Re: [PATCH v2 08/12] mempolicy: remove confusing MPOL_MF_LAZY dead code

[Yang Shi <shy828301@gmail.com>]

On Tue, Oct 3, 2023 at 2:24 AM Hugh Dickins <hughd@google.com> wrote:
>
> v3.8 commit b24f53a0bea3 ("mm: mempolicy: Add MPOL_MF_LAZY") introduced
> MPOL_MF_LAZY, and included it in the MPOL_MF_VALID flags; but a720094ded8
> ("mm: mempolicy: Hide MPOL_NOOP and MPOL_MF_LAZY from userspace for now")
> immediately removed it from MPOL_MF_VALID flags, pending further review.
> "This will need to be revisited", but it has not been reinstated.
>
> The present state is confusing: there is dead code in mm/mempolicy.c to
> handle MPOL_MF_LAZY cases which can never occur.  Remove that: it can be
> resurrected later if necessary.  But keep the definition of MPOL_MF_LAZY,
> which must remain in the UAPI, even though it always fails with EINVAL.
>
> https://lore.kernel.org/linux-mm/1553041659-46787-1-git-send-email-yang.shi@linux.alibaba.com/
> links to a previous request to remove MPOL_MF_LAZY.

Thanks for mentioning my work. I'm glad to see the dead code go away.

Reviewed-by: Yang Shi <shy828301@gmail.com>

>
> Signed-off-by: Hugh Dickins <hughd@google.com>
> Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> ---
>  include/uapi/linux/mempolicy.h |  2 +-
>  mm/mempolicy.c                 | 18 ------------------
>  2 files changed, 1 insertion(+), 19 deletions(-)
>
> diff --git a/include/uapi/linux/mempolicy.h b/include/uapi/linux/mempolicy.h
> index 046d0ccba4cd..a8963f7ef4c2 100644
> --- a/include/uapi/linux/mempolicy.h
> +++ b/include/uapi/linux/mempolicy.h
> @@ -48,7 +48,7 @@ enum {
>  #define MPOL_MF_MOVE    (1<<1) /* Move pages owned by this process to conform
>                                    to policy */
>  #define MPOL_MF_MOVE_ALL (1<<2)        /* Move every page to conform to policy */
> -#define MPOL_MF_LAZY    (1<<3) /* Modifies '_MOVE:  lazy migrate on fault */
> +#define MPOL_MF_LAZY    (1<<3) /* UNSUPPORTED FLAG: Lazy migrate on fault */
>  #define MPOL_MF_INTERNAL (1<<4)        /* Internal flags start here */
>
>  #define MPOL_MF_VALID  (MPOL_MF_STRICT   |     \
> diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> index 5d99fd5cd60b..f3224a8b0f6c 100644
> --- a/mm/mempolicy.c
> +++ b/mm/mempolicy.c
> @@ -636,12 +636,6 @@ unsigned long change_prot_numa(struct vm_area_struct *vma,
>
>         return nr_updated;
>  }
> -#else
> -static unsigned long change_prot_numa(struct vm_area_struct *vma,
> -                       unsigned long addr, unsigned long end)
> -{
> -       return 0;
> -}
>  #endif /* CONFIG_NUMA_BALANCING */
>
>  static int queue_pages_test_walk(unsigned long start, unsigned long end,
> @@ -680,14 +674,6 @@ static int queue_pages_test_walk(unsigned long start, unsigned long end,
>         if (endvma > end)
>                 endvma = end;
>
> -       if (flags & MPOL_MF_LAZY) {
> -               /* Similar to task_numa_work, skip inaccessible VMAs */
> -               if (!is_vm_hugetlb_page(vma) && vma_is_accessible(vma) &&
> -                       !(vma->vm_flags & VM_MIXEDMAP))
> -                       change_prot_numa(vma, start, endvma);
> -               return 1;
> -       }
> -
>         /*
>          * Check page nodes, and queue pages to move, in the current vma.
>          * But if no moving, and no strict checking, the scan can be skipped.
> @@ -1274,9 +1260,6 @@ static long do_mbind(unsigned long start, unsigned long len,
>         if (IS_ERR(new))
>                 return PTR_ERR(new);
>
> -       if (flags & MPOL_MF_LAZY)
> -               new->flags |= MPOL_F_MOF;
> -
>         /*
>          * If we are using the default policy then operation
>          * on discontinuous address spaces is okay after all
> @@ -1321,7 +1304,6 @@ static long do_mbind(unsigned long start, unsigned long len,
>
>         if (!err) {
>                 if (!list_empty(&pagelist)) {
> -                       WARN_ON_ONCE(flags & MPOL_MF_LAZY);
>                         nr_failed |= migrate_pages(&pagelist, new_folio, NULL,
>                                 start, MIGRATE_SYNC, MR_MEMPOLICY_MBIND, NULL);
>                 }
> --
> 2.35.3
>