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Date: Thu, 30 Nov 2023 11:18:45 +0100
From: Jan Kara <jack@...e.cz>
To: Ritesh Harjani <ritesh.list@...il.com>
Cc: Christoph Hellwig <hch@...radead.org>, Jan Kara <jack@...e.cz>,
	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 30-11-23 13:15:58, Ritesh Harjani wrote:
> Ritesh Harjani (IBM) <ritesh.list@...il.com> writes:
> 
> > Ritesh Harjani (IBM) <ritesh.list@...il.com> writes:
> >
> >> Christoph Hellwig <hch@...radead.org> writes:
> >>
> >>> 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).
> 
> ok. So I am re-thinknig over this on why will a filesystem like ext2
> would require sequence counter check. We don't have collapse range
> or COW sort of operations, it is only the truncate which can race,
> but that should be taken care by folio_lock. And even if the partial
> truncate happens on a folio, since the logical to physical block mapping
> never changes, it should not matter if the writeback wrote data to a
> cached entry, right?

Yes, so this is what I think I've already mentioned. As long as we map just
the block at the current offset (or a block under currently locked folio),
we are fine and we don't need any kind of sequence counter. But as soon as
we start caching any kind of mapping in iomap_writepage_ctx we need a way
to protect from races with truncate. So something like the sequence counter.

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR

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