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

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Date: Mon, 10 Jun 2024 16:13:23 +0200
From: Cyril Hrubis <chrubis@...e.cz>
To: Christoph Hellwig <hch@...radead.org>
Cc: Jens Axboe <axboe@...nel.dk>, linux-block@...r.kernel.org,
	linux-kernel@...r.kernel.org, Jan Kara <jack@...e.cz>
Subject: Re: [PATCH] loop: Disable fallocate() zero and discard if not
 supported

Hi!
> > 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?

Will try.

> > +	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.

Will do.

Do we need noinline attribute for the function as well or unlikely() in
the if condition?

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

One of my ideas was to try fallocate() with zero size in the
loop_reconfigure_limits() to see if we get EOPNOTSUPP but for that to
work we would have to make sure that we do not bail early on zero size
in the vfs layer...

-- 
Cyril Hrubis
chrubis@...e.cz