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SubjectRe: [PATCH] mm: Fix mmap MAP_POPULATE for DAX pmd mapping
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On Mon, 2015-11-30 at 14:08 -0800, Dan Williams wrote:
> On Mon, Nov 23, 2015 at 12:04 PM, Toshi Kani <toshi.kani@hpe.com> wrote:
> > The following oops was observed when mmap() with MAP_POPULATE
> > pre-faulted pmd mappings of a DAX file. follow_trans_huge_pmd()
> > expects that a target address has a struct page.
> >
> > BUG: unable to handle kernel paging request at ffffea0012220000
> > follow_trans_huge_pmd+0xba/0x390
> > follow_page_mask+0x33d/0x420
> > __get_user_pages+0xdc/0x800
> > populate_vma_page_range+0xb5/0xe0
> > __mm_populate+0xc5/0x150
> > vm_mmap_pgoff+0xd5/0xe0
> > SyS_mmap_pgoff+0x1c1/0x290
> > SyS_mmap+0x1b/0x30
> >
> > Fix it by making the PMD pre-fault handling consistent with PTE.
> > After pre-faulted in faultin_page(), follow_page_mask() calls
> > follow_trans_huge_pmd(), which is changed to call follow_pfn_pmd()
> > for VM_PFNMAP or VM_MIXEDMAP. follow_pfn_pmd() handles FOLL_TOUCH
> > and returns with -EEXIST.
> >
> > Reported-by: Mauricio Porto <mauricio.porto@hpe.com>
> > Signed-off-by: Toshi Kani <toshi.kani@hpe.com>
> > Cc: Andrew Morton <akpm@linux-foundation.org>
> > Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> > Cc: Matthew Wilcox <willy@linux.intel.com>
> > Cc: Dan Williams <dan.j.williams@intel.com>
> > Cc: Ross Zwisler <ross.zwisler@linux.intel.com>
> > ---
>
> 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.

I also noticed that reason is not set in the case below.

if (length < PMD_SIZE
|| (pfn_t_to_pfn(dax.pfn) & PG_PMD_COLOUR)) {
dax_unmap_atomic(bdev, &dax);
goto fallback;
}

Thanks,
-Toshi


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