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From: Andrzej Pietrasiewicz <andrzej.p@collabora.com>
To: linux-block@vger.kernel.org
Cc: Jens Axboe <axboe@kernel.dk>,
	kernel@collabora.com, andrzej.p@collabora.com,
	Evan Green <evgreen@chromium.org>
Subject: [PATCH 2/2] loop: Better discard support for block devices
Date: Tue, 17 Mar 2020 16:11:11 +0100
Message-ID: <20200317151111.25749-3-andrzej.p@collabora.com> (raw)
In-Reply-To: <20200317151111.25749-1-andrzej.p@collabora.com>

From: Evan Green <evgreen@chromium.org>

If the backing device for a loop device is a block device,
then mirror the "write zeroes" capabilities of the underlying
block device into the loop device. Copy this capability into both
max_write_zeroes_sectors and max_discard_sectors of the loop device.

The reason for this is that REQ_OP_DISCARD on a loop device translates
into blkdev_issue_zeroout(), rather than blkdev_issue_discard(). This
presents a consistent interface for loop devices (that discarded data
is zeroed), regardless of the backing device type of the loop device.
There should be no behavior change for loop devices backed by regular
files.

This change fixes blktest block/003, and removes an extraneous
error print in block/013 when testing on a loop device backed
by a block device that does not support discard.

Signed-off-by: Evan Green <evgreen@chromium.org>
[rebase onto v5.6-rc6]
Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@collabora.com>

---

Changes in v5:
- Don't mirror discard if lo_encrypt_key_size is non-zero (Gwendal)

Changes in v4:
- Mirror blkdev's write_zeroes into loopdev's discard_sectors.

Changes in v3:
- Updated commit description

Changes in v2: None
---
 drivers/block/loop.c | 42 +++++++++++++++++++++++++++++++-----------
 1 file changed, 31 insertions(+), 11 deletions(-)

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index 87307454f768..8de3b2b098d3 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -427,11 +427,12 @@ static int lo_fallocate(struct loop_device *lo, struct request *rq, loff_t pos,
 	 * information.
 	 */
 	struct file *file = lo->lo_backing_file;
+	struct request_queue *q = lo->lo_queue;
 	int ret;
 
 	mode |= FALLOC_FL_KEEP_SIZE;
 
-	if ((!file->f_op->fallocate) || lo->lo_encrypt_key_size) {
+	if (!blk_queue_discard(q)) {
 		ret = -EOPNOTSUPP;
 		goto out;
 	}
@@ -867,28 +868,47 @@ static void loop_config_discard(struct loop_device *lo)
 	struct inode *inode = file->f_mapping->host;
 	struct request_queue *q = lo->lo_queue;
 
+
+	/*
+	 * If the backing device is a block device, mirror its zeroing
+	 * capability. REQ_OP_DISCARD translates to a zero-out even when backed
+	 * by block devices to keep consistent behavior with file-backed loop
+	 * devices.
+	 */
+	if (S_ISBLK(inode->i_mode) && !lo->lo_encrypt_key_size) {
+		struct request_queue *backingq;
+
+		backingq = bdev_get_queue(inode->i_bdev);
+		blk_queue_max_discard_sectors(q,
+			backingq->limits.max_write_zeroes_sectors);
+
+		blk_queue_max_write_zeroes_sectors(q,
+			backingq->limits.max_write_zeroes_sectors);
+
 	/*
 	 * We use punch hole to reclaim the free space used by the
 	 * image a.k.a. discard. However we do not support discard if
 	 * encryption is enabled, because it may give an attacker
 	 * useful information.
 	 */
-	if ((!file->f_op->fallocate) ||
-	    lo->lo_encrypt_key_size) {
+	} else if ((!file->f_op->fallocate) || lo->lo_encrypt_key_size) {
 		q->limits.discard_granularity = 0;
 		q->limits.discard_alignment = 0;
 		blk_queue_max_discard_sectors(q, 0);
 		blk_queue_max_write_zeroes_sectors(q, 0);
-		blk_queue_flag_clear(QUEUE_FLAG_DISCARD, q);
-		return;
-	}
 
-	q->limits.discard_granularity = inode->i_sb->s_blocksize;
-	q->limits.discard_alignment = 0;
+	} else {
+		q->limits.discard_granularity = inode->i_sb->s_blocksize;
+		q->limits.discard_alignment = 0;
+
+		blk_queue_max_discard_sectors(q, UINT_MAX >> 9);
+		blk_queue_max_write_zeroes_sectors(q, UINT_MAX >> 9);
+	}
 
-	blk_queue_max_discard_sectors(q, UINT_MAX >> 9);
-	blk_queue_max_write_zeroes_sectors(q, UINT_MAX >> 9);
-	blk_queue_flag_set(QUEUE_FLAG_DISCARD, q);
+	if (q->limits.max_write_zeroes_sectors)
+		blk_queue_flag_set(QUEUE_FLAG_DISCARD, q);
+	else
+		blk_queue_flag_clear(QUEUE_FLAG_DISCARD, q);
 }
 
 static void loop_unprepare_queue(struct loop_device *lo)
-- 
2.17.1


  parent reply index

Thread overview: 22+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-03-17 15:11 [PATCH 0/2] " Andrzej Pietrasiewicz
2020-03-17 15:11 ` [PATCH 1/2] loop: Report EOPNOTSUPP properly Andrzej Pietrasiewicz
2020-03-17 16:51   ` [PATCH RESEND " Andrzej Pietrasiewicz
2020-03-26 14:52     ` Christoph Hellwig
2020-03-26 15:51       ` Evan Green
2020-03-26 15:55         ` Christoph Hellwig
2020-03-26 16:36           ` Andrzej Pietrasiewicz
2020-03-17 15:11 ` Andrzej Pietrasiewicz [this message]
2020-03-26 11:48 ` [PATCH 0/2] loop: Better discard support for block devices Andrzej Pietrasiewicz
  -- strict thread matches above, loose matches on Subject: below --
2018-10-30 23:06 [PATCH 0/2] loop: Better discard " Evan Green
2018-10-30 23:06 ` [PATCH 2/2] loop: Better discard support " Evan Green
2018-11-26 18:53   ` Evan Green
2018-11-27  2:55     ` Ming Lei
2018-11-27 23:34       ` Evan Green
2018-11-28  1:28         ` Ming Lei
2018-11-28  1:26   ` Ming Lei
2018-12-04 22:19     ` Evan Green
2018-12-05  1:10       ` Ming Lei
2018-12-05 19:35         ` Evan Green
2018-12-06  0:22           ` Ming Lei
2018-12-06  3:15           ` Martin K. Petersen
2018-12-10 17:31             ` Evan Green
2018-12-18 23:48               ` Evan Green

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