linux-ext4 - Re: [RFC 2/3] ext2: Convert ext2 regular file buffered I/O to use iomap

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Date: Wed, 22 Nov 2023 20:09:44 -0800
From: "Darrick J. Wong" <djwong@...nel.org>
To: Dave Chinner <david@...morbit.com>
Cc: Christoph Hellwig <hch@...radead.org>, Jan Kara <jack@...e.cz>,
	"Ritesh Harjani (IBM)" <ritesh.list@...il.com>,
	linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [RFC 2/3] ext2: Convert ext2 regular file buffered I/O to use
 iomap

On Thu, Nov 23, 2023 at 09:26:44AM +1100, Dave Chinner wrote:
> On Wed, Nov 22, 2023 at 05:11:18AM -0800, Christoph Hellwig wrote:
> > On Wed, Nov 22, 2023 at 01:29:46PM +0100, Jan Kara wrote:
> > > writeback bit set. XFS plays the revalidation sequence counter games
> > > because of this so we'd have to do something similar for ext2. Not that I'd
> > > care as much about ext2 writeback performance but it should not be that
> > > hard and we'll definitely need some similar solution for ext4 anyway. Can
> > > you give that a try (as a followup "performance improvement" patch).
> > 
> > Darrick has mentioned that he is looking into lifting more of the
> > validation sequence counter validation into iomap.
> 
> I think that was me, as part of aligning the writeback path with
> the ->iomap_valid() checks in the write path after we lock the folio
> we instantiated for the write.
> 
> It's basically the same thing - once we have a locked folio, we have
> to check that the cached iomap is still valid before we use it for
> anything.
> 
> I need to find the time to get back to that, though.

Heh, we probably both have been chatting with willy on and off about
iomap.

The particular idea I had is to add a u64 counter to address_space that
we can bump in the same places where we bump xfs_inode_fork::if_seq
right now..  ->iomap_begin would sample this address_space::i_mappingseq
counter (with locks held), and now buffered writes and writeback can
check iomap::mappingseq == address_space::i_mappingseq to decide if it's
time to revalidate.

Anyway, I'll have time to go play with that (and further purging of
function pointers) next week or whenever is "after I put out v28 of
online repair".  ATM I have a rewrite of log intent recovery cooking
too, but that's going to need at least a week or two of recoveryloop
testing before I put that on the list.

--D

> -Dave.
> -- 
> Dave Chinner
> david@...morbit.com
>