Re: [PATCH v2] mm/mempolicy: support MPOL_MF_STRICT for huge page mapping

["HORIGUCHI NAOYA(堀口 直也)" <naoya.horiguchi@nec.com>]

On Thu, Feb 13, 2020 at 02:07:07AM +0000, Li Xinhai wrote:
> MPOL_MF_STRICT is used in mbind() for purposes:
> (1) MPOL_MF_STRICT is set alone without MPOL_MF_MOVE or MPOL_MF_MOVE_ALL,
>     to check if there is misplaced page and return -EIO;
> (2) MPOL_MF_STRICT is set with MPOL_MF_MOVE or MPOL_MF_MOVE_ALL, to check
>     if there is misplaced page which is failed to isolate, or page is
>     success on isolate but failed to move, and return -EIO.
> 
> For non hugepage mapping, (1) and (2) are implemented as expectation.
> For hugepage mapping, (1) is not implemented. And in (2), the part about
> failed to isolate and report -EIO is not implemented.
> 
> This patch implements the missed parts for hugepage mapping. Benefits
> with it applied:
> - User space can apply same code logic to handle mbind() on hugepage and
>   non hugepage mapping;
> - Reliably using MPOL_MF_STRICT alone to check whether there is misplaced
>   page or not when bind policy on address range, especially for address
>   range which contains both hugepage and non hugepage mapping.
> 
> Analysis of potential impact to existing users:
> - If MPOL_MF_STRICT alone was previously used, hugetlb pages not following
>   the memory policy would not cause an EIO error.  After this change,
>   hugetlb pages are treated like all other pages. If  MPOL_MF_STRICT alone
>   is used and hugetlb pages do not follow memory policy an EIO error will
>   be returned.
> - For users who using MPOL_MF_STRICT with MPOL_MF_MOVE or
>   MPOL_MF_MOVE_ALL, the semantic about some pages could not be moved will
>   not be changed by this patch, because failed to isolate and failed to
>   move have same effects to users, so their existing code will not be
>   impacted.
> 
> In mbind man page, the note about 'MPOL_MF_STRICT is ignored on huge page
> mappings' can be removed after this patch is applied.
> 
> Signed-off-by: Li Xinhai <lixinhai.lxh@gmail.com>
> Cc: Michal Hocko <mhocko@suse.com>
> Cc: Mike Kravetz <mike.kravetz@oracle.com>
> Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
> Cc: linux-man <linux-man@vger.kernel.org>

Thanks,

Reviewed-by: Naoya Horiguchi <naoya.horiguchi@nec.com>