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

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Date:   Tue, 17 Jan 2023 23:21:38 -0800
From:   Christoph Hellwig <hch@...radead.org>
To:     "Darrick J. Wong" <djwong@...nel.org>
Cc:     Andreas Grünbacher 
        <andreas.gruenbacher@...il.com>,
        Dave Chinner <david@...morbit.com>,
        Andreas Gruenbacher <agruenba@...hat.com>,
        Christoph Hellwig <hch@...radead.org>,
        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 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.

> 
> 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.

> 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.