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

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Date:   Wed, 18 Jan 2023 18:11:03 +0900
From:   Damien Le Moal <damien.lemoal@...nsource.wdc.com>
To:     Christoph Hellwig <hch@...radead.org>,
        "Darrick J. Wong" <djwong@...nel.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 1/18/23 16:21, 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.
> 
>>
>> 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.

zonefs has a static mapping of file blocks that never changes and is fully
populated up to a file max size from mount. So zonefs is not using the
iomap_valid page operation. In fact, zonefs is not even using struct
iomap_page_ops.

> 
>> 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.

-- 
Damien Le Moal
Western Digital Research