Re: [PATCH] mm, dax: fix DAX deadlocks (COW fault)

[Dan Williams]

On Mon, Nov 16, 2015 at 4:09 AM, Yigal Korman <yigal@xxxxxxxxxxxxx> wrote:
> DAX handling of COW faults has wrong locking sequence:
>         dax_fault does i_mmap_lock_read
>         do_cow_fault does i_mmap_unlock_write
>
> Ross's commit[1] missed a fix[2] that Kirill added to Matthew's
> commit[3].
>
> Original COW locking logic was introduced by Matthew here[4].
>
> This should be applied to v4.3 as well.
>
> [1] 0f90cc6609c7 mm, dax: fix DAX deadlocks
> [2] 52a2b53ffde6 mm, dax: use i_mmap_unlock_write() in do_cow_fault()
> [3] 843172978bb9 dax: fix race between simultaneous faults
> [4] 2e4cdab0584f mm: allow page fault handlers to perform the COW
>
> Signed-off-by: Yigal Korman <yigal@xxxxxxxxxxxxx>
>
> Cc: Stable Tree <stable@xxxxxxxxxxxxxxx>
> Cc: Boaz Harrosh <boaz@xxxxxxxxxxxxx>
> Cc: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx>
> Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
> Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
> Cc: Dave Chinner <dchinner@xxxxxxxxxx>
> Cc: Jan Kara <jack@xxxxxxxx>
> Cc: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
> Cc: Matthew Wilcox <matthew.r.wilcox@xxxxxxxxx>
> ---
>  mm/memory.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/mm/memory.c b/mm/memory.c
> index c716913..e5071af 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -3015,9 +3015,9 @@ static int do_cow_fault(struct mm_struct *mm, struct vm_area_struct *vma,
>                 } else {
>                         /*
>                          * The fault handler has no page to lock, so it holds
> -                        * i_mmap_lock for write to protect against truncate.
> +                        * i_mmap_lock for read to protect against truncate.
>                          */
> -                       i_mmap_unlock_write(vma->vm_file->f_mapping);
> +                       i_mmap_unlock_read(vma->vm_file->f_mapping);
>                 }
>                 goto uncharge_out;
>         }
> @@ -3031,9 +3031,9 @@ static int do_cow_fault(struct mm_struct *mm, struct vm_area_struct *vma,
>         } else {
>                 /*
>                  * The fault handler has no page to lock, so it holds
> -                * i_mmap_lock for write to protect against truncate.
> +                * i_mmap_lock for read to protect against truncate.
>                  */
> -               i_mmap_unlock_write(vma->vm_file->f_mapping);
> +               i_mmap_unlock_read(vma->vm_file->f_mapping);
>         }
>         return ret;
>  uncharge_out:

Looks good to me.  I'll include this with some other DAX fixes I have pending.
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