lkml.org 
[lkml]   [2019]   [Mar]   [27]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH v3 2/2] loop: Better discard support for block devices
On Wed, Mar 27, 2019 at 03:28:41PM -0700, Evan Green wrote:
> If the backing device for a loop device is a block device,
> then mirror the discard properties of the underlying block
> device into the loop device. This new change only applies to
> loop devices backed directly by a block device, not 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.

I saw such issue many times, I believe it needs the fix.

>
> Signed-off-by: Evan Green <evgreen@chromium.org>
> ---
>
> Changes in v3:
> - Updated commit description
>
> Changes in v2: None
>
> drivers/block/loop.c | 61 +++++++++++++++++++++++++++++---------------
> 1 file changed, 41 insertions(+), 20 deletions(-)
>
> diff --git a/drivers/block/loop.c b/drivers/block/loop.c
> index bbf21ebeccd3..e1edd004298a 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 +854,25 @@ 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 discard
> + * capabilities.
> + */
> + if (S_ISBLK(inode->i_mode)) {
> + backingq = bdev_get_queue(inode->i_bdev);
> + blk_queue_max_discard_sectors(q,
> + backingq->limits.max_discard_sectors);
> +
> + blk_queue_max_write_zeroes_sectors(q,
> + backingq->limits.max_write_zeroes_sectors);
> +
> + q->limits.discard_granularity =
> + backingq->limits.discard_granularity;
> +
> + q->limits.discard_alignment =
> + backingq->limits.discard_alignment;

Loop usually doesn't mirror backing queue's limits, and I believe
it isn't necessary for this case too, just wondering why the
following simple setting can't work?

if (S_ISBLK(inode->i_mode)) {
backingq = bdev_get_queue(inode->i_bdev);

q->limits.discard_alignment = 0;
if (!blk_queue_discard(backingq)) {
q->limits.discard_granularity = 0;
blk_queue_max_discard_sectors(q, 0);
blk_queue_max_write_zeroes_sectors(q, 0);
blk_queue_flag_clear(QUEUE_FLAG_DISCARD, q);
} else {
q->limits.discard_granularity = inode->i_sb->s_blocksize;
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);
}
} else if ((!file->f_op->fallocate) || lo->lo_encrypt_key_size) {
...
}

I remembered you mentioned the above code doesn't work in some of your
tests, but never explain the reason. However, it is supposed to work
given bio splitting does handle/respect the discard limits. Or is there
bug in bio splitting on discard IO?

Thanks,
Ming

\
 
 \ /
  Last update: 2019-03-28 03:39    [W:0.046 / U:23.092 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site