linux-kernel - Re: [PATCH 08/18] MM: Add AS_CAN

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Message-id: <164256448911.8775.16853022981145854177@noble.neil.brown.name>
Date:   Wed, 19 Jan 2022 14:54:49 +1100
From:   "NeilBrown" <neilb@...e.de>
To:     "Christoph Hellwig" <hch@...radead.org>
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 08/18] MM: Add AS_CAN_DIO mapping flag

On Tue, 21 Dec 2021, Christoph Hellwig wrote:
> On Fri, Dec 17, 2021 at 10:48:23AM +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 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.
> > 
> > NFS *is* changed, to set the flag explicitly and discard the direct_IO
> > entry in the address_space_operations for files.
> 
> For other can flags related to file operations we usuall stash them into
> file->f_mode.  Any reason to treat this different?

f_mode is a much better place to put the flag!  Thanks for that
suggestion and all the other feedback.  I'll post a new series early
next week.

Thanks,
NeilBrown