linux-kernel - Re: [PATCH] mm: Fix mmap MAP_POPULATE for DAX pmd mapping

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Date:	Wed, 2 Dec 2015 14:09:10 -0800
From:	Dave Hansen <dave.hansen@...el.com>
To:	Dan Williams <dan.j.williams@...el.com>
Cc:	Toshi Kani <toshi.kani@....com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Matthew Wilcox <willy@...ux.intel.com>,
	Ross Zwisler <ross.zwisler@...ux.intel.com>,
	mauricio.porto@....com, Linux MM <linux-mm@...ck.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	"linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: Fix mmap MAP_POPULATE for DAX pmd mapping

On 12/02/2015 02:03 PM, Dan Williams wrote:
>>> >> Is pfn_valid() a reliable check?  It seems to be based on a max_pfn
>>> >> per node... what happens when pmem is located below that point.  I
>>> >> haven't been able to convince myself that we won't get false
>>> >> positives, but maybe I'm missing something.
>> >
>> > With sparsemem at least, it makes sure that you're looking at a valid
>> > _section_.  See the pfn_valid() at ~include/linux/mmzone.h:1222.
> At a minimum we would need to add "depends on SPARSEMEM" to "config FS_DAX_PMD".

Yeah, it seems like an awful layering violation.  But, sparsemem is
turned on everywhere (all the distros/users) that we care about, as far
as I know.
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