linux-kernel - Re: [PATCH 11/23] VFS: Add FMODE_CAN

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Date:   Mon, 24 Jan 2022 00:56:24 -0800
From:   Christoph Hellwig <hch@...radead.org>
To:     NeilBrown <neilb@...e.de>
Cc:     Trond Myklebust <trond.myklebust@...merspace.com>,
        Anna Schumaker <anna.schumaker@...app.com>,
        Chuck Lever <chuck.lever@...cle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mel Gorman <mgorman@...e.de>,
        Christoph Hellwig <hch@...radead.org>,
        David Howells <dhowells@...hat.com>, linux-nfs@...r.kernel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 11/23] VFS: Add FMODE_CAN_ODIRECT file flag

On Mon, Jan 24, 2022 at 02:48:32PM +1100, NeilBrown wrote:
> Currently various places test if direct IO is possible on a file by
> checking for the existence of the direct_IO address space operation.
> This is a poor choice, as the direct_IO operation may not be used - it is
> only used if the generic_file_*_iter functions are called for direct IO
> and some filesystems - particularly NFS - don't do this.
> 
> Instead, introduce a new f_mode flag: FMODE_CAN_ODIRECT and change the
> various places to check this (avoiding pointer dereferences).
> do_dentry_open() will set this flag if ->direct_IO is present, so
> filesystems do not need to be changed.
> 
> NFS *is* changed, to set the flag explicitly and discard the direct_IO
> entry in the address_space_operations for files.

Looks good:

Reviewed-by: Christoph Hellwig <hch@....de>

It would be nice to throw in a cleanup to remove noop_direct_IO as well.