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Date:   Fri, 24 Sep 2021 21:10:00 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     David Howells <dhowells@redhat.com>
Cc:     hch@lst.de, trond.myklebust@primarydata.com,
        "Darrick J. Wong" <djwong@kernel.org>,
        Ilya Dryomov <idryomov@gmail.com>,
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Subject: Re: [PATCH v3 2/9] mm: Add 'supports' field to the
 address_space_operations to list features
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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.