Re: [PATCH] mm: take i_mmap_lock in unmap_mapping_range() for DAX

[Dan Williams]

Hi Kirill,

On Fri, Aug 7, 2015 at 4:53 AM, Kirill A. Shutemov
<kirill.shutemov@xxxxxxxxxxxxxxx> wrote:
> DAX is not so special: we need i_mmap_lock to protect mapping->i_mmap.
>
> __dax_pmd_fault() uses unmap_mapping_range() shoot out zero page from
> all mappings. We need to drop i_mmap_lock there to avoid lock deadlock.
>
> Re-aquiring the lock should be fine since we check i_size after the
> point.
>
> Not-yet-signed-off-by: Matthew Wilcox <willy@xxxxxxxxxxxxxxx>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> ---
>  fs/dax.c    | 35 +++++++++++++++++++----------------
>  mm/memory.c | 11 ++---------
>  2 files changed, 21 insertions(+), 25 deletions(-)
>
> diff --git a/fs/dax.c b/fs/dax.c
> index 9ef9b80cc132..ed54efedade6 100644
> --- a/fs/dax.c
> +++ b/fs/dax.c
> @@ -554,6 +554,25 @@ int __dax_pmd_fault(struct vm_area_struct *vma, unsigned long address,
>         if (!buffer_size_valid(&bh) || bh.b_size < PMD_SIZE)
>                 goto fallback;
>
> +       if (buffer_unwritten(&bh) || buffer_new(&bh)) {
> +               int i;
> +               for (i = 0; i < PTRS_PER_PMD; i++)
> +                       clear_page(kaddr + i * PAGE_SIZE);

This patch, now upstream as commit 46c043ede471, moves the call to
clear_page() earlier in __dax_pmd_fault().  However, 'kaddr' is not
set at this point, so I'm not sure this path was ever tested.  I'm
also not sure why the compiler is not complaining about an
uninitialized variable?
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