linux-kernel - Re: [PATCH v3 2/9] mm: Add 'supports' field to the address_space

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Date:   Fri, 24 Sep 2021 21:10:00 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     David Howells <dhowells@...hat.com>
Cc:     hch@....de, trond.myklebust@...marydata.com,
        "Darrick J. Wong" <djwong@...nel.org>,
        Ilya Dryomov <idryomov@...il.com>,
        Jeff Layton <jlayton@...nel.org>, ceph-devel@...r.kernel.org,
        Steve French <sfrench@...ba.org>, linux-cifs@...r.kernel.org,
        linux-xfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-mm@...ck.org, darrick.wong@...cle.com,
        viro@...iv.linux.org.uk, torvalds@...ux-foundation.org,
        linux-nfs@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 2/9] mm: Add 'supports' field to the
 address_space_operations to list features

On Fri, Sep 24, 2021 at 06:18:14PM +0100, David Howells wrote:
> Rather than depending on .direct_IO to point to something to indicate that
> direct I/O is supported, add a 'supports' bitmask that we can test, since
> we only need one bit.

Why would you add mapping->aops->supports instead of using one of the free
bits in mapping->flags?  enum mapping_flags in pagemap.h.

It could also be a per-fs flag, or per-sb flag, but it's fewer
dereferences at check time if it's in mapping->flags.