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

[Dan Williams]

On Tue, Dec 1, 2015 at 6:19 PM, Toshi Kani <toshi.kani@xxxxxxx> 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@xxxxxxx> 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@xxxxxxx>
>> > Signed-off-by: Toshi Kani <toshi.kani@xxxxxxx>
>> > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
>> > Cc: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
>> > Cc: Matthew Wilcox <willy@xxxxxxxxxxxxxxx>
>> > Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
>> > Cc: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx>
>> > ---
>>
>> 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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