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From:   "NeilBrown" <neilb@suse.de>
To:     "Christoph Hellwig" <hch@infradead.org>
Cc:     "Trond Myklebust" <trond.myklebust@hammerspace.com>,
        "Anna Schumaker" <anna.schumaker@netapp.com>,
        "Chuck Lever" <chuck.lever@oracle.com>,
        "Andrew Morton" <akpm@linux-foundation.org>,
        "Mel Gorman" <mgorman@suse.de>,
        "Christoph Hellwig" <hch@infradead.org>,
        "David Howells" <dhowells@redhat.com>, linux-nfs@vger.kernel.org,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 11/23] VFS: Add FMODE_CAN_ODIRECT file flag
In-reply-to: <Ye5puPat8w9/nQ6R@infradead.org>
References: <164299573337.26253.7538614611220034049.stgit@noble.brown>,
 <164299611280.26253.2845018521780218144.stgit@noble.brown>,
 <Ye5puPat8w9/nQ6R@infradead.org>
Date:   Thu, 27 Jan 2022 09:14:41 +1100
Message-id: <164323528189.5493.7196087998245554506@noble.neil.brown.name>
Precedence: bulk

On Mon, 24 Jan 2022, Christoph Hellwig wrote:
> 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@lst.de>
> 
> It would be nice to throw in a cleanup to remove noop_direct_IO as well.

I don't want to add this to the present series.  When it lands I'll send
patches to the various filesystems to switch to using FMODE_CAN_ODIRECT.

Thanks,
NeilBrown