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

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Date:   Mon, 18 Jun 2018 23:50:29 -0700
From:   "Darrick J. Wong" <darrick.wong@...cle.com>
To:     Christoph Hellwig <hch@....de>
Cc:     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 Tue, Jun 19, 2018 at 08:44:51AM +0200, Christoph Hellwig wrote:
> 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.

Indeed.

> > 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.

For now all we do with it is compare iomap->bdev to the swapfile bdev so
it'll effectively disable DAX swapfiles in a weird and scary way...

...but maybe we should ask around if anyone cares about DAX swapfiles.
Everyone may just run away, but at least we will have tried. :)

> > 
> > 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.

<nod>

--D

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