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Message-ID: <CAPUE2uuGM_JgD8CFkQcjMeVk_q17sO3Jfpr8OO-N9h6aNUwqBA@mail.gmail.com>
Date: Tue, 7 May 2019 09:47:09 -0700
From: Gwendal Grignou <gwendal@...omium.org>
To: Evan Green <evgreen@...omium.org>
Cc: Jens Axboe <axboe@...nel.dk>,
Martin K Petersen <martin.petersen@...cle.com>,
Bart Van Assche <bvanassche@....org>,
Alexis Savery <asavery@...omium.org>,
Ming Lei <ming.lei@...hat.com>, linux-block@...r.kernel.org,
linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5 2/2] loop: Better discard support for block devices
Reviewed-by: Gwendal Grignou <gwendal@...omium.org>
On Mon, May 6, 2019 at 11:30 AM Evan Green <evgreen@...omium.org> wrote:
>
> 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.
>
> While in there, differentiate between REQ_OP_DISCARD and
> REQ_OP_WRITE_ZEROES, which are different for block devices,
> but which the loop device had just been lumping together, since
> they're largely the same for 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@...omium.org>
> ---
>
> 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 | 57 ++++++++++++++++++++++++++++----------------
> 1 file changed, 37 insertions(+), 20 deletions(-)
>
> diff --git a/drivers/block/loop.c b/drivers/block/loop.c
> index bbf21ebeccd3..a147210ed009 100644
> --- a/drivers/block/loop.c
> +++ b/drivers/block/loop.c
> @@ -417,19 +417,14 @@ static int lo_read_transfer(struct loop_device *lo, struct request *rq,
> return ret;
> }
>
> -static int lo_discard(struct loop_device *lo, struct request *rq, loff_t pos)
> +static int lo_discard(struct loop_device *lo, struct request *rq,
> + int mode, loff_t pos)
> {
> - /*
> - * 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.
> - */
> struct file *file = lo->lo_backing_file;
> - int mode = FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE;
> + struct request_queue *q = lo->lo_queue;
> int ret;
>
> - if ((!file->f_op->fallocate) || lo->lo_encrypt_key_size) {
> + if (!blk_queue_discard(q)) {
> ret = -EOPNOTSUPP;
> goto out;
> }
> @@ -599,8 +594,13 @@ static int do_req_filebacked(struct loop_device *lo, struct request *rq)
> case REQ_OP_FLUSH:
> return lo_req_flush(lo, rq);
> case REQ_OP_DISCARD:
> + return lo_discard(lo, rq,
> + FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE, pos);
> +
> case REQ_OP_WRITE_ZEROES:
> - return lo_discard(lo, rq, pos);
> + return lo_discard(lo, rq,
> + FALLOC_FL_ZERO_RANGE | FALLOC_FL_KEEP_SIZE, pos);
> +
> case REQ_OP_WRITE:
> if (lo->transfer)
> return lo_write_transfer(lo, rq, pos);
> @@ -854,6 +854,21 @@ static void loop_config_discard(struct loop_device *lo)
> struct file *file = lo->lo_backing_file;
> struct inode *inode = file->f_mapping->host;
> struct request_queue *q = lo->lo_queue;
> + struct request_queue *backingq;
> +
> + /*
> + * 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) {
> + 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
> @@ -861,22 +876,24 @@ static void loop_config_discard(struct loop_device *lo)
> * 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)
> --
> 2.20.1
>
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