linux-ext4 - Re: [RFC v6 08/10] iomap/xfs: Eliminate the iomap

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Message-ID: <Y8hCq+fIWgHfUufe@magnolia>
Date:   Wed, 18 Jan 2023 11:04:11 -0800
From:   "Darrick J. Wong" <djwong@...nel.org>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Andreas Grünbacher 
        <andreas.gruenbacher@...il.com>,
        Dave Chinner <david@...morbit.com>,
        Andreas Gruenbacher <agruenba@...hat.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Damien Le Moal <damien.lemoal@....com>,
        Matthew Wilcox <willy@...radead.org>,
        linux-xfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-ext4@...r.kernel.org, cluster-devel@...hat.com
Subject: Re: [RFC v6 08/10] iomap/xfs: Eliminate the iomap_valid handler

On Tue, Jan 17, 2023 at 11:21:38PM -0800, Christoph Hellwig wrote:
> On Sun, Jan 15, 2023 at 09:29:58AM -0800, Darrick J. Wong wrote:
> > I don't have any objections to pulling everything except patches 8 and
> > 10 for testing this week. 
> 
> That would be great.  I now have a series to return the ERR_PTR
> from __filemap_get_folio which will cause a minor conflict, but
> I think that's easy enough for Linux to handle.

Ok, done.

> > 
> > 1. Does zonefs need to revalidate mappings?  The mappings are 1:1 so I
> > don't think it does, but OTOH zone pointer management might complicate
> > that.
> 
> Adding Damien.
> 
> > 2. How about porting the writeback iomap validation to use this
> > mechanism?  (I suspect Dave might already be working on this...)
> 
> What is "this mechanism"?  Do you mean the here removed ->iomap_valid
> ?   writeback calls into ->map_blocks for every block while under the
> folio lock, so the validation can (and for XFS currently is) done
> in that.  Moving it out into a separate method with extra indirect
> functiona call overhead and interactions between the methods seems
> like a retrograde step to me.

Sorry, I should've been more specific -- can xfs writeback use the
validity cookie in struct iomap and thereby get rid of struct
xfs_writepage_ctx entirely?

> > 2. Do we need to revalidate mappings for directio writes?  I think the
> > answer is no (for xfs) because the ->iomap_begin call will allocate
> > whatever blocks are needed and truncate/punch/reflink block on the
> > iolock while the directio writes are pending, so you'll never end up
> > with a stale mapping.
> 
> Yes.

Er... yes as in "Yes, we *do* need to revalidate directio writes", or
"Yes, your reasoning is correct"?

--D