Re: [PATCH] dax: Split pmd map when fallback on COW

[Toshi Kani]

On Mon, 2015-11-23 at 12:45 -0800, Dan Williams wrote:
> On Mon, Nov 23, 2015 at 12:05 PM, Toshi Kani <toshi.kani@xxxxxxx> wrote:
> > An infinite loop of PMD faults was observed when attempted to
> > mlock() a private read-only PMD mmap'd range of a DAX file.
> > 
> > __dax_pmd_fault() simply returns with VM_FAULT_FALLBACK when
> > falling back to PTE on COW.  However, __handle_mm_fault()
> > returns without falling back to handle_pte_fault() because
> > a PMD map is present in this case.
> > 
> > Change __dax_pmd_fault() to split the PMD map, if present,
> > before returning with VM_FAULT_FALLBACK.
> > 
> > Signed-off-by: Toshi Kani <toshi.kani@xxxxxxx>
> > Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
> > Cc: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> > Cc: Matthew Wilcox <willy@xxxxxxxxxxxxxxx>
> > Cc: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx>
> 
> I thought the patch from Ross already addressed the infinite loop:
> 
> https://patchwork.kernel.org/patch/7653731/

This fixes a different issue.  I hit this one while testing my other patch along
with the Ross's patch.

> > ---
> >  fs/dax.c |    4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> > 
> > diff --git a/fs/dax.c b/fs/dax.c
> > index 43671b6..3405583 100644
> > --- a/fs/dax.c
> > +++ b/fs/dax.c
> > @@ -546,8 +546,10 @@ int __dax_pmd_fault(struct vm_area_struct *vma,
> > unsigned long address,
> >                 return VM_FAULT_FALLBACK;
> > 
> >         /* Fall back to PTEs if we're going to COW */
> > -       if (write && !(vma->vm_flags & VM_SHARED))
> > +       if (write && !(vma->vm_flags & VM_SHARED)) {
> > +               split_huge_page_pmd(vma, address, pmd);
> >                 return VM_FAULT_FALLBACK;
> > +       }
> >         /* If the PMD would extend outside the VMA */
> >         if (pmd_addr < vma->vm_start)
> >                 return VM_FAULT_FALLBACK;
> 
> This is a nop if CONFIG_TRANSPARENT_HUGEPAGE=n, so I don't think it's
> a complete fix.

Well, __dax_pmd_fault() itself depends on CONFIG_TRANSPARENT_HUGEPAGE.

Thanks,
-Toshi

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