do actually, so this patch remove these codes.

Signed-off-by: tang.junhui <tang.junhui@....com.cn>
Reviewed-by: Kent Overstreet <kent.overstreet@...il.com>
Reviewed-by: Eric Wheeler <bcache@...ux.ewheeler.net>
Signed-off-by: Jens Axboe <axboe@...nel.dk>
[bwh: Backported to 3.16: deleted code is slightly different]
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 drivers/md/bcache/request.c | 6 ------
 1 file changed, 6 deletions(-)

--- a/drivers/md/bcache/request.c
+++ b/drivers/md/bcache/request.c
@@ -395,12 +395,6 @@ static bool check_should_bypass(struct c
 	if (!congested && !dc->sequential_cutoff)
 		goto rescale;
 
-	if (!congested &&
-	    mode == CACHE_MODE_WRITEBACK &&
-	    (bio->bi_rw & REQ_WRITE) &&
-	    (bio->bi_rw & REQ_SYNC))
-		goto rescale;
-
 	spin_lock(&dc->io_lock);
 
 	hlist_for_each_entry(i, iohash(dc, bio->bi_iter.bi_sector), hash)