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

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Message-ID: <1449102105.9855.15.camel@hpe.com>
Date:	Wed, 02 Dec 2015 17:21:45 -0700
From:	Toshi Kani <toshi.kani@....com>
To:	Dan Williams <dan.j.williams@...el.com>
Cc:	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 Wed, 2015-12-02 at 10:43 -0700, Toshi Kani wrote:
> On Tue, 2015-12-01 at 19:45 -0800, Dan Williams wrote:
> > On Tue, Dec 1, 2015 at 6:19 PM, Toshi Kani <toshi.kani@....com> wrote:
> > > On Mon, 2015-11-30 at 14:08 -0800, Dan Williams wrote:
 :
> > > > 
> > > > Hey Toshi,
> > > > 
> > > > I ended up fixing this differently with follow_pmd_devmap() introduced
> > > > in this series:
> > > > 
> > > > https://lists.01.org/pipermail/linux-nvdimm/2015-November/003033.html
> > > > 
> > > > Does the latest libnvdimm-pending branch [1] pass your test case?
> > > 
> > > Hi Dan,
> > > 
> > > I ran several test cases, and they all hit the case "pfn not in memmap" in
> > > __dax_pmd_fault() during mmap(MAP_POPULATE).  Looking at the dax.pfn,
> > > PFN_DEV is set but PFN_MAP is not.  I have not looked into why, but I 
> > > thought I let you know first.  I've also seen the test thread got hung up 
> > > at the end sometime.
> > 
> > That PFN_MAP flag will not be set by default for NFIT-defined
> > persistent memory.  See pmem_should_map_pages() for pmem namespaces
> > that will have it set by default, currently only e820 type-12 memory
> > ranges.
> > 
> > NFIT-defined persistent memory can have a memmap array dynamically
> > allocated by setting up a pfn device (similar to setting up a btt).
> > We don't map it by default because the NFIT may describe hundreds of
> > gigabytes of persistent and the overhead of the memmap may be too
> > large to locate the memmap in ram.
> 
> Oh, I see.  I will setup the memmap array and run the tests again.

I setup a pfn device, and ran a few test cases again.  Yes, it solved the
PFN_MAP issue.  However, I am no longer able to allocate FS blocks aligned by
2MB, so PMD faults fall back to PTE.  They are off by 2 pages, which I suspect
due to the pfn metadata.  If I pass a 2MB-aligned+2pages virtual address to
mmap(MAP_POPULATE), the mmap() call gets hung up.

Thanks,
-Toshi


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