linux-kernel - Re: [PATCH 14/15] mm, dax, pmem: introduce {get|put}_dev

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Message-ID: <CAPcyv4jbRn8tTbuqKYCzd4Qs5FjvJCN_Jy5pjHfwX+Kw9bub0Q@mail.gmail.com>
Date:	Fri, 2 Oct 2015 15:55:23 -0700
From:	Dan Williams <dan.j.williams@...el.com>
To:	Logan Gunthorpe <logang@...tatee.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Dave Hansen <dave@...1.net>,
	"linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linux MM <linux-mm@...ck.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Matthew Wilcox <willy@...ux.intel.com>,
	Ross Zwisler <ross.zwisler@...ux.intel.com>,
	Stephen Bates <Stephen.Bates@...s.com>
Subject: Re: [PATCH 14/15] mm, dax, pmem: introduce {get|put}_dev_pagemap()
 for dax-gup

On Fri, Oct 2, 2015 at 3:42 PM, Logan Gunthorpe <logang@...tatee.com> wrote:
>
>
> On 02/10/15 03:53 PM, Dan Williams wrote:
>>
>> Yes, this location for dev_pagemap will not work.  I've since moved it
>> to a union with the lru list_head since ZONE_DEVICE pages memory
>> should always have an elevated page count and never land on a slab
>> allocator lru.
>
>
> Oh, also, I was actually hoping to make use of the lru list_head in the
> future with ZONE_DEVICE memory. One thought I had was once we have a PCIe
> device with a BAR space, we'd then need to have a way of allocating these
> buffers when user space needs them. The simple way I was thinking was to
> just use the lru list head to store lists of used and unused pages -- though
> there are probably other solutions to this that don't require using struct
> pages.
>

The current assumption is the ZONE_DEVICE ranges are being managed by
a physical address allocator.  In the case of persistent memory this
is the block allocator of the filesystem sitting on top of a pmem
block device.  The struct page is really only there to facilitate
in-flight I/O requests.  If it weren't for complexity we'd allocate
them on demand.  So you're "unused" case should be a raw pfn and then
for the time-limited duration it is in use as a struct page it should
hold a reference against the mapping.
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