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To:     Evan Green <evgreen@chromium.org>
Cc:     Jens Axboe <axboe@kernel.dk>, Bart Van Assche <bvanassche@acm.org>,
        Gwendal Grignou <gwendal@chromium.org>,
        Martin K Petersen <martin.petersen@oracle.com>,
        Alexis Savery <asavery@chromium.org>,
        Ming Lei <ming.lei@redhat.com>, linux-block@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 2/2] loop: Better discard support for block devices
From:   "Martin K. Petersen" <martin.petersen@oracle.com>
Organization: Oracle Corporation
References: <20190207205730.199332-1-evgreen@chromium.org>
        <20190207205730.199332-3-evgreen@chromium.org>
Date:   Wed, 13 Feb 2019 21:39:50 -0500
In-Reply-To: <20190207205730.199332-3-evgreen@chromium.org> (Evan Green's
        message of "Thu, 7 Feb 2019 12:57:29 -0800")
Message-ID: <yq1d0nvqeix.fsf@oracle.com>
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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.

-- 
Martin K. Petersen	Oracle Linux Engineering