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

[Toshi Kani]

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.  

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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