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From:   Evan Green <evgreen@chromium.org>
Date:   Tue, 26 Feb 2019 09:23:13 -0800
Message-ID: <CAE=gft4OeQwokacbX8D1t7JrawKSZGjfjNm0xZc=Qx92HqieWg@mail.gmail.com>
Subject: Re: [PATCH v2 2/2] loop: Better discard support for block devices
To:     "Martin K. Petersen" <martin.petersen@oracle.com>
Cc:     Jens Axboe <axboe@kernel.dk>, Bart Van Assche <bvanassche@acm.org>,
        Gwendal Grignou <gwendal@chromium.org>,
        Alexis Savery <asavery@chromium.org>,
        Ming Lei <ming.lei@redhat.com>,
        linux-block <linux-block@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>
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On Thu, Feb 14, 2019 at 10:00 AM Evan Green <evgreen@chromium.org> wrote:
>
> On Wed, Feb 13, 2019 at 6:40 PM Martin K. Petersen
> <martin.petersen@oracle.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

Hi Martin,
Did you get a chance to take a look at my reply? This error log
plagues our Chrome OS installer, and I'm hopeful I've found an
approach here that's upstream-friendly. But if it's not, let me know
and I can fix it.
-Evan