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

[Christoph Hellwig <hch@infradead.org>]

On Fri, Jun 07, 2024 at 11:15:55AM +0200, Cyril Hrubis wrote:
> If fallcate is implemented but zero and discard operations are not
> 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

Can you wire this up for blktests?

> +	if (ret == -EOPNOTSUPP) {
> +		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);

Please split this out into a separate helper to keep it out of the
main fast path I/O handling.  A little comment that we are
optimistically trying these if ->fallocate is support and might have
to paddle back here would also be useful.

(and maybe one day we figure out a way for the file system to
advertise what fallocate modes it actually supports..)