linux-kernel - Re: [PATCH v3 02/13] dax: require 'struct page' for filesystem dax

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Date:   Fri, 20 Oct 2017 21:16:21 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Christoph Hellwig <hch@....de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jan Kara <jack@...e.cz>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        linux-xfs@...r.kernel.org, Linux MM <linux-mm@...ck.org>,
        Jeff Moyer <jmoyer@...hat.com>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>,
        Gerald Schaefer <gerald.schaefer@...ibm.com>
Subject: Re: [PATCH v3 02/13] dax: require 'struct page' for filesystem dax

On Fri, Oct 20, 2017 at 8:20 PM, Matthew Wilcox <willy@...radead.org> wrote:
> On Fri, Oct 20, 2017 at 03:29:57PM -0700, Dan Williams wrote:
>> Ok, I'd also like to kill DAX support in the brd driver. It's a source
>> of complexity and maintenance burden for zero benefit. It's the only
>> ->direct_access() implementation that sleeps and it's the only
>> implementation where there is a non-linear relationship between
>> sectors and pfns. Having a 1:1 sector to pfn relationship will help
>> with the dma-extent-busy management since we don't need to keep
>> calling into the driver to map pfns back to sectors once we know the
>> pfn[0] sector[0] relationship.
>
> But these are important things that other block devices may / will want.
>
> For example, I think it's entirely sensible to support ->direct_access
> for RAID-0.  Dell are looking at various different options for having
> one pmemX device per DIMM and using RAID to lash them together.
> ->direct_access makes no sense for RAID-5 or RAID-1, but RAID-0 makes
> sense to me.
>
> Last time we tried to take sleeping out, there were grumblings from people
> with network block devices who thought they'd want to bring pages in
> across the network.  I'm a bit less sympathetic to this because I don't
> know anyone actively working on it, but the RAID-0 case is something I
> think we should care about.

True, good point. In fact we already support device-mapper striping
with ->direct_access(). I'd still like to go ahead with the sleeping
removal. When those folks come back and add network direct_access they
can do the hard work of figuring out cases where we need to call
direct_access in atomic contexts.