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From:   "NeilBrown" <neilb@suse.de>
To:     "Mark Hemment" <markhemm@googlemail.com>
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 08/18] MM: Add AS_CAN_DIO mapping flag
In-reply-to: <CANe_+Uj0ooR2QzNQLXihuoaWMCJMpo3yJNhP_9DyPaCzeO7v7w@mail.gmail.com>
References: <163969801519.20885.3977673503103544412.stgit@noble.brown>,
 <163969850302.20885.17124747377211907111.stgit@noble.brown>,
 <CANe_+Uj0ooR2QzNQLXihuoaWMCJMpo3yJNhP_9DyPaCzeO7v7w@mail.gmail.com>
Date:   Mon, 20 Dec 2021 07:59:56 +1100
Message-id: <163994759613.25899.17650220291128318661@noble.neil.brown.name>
Precedence: bulk

On Mon, 20 Dec 2021, Mark Hemment wrote:
> On Thu, 16 Dec 2021 at 23:57, NeilBrown <neilb@suse.de> 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 mapping flag: AS_CAN_DIO and change the various
> > places to check this (avoiding a pointer dereference).
> > unlock_new_inode() will set this flag if ->direct_IO is present, so
> > filesystems do not need to be changed.
> ...
> > diff --git a/fs/inode.c b/fs/inode.c
> > index 6b80a51129d5..bae65ccecdb1 100644
> > --- a/fs/inode.c
> > +++ b/fs/inode.c
> > @@ -1008,6 +1008,9 @@ EXPORT_SYMBOL(lockdep_annotate_inode_mutex_key);
> >  void unlock_new_inode(struct inode *inode)
> >  {
> >         lockdep_annotate_inode_mutex_key(inode);
> > +       if (inode->i_mapping->a_ops &&
> > +           inode->i_mapping->a_ops->direct_IO)
> > +               set_bit(AS_CAN_DIO, &inode->i_mapping->flags);
> >         spin_lock(&inode->i_lock);
> >         WARN_ON(!(inode->i_state & I_NEW));
> >         inode->i_state &= ~I_NEW & ~I_CREATING;
> 
> Does d_instantiate_new() also need to set AS_CAN_DIO?

Yes it does - thanks for catching that.  I'll update my patch.

Thanks,
NeilBrown