Re: [PATCHv3 12/41] thp: handle write-protection faults for file THP

Jan Kara
2016-10-11T15:47:50Z

On Thu 15-09-16 14:54:54, Kirill A. Shutemov wrote:
> For filesystems that wants to be write-notified (has mkwrite), we will
> encount write-protection faults for huge PMDs in shared mappings.
>
> The easiest way to handle them is to clear the PMD and let it refault as
> wriable.
>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> ---
> mm/memory.c | 11 ++++++++++-
> 1 file changed, 10 insertions(+), 1 deletion(-)
>
> diff --git a/mm/memory.c b/mm/memory.c
> index 83be99d9d8a1..aad8d5c6311f 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -3451,8 +3451,17 @@ static int wp_huge_pmd(struct fault_env *fe, pmd_t orig_pmd)
> return fe->vma->vm_ops->pmd_fault(fe->vma, fe->address, fe->pmd,
> fe->flags);
>
> + if (fe->vma->vm_flags & VM_SHARED) {
> + /* Clear PMD */
> + zap_page_range_single(fe->vma, fe->address,
> + HPAGE_PMD_SIZE, NULL);
> + VM_BUG_ON(!pmd_none(*fe->pmd));
> +
> + /* Refault to establish writable PMD */
> + return 0;
> + }
> +

Since we want to write-protect the page table entry on each page writeback
and write-enable then on the next write, this is relatively expensive.
Would it be that complicated to handle this fully in ->pmd_fault handler
like we do for DAX?

Maybe it doesn't have to be done now but longer term I guess it might make
sense.

Otherwise the patch looks good so feel free to add:

Reviewed-by: Jan Kara <jack@suse.cz>

Honza
--
Jan Kara <jack@suse.com>
SUSE Labs, CR