linux-kernel - Re: [PATCH 1/2] Block layer filter

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Message-ID: <20201021114438.GK20115@casper.infradead.org>
Date:   Wed, 21 Oct 2020 12:44:38 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Damien Le Moal <Damien.LeMoal@....com>
Cc:     Sergei Shtepa <sergei.shtepa@...am.com>,
        "axboe@...nel.dk" <axboe@...nel.dk>,
        "viro@...iv.linux.org.uk" <viro@...iv.linux.org.uk>,
        "hch@...radead.org" <hch@...radead.org>,
        "darrick.wong@...cle.com" <darrick.wong@...cle.com>,
        "linux-xfs@...r.kernel.org" <linux-xfs@...r.kernel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "rjw@...ysocki.net" <rjw@...ysocki.net>,
        "len.brown@...el.com" <len.brown@...el.com>,
        "pavel@....cz" <pavel@....cz>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        Johannes Thumshirn <Johannes.Thumshirn@....com>,
        "ming.lei@...hat.com" <ming.lei@...hat.com>,
        "jack@...e.cz" <jack@...e.cz>, "tj@...nel.org" <tj@...nel.org>,
        "gustavo@...eddedor.com" <gustavo@...eddedor.com>,
        "bvanassche@....org" <bvanassche@....org>,
        "osandov@...com" <osandov@...com>,
        "koct9i@...il.com" <koct9i@...il.com>,
        "steve@....org" <steve@....org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: [PATCH 1/2] Block layer filter - second version

On Wed, Oct 21, 2020 at 09:21:36AM +0000, Damien Le Moal wrote:
> > + * submit_bio_direct - submit a bio to the block device layer for I/O
> > + * bypass filter.
> > + * @bio:  The bio describing the location in memory and on the device.
> >   *
> > + * Description:

You don't need this line.

> > + *    This is a version of submit_bio() that shall only be used for I/O
> > + *    that cannot be intercepted by block layer filters.
> > + *    All file systems and other upper level users of the block layer
> > + *    should use submit_bio() instead.
> > + *    Use this function to access the swap partition and directly access
> > + *    the block device file.

I don't understand why O_DIRECT gets to bypass the block filter.  Nor do
I understand why anybody would place a block filter on the swap device.
But if somebody did place a filter on the swap device, why should swap
be able to bypass the filter?