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SubjectRe: [PATCH] mm: Fix mmap MAP_POPULATE for DAX pmd mapping
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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@hpe.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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