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

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Date:   Fri, 20 Oct 2017 15:29:57 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Christoph Hellwig <hch@....de>
Cc:     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 9:29 AM, Christoph Hellwig <hch@....de> wrote:
> On Fri, Oct 20, 2017 at 08:23:02AM -0700, Dan Williams wrote:
>> Yes, however it seems these drivers / platforms have been living with
>> the lack of struct page for a long time. So they either don't use DAX,
>> or they have a constrained use case that never triggers
>> get_user_pages(). If it is the latter then they could introduce a new
>> configuration option that bypasses the pfn_t_devmap() check in
>> bdev_dax_supported() and fix up the get_user_pages() paths to fail.
>> So, I'd like to understand how these drivers have been using DAX
>> support without struct page to see if we need a workaround or we can
>> go ahead delete this support. If the usage is limited to
>> execute-in-place perhaps we can do a constrained ->direct_access() for
>> just that case.
>
> For axonram I doubt anyone is using it any more - it was a very for
> the IBM Cell blades, which were produceѕ in a rather limited number.
> And Cell basically seems to be dead as far as I can tell.
>
> For S/390 Martin might be able to help out what the status of xpram
> in general and DAX support in particular is.

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.