linux-kernel - Re: [PATCH 2/2] mm: Make swap_readpage() for SWP_FS_OPS use ->direct

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Date:   Fri, 13 Aug 2021 08:54:26 +0200
From:   Christoph Hellwig <hch@....de>
To:     "Darrick J. Wong" <djwong@...nel.org>
Cc:     Christoph Hellwig <hch@....de>,
        Matthew Wilcox <willy@...radead.org>,
        David Howells <dhowells@...hat.com>,
        trond.myklebust@...marydata.com, darrick.wong@...cle.com,
        jlayton@...nel.org, sfrench@...ba.org,
        torvalds@...ux-foundation.org, linux-nfs@...r.kernel.org,
        linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] mm: Make swap_readpage() for SWP_FS_OPS use
 ->direct_IO() not ->readpage()

On Thu, Aug 12, 2021 at 10:48:18AM -0700, Darrick J. Wong wrote:
> On Thu, Aug 12, 2021 at 07:02:33PM +0200, Christoph Hellwig wrote:
> > On Thu, Aug 12, 2021 at 04:39:40PM +0100, Matthew Wilcox wrote:
> > > I agree with David; we want something lower-level for swap to call into.
> > > I'd suggest aops->swap_rw and an implementation might well look
> > > something like:
> > > 
> > > static ssize_t ext4_swap_rw(struct kiocb *iocb, struct iov_iter *iter)
> > > {
> > > 	return iomap_dio_rw(iocb, iter, &ext4_iomap_ops, NULL, 0);
> > > }
> > 
> > Yes, that might make sense and would also replace the awkward IOCB_SWAP
> > flag for the write side.
> > 
> > For file systems like ext4 and xfs that have an in-memory block mapping
> > tree this would be way better than the current version and also support
> > swap on say multi-device file systems properly.  We'd just need to be
> > careful to read the extent information in at extent_activate time,
> > by doing xfs_iread_extents for XFS or the equivalents in other file
> > systems.
> 
> You'd still want to walk the extent map at activation time to reject
> swapfiles with holes, shared extents, etc., right?

Yes.  While direct I/O code could do allocation at swap I/O time that
probably is not a good idea due to the memory requirements.