linux-kernel - Re: [PATCH v2 2/2] loop: Better discard support for block devices

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Date:   Thu, 14 Feb 2019 10:00:05 -0800
From:   Evan Green <evgreen@...omium.org>
To:     "Martin K. Petersen" <martin.petersen@...cle.com>
Cc:     Jens Axboe <axboe@...nel.dk>, Bart Van Assche <bvanassche@....org>,
        Gwendal Grignou <gwendal@...omium.org>,
        Alexis Savery <asavery@...omium.org>,
        Ming Lei <ming.lei@...hat.com>,
        linux-block <linux-block@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 2/2] loop: Better discard support for block devices

On Wed, Feb 13, 2019 at 6:40 PM Martin K. Petersen
<martin.petersen@...cle.com> wrote:
>
>
> Evan,
>
> > 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. 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.
>
> Bubbling up the queue limits from the backing device is fine. However,
> I'm not sure why you are requiring a filesystem to be on a
> discard-capable device for REQ_OP_DISCARD to have an effect? Punching a
> hole in a file is semantically the same as discarding.
>

Hi Martin,
Thanks so much for taking a look at this patch, I was getting nervous
it was languishing again. I got confused by this comment though. My
intention was to not change behavior for loop devices backed by a
regular file system file. The changes in loop_config_discard() should
result in QUEUE_FLAG_DISCARD being set for backings of regular files
that support f_op->fallocate(), same as before my patch. The change in
lo_discard() to call blk_queue_discard() on the loopback queue itself
was just shorthand to avoid duplicating all those if statements from
loop_config_discard again. Am I missing a spot where I've implicitly
changed the behavior for file-backed loop devices?
-Evan