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

    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.

    I have a pending patch in libnvdimm-pending that allows the capacity
    for the memmap to come from pmem instead of ram:

    https://git.kernel.org/cgit/linux/kernel/git/djbw/nvdimm.git/commit/?h=libnvdimm-pending&id=3117a24e07fe

    > 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, I'll fix that up.


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