linux-kernel - Re: [PATCH v2] loop: Disable fallocate() zero and discard if not supported

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Date: Fri, 14 Jun 2024 13:21:29 +0200
From: Jan Kara <jack@...e.cz>
To: Cyril Hrubis <chrubis@...e.cz>
Cc: Christoph Hellwig <hch@...radead.org>, Jens Axboe <axboe@...nel.dk>,
	linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
	Jan Kara <jack@...e.cz>
Subject: Re: [PATCH v2] loop: Disable fallocate() zero and discard if not
 supported

On Thu 13-06-24 18:38:17, Cyril Hrubis wrote:
> If fallcate is implemented but zero and discard operations are not
     ^^^ fallocate

> supported by the filesystem the backing file is on we continue to fill
> dmesg with errors from the blk_mq_end_request() since each time we call
> fallocate() on the loop device the EOPNOTSUPP error from lo_fallocate()
> ends up propagated into the block layer. In the end syscall succeeds
> since the blkdev_issue_zeroout() falls back to writing zeroes which
> makes the errors even more misleading and confusing.
> 
> How to reproduce:
> 
> 1. make sure /tmp is mounted as tmpfs
> 2. dd if=/dev/zero of=/tmp/disk.img bs=1M count=100
> 3. losetup /dev/loop0 /tmp/disk.img
> 4. mkfs.ext2 /dev/loop0
> 5. dmesg |tail
> 
> [710690.898214] operation not supported error, dev loop0, sector 204672 op 0x9:(WRITE_ZEROES) flags 0x8000800 phys_seg 0 prio class 0
> [710690.898279] operation not supported error, dev loop0, sector 522 op 0x9:(WRITE_ZEROES) flags 0x8000800 phys_seg 0 prio class 0
> [710690.898603] operation not supported error, dev loop0, sector 16906 op 0x9:(WRITE_ZEROES) flags 0x8000800 phys_seg 0 prio class 0
> [710690.898917] operation not supported error, dev loop0, sector 32774 op 0x9:(WRITE_ZEROES) flags 0x8000800 phys_seg 0 prio class 0
> [710690.899218] operation not supported error, dev loop0, sector 49674 op 0x9:(WRITE_ZEROES) flags 0x8000800 phys_seg 0 prio class 0
> [710690.899484] operation not supported error, dev loop0, sector 65542 op 0x9:(WRITE_ZEROES) flags 0x8000800 phys_seg 0 prio class 0
> [710690.899743] operation not supported error, dev loop0, sector 82442 op 0x9:(WRITE_ZEROES) flags 0x8000800 phys_seg 0 prio class 0
> [710690.900015] operation not supported error, dev loop0, sector 98310 op 0x9:(WRITE_ZEROES) flags 0x8000800 phys_seg 0 prio class 0
> [710690.900276] operation not supported error, dev loop0, sector 115210 op 0x9:(WRITE_ZEROES) flags 0x8000800 phys_seg 0 prio class 0
> [710690.900546] operation not supported error, dev loop0, sector 131078 op 0x9:(WRITE_ZEROES) flags 0x8000800 phys_seg 0 prio class 0
> 
> This patch changes the lo_fallocate() to clear the flags for zero and
> discard operations if we get EOPNOTSUPP from the backing file fallocate
> callback, that way we at least stop spewing errors after the first
> unsuccessful try.
> 
> CC: Jan Kara <jack@...e.cz>
> Signed-off-by: Cyril Hrubis <chrubis@...e.cz>

Thanks. Besides the spelling fix the patch looks good to me. Feel free to
add:

Reviewed-by: Jan Kara <jack@...e.cz>

								Honza

>  drivers/block/loop.c | 23 +++++++++++++++++++++++
>  1 file changed, 23 insertions(+)
> 
> diff --git a/drivers/block/loop.c b/drivers/block/loop.c
> index 93780f41646b..1153721bc7c2 100644
> --- a/drivers/block/loop.c
> +++ b/drivers/block/loop.c
> @@ -302,6 +302,21 @@ static int lo_read_simple(struct loop_device *lo, struct request *rq,
>  	return 0;
>  }
>  
> +static void loop_clear_limits(struct loop_device *lo, int mode)
> +{
> +	struct queue_limits lim = queue_limits_start_update(lo->lo_queue);
> +
> +	if (mode & FALLOC_FL_ZERO_RANGE)
> +		lim.max_write_zeroes_sectors = 0;
> +
> +	if (mode & FALLOC_FL_PUNCH_HOLE) {
> +		lim.max_hw_discard_sectors = 0;
> +		lim.discard_granularity = 0;
> +	}
> +
> +	queue_limits_commit_update(lo->lo_queue, &lim);
> +}
> +
>  static int lo_fallocate(struct loop_device *lo, struct request *rq, loff_t pos,
>  			int mode)
>  {
> @@ -320,6 +335,14 @@ static int lo_fallocate(struct loop_device *lo, struct request *rq, loff_t pos,
>  	ret = file->f_op->fallocate(file, mode, pos, blk_rq_bytes(rq));
>  	if (unlikely(ret && ret != -EINVAL && ret != -EOPNOTSUPP))
>  		return -EIO;
> +
> +	/*
> +	 * We initially configure the limits in a hope that fallocate is
> +	 * supported and clear them here if that turns out not to be true.
> +	 */
> +	if (unlikely(ret == -EOPNOTSUPP))
> +		loop_clear_limits(lo, mode);
> +
>  	return ret;
>  }
>  
> -- 
> 2.44.2
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR