linux-ext4 - Re: [PATCH 2/6] iomap: move bdev and dax

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Date:   Tue, 19 Jun 2018 08:44:51 +0200
From:   Christoph Hellwig <hch@....de>
To:     "Darrick J. Wong" <darrick.wong@...cle.com>
Cc:     Christoph Hellwig <hch@....de>,
        Andreas Gruenbacher <agruenba@...hat.com>,
        linux-xfs@...r.kernel.org, Dan Williams <dan.j.williams@...el.com>,
        linux-fsdevel@...r.kernel.org, cluster-devel@...hat.com,
        linux-ext4@...r.kernel.org
Subject: Re: [PATCH 2/6] iomap: move bdev and dax_dev in a union

On Mon, Jun 18, 2018 at 11:25:57PM -0700, Darrick J. Wong wrote:
> Is this going to blow up iomap_dax_zero?  It seems to use both bdev and
> dax_dev on __dax_zero_page_range, which definitely uses both.
> 
> (Or did all that get rearranged when I wasn't looking?)

Ouch, it does.  And that looks pretty broken.

> Also, I guess this will break iomap swapfiles since it checks
> iomap->bdev which we stop supplying with this patch...
> though I have no idea if DAX swapfiles are even supported.

Not sure if we support it.  We didn't use to support it when
swap used ->bmap, so until someone volunteers to test it we
should disable it with the iomap swapfile code as well.  But
even then doing a detour through the block layer and thus
the bdev makes very little sense.

> 
> What's the harm in supplying both pointers?

Just blowing up the size of the iomap.  Especially once we add
the inline data as the third option.