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

[Dan Williams]

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.

As of 4.4.-rc2 DAX pmd mappings are disabled.  So we have time to do
something more comprehensive in 4.5.

>
> 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>
> ---
>  mm/huge_memory.c |   34 ++++++++++++++++++++++++++++++++++
>  1 file changed, 34 insertions(+)
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index d5b8920..f56e034 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
[..]
> @@ -1288,6 +1315,13 @@ struct page *follow_trans_huge_pmd(struct vm_area_struct *vma,
>         if ((flags & FOLL_NUMA) && pmd_protnone(*pmd))
>                 goto out;
>
> +       /* pfn map does not have a struct page */
> +       if (vma->vm_flags & (VM_PFNMAP | VM_MIXEDMAP)) {
> +               ret = follow_pfn_pmd(vma, addr, pmd, flags);
> +               page = ERR_PTR(ret);
> +               goto out;
> +       }
> +
>         page = pmd_page(*pmd);
>         VM_BUG_ON_PAGE(!PageHead(page), page);
>         if (flags & FOLL_TOUCH) {

I think it is already problematic that dax pmd mappings are getting
confused with transparent huge pages.  They're more closely related to
a hugetlbfs pmd mappings in that they are mapping an explicit
allocation.  I have some pending patches to address this dax-pmd vs
hugetlb-pmd vs thp-pmd classification that I will post shortly.

By the way, I'm collecting DAX pmd regression tests [1], is this just
a simple crash upon using MAP_POPULATE?

[1]: https://github.com/pmem/ndctl/blob/master/lib/test-dax-pmd.c
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