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SubjectRe: [PATCH 15/31] mm/mpol: Add MPOL_MF_LAZY
On Thu, Oct 25, 2012 at 02:16:32PM +0200, Peter Zijlstra wrote:
> From: Lee Schermerhorn <lee.schermerhorn@hp.com>
>
> This patch adds another mbind() flag to request "lazy migration". The
> flag, MPOL_MF_LAZY, modifies MPOL_MF_MOVE* such that the selected
> pages are marked PROT_NONE. The pages will be migrated in the fault
> path on "first touch", if the policy dictates at that time.
>
> "Lazy Migration" will allow testing of migrate-on-fault via mbind().
> Also allows applications to specify that only subsequently touched
> pages be migrated to obey new policy, instead of all pages in range.
> This can be useful for multi-threaded applications working on a
> large shared data area that is initialized by an initial thread
> resulting in all pages on one [or a few, if overflowed] nodes.
> After PROT_NONE, the pages in regions assigned to the worker threads
> will be automatically migrated local to the threads on 1st touch.
>

I like the idea. Applications will need some way of progamatically detecting
if MPOL_MF_LAZY is available. I guess they could always try the flag and if
it fails, do nothing. When/if the manual page for it happens it probably
will be a lot more obvious. The changelog does not actually describe what
this patch does though

This patch is a preparation step for "lazy migration". When the flag is
used, the range is marked prot_none. In a later patch it will be
detected at fault time that the page is misplaced and migrate it at that
point.

> Signed-off-by: Lee Schermerhorn <lee.schermerhorn@hp.com>
> Reviewed-by: Rik van Riel <riel@redhat.com>
> Cc: Lee Schermerhorn <Lee.Schermerhorn@hp.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Linus Torvalds <torvalds@linux-foundation.org>
> [ nearly complete rewrite.. ]
> Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
> Signed-off-by: Ingo Molnar <mingo@kernel.org>

> ---
> include/uapi/linux/mempolicy.h | 13 ++++++++--
> mm/mempolicy.c | 49 ++++++++++++++++++++++++++---------------
> 2 files changed, 42 insertions(+), 20 deletions(-)
>
> Index: tip/include/uapi/linux/mempolicy.h
> ===================================================================
> --- tip.orig/include/uapi/linux/mempolicy.h
> +++ tip/include/uapi/linux/mempolicy.h
> @@ -49,9 +49,16 @@ enum mpol_rebind_step {
>
> /* Flags for mbind */
> #define MPOL_MF_STRICT (1<<0) /* Verify existing pages in the mapping */
> -#define MPOL_MF_MOVE (1<<1) /* Move pages owned by this process to conform to mapping */
> -#define MPOL_MF_MOVE_ALL (1<<2) /* Move every page to conform to mapping */
> -#define MPOL_MF_INTERNAL (1<<3) /* Internal flags start here */
> +#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_INTERNAL (1<<4) /* Internal flags start here */
> +
> +#define MPOL_MF_VALID (MPOL_MF_STRICT | \
> + MPOL_MF_MOVE | \
> + MPOL_MF_MOVE_ALL | \
> + MPOL_MF_LAZY)
>
> /*
> * Internal flags that share the struct mempolicy flags word with
> Index: tip/mm/mempolicy.c
> ===================================================================
> --- tip.orig/mm/mempolicy.c
> +++ tip/mm/mempolicy.c
> @@ -583,22 +583,32 @@ check_range(struct mm_struct *mm, unsign
> return ERR_PTR(-EFAULT);
> prev = NULL;
> for (vma = first; vma && vma->vm_start < end; vma = vma->vm_next) {
> + unsigned long endvma = vma->vm_end;
> +
> + if (endvma > end)
> + endvma = end;
> + if (vma->vm_start > start)
> + start = vma->vm_start;
> +
> if (!(flags & MPOL_MF_DISCONTIG_OK)) {
> if (!vma->vm_next && vma->vm_end < end)
> return ERR_PTR(-EFAULT);
> if (prev && prev->vm_end < vma->vm_start)
> return ERR_PTR(-EFAULT);
> }
> - if (!is_vm_hugetlb_page(vma) &&
> - ((flags & MPOL_MF_STRICT) ||
> +
> + if (is_vm_hugetlb_page(vma))
> + goto next;
> +
> + if (flags & MPOL_MF_LAZY) {
> + change_prot_none(vma, start, endvma);
> + goto next;
> + }
> +

More hard-coding of prot_none.

> + if ((flags & MPOL_MF_STRICT) ||
> ((flags & (MPOL_MF_MOVE | MPOL_MF_MOVE_ALL)) &&
> - vma_migratable(vma)))) {
> - unsigned long endvma = vma->vm_end;
> + vma_migratable(vma))) {
>
> - if (endvma > end)
> - endvma = end;
> - if (vma->vm_start > start)
> - start = vma->vm_start;
> err = check_pgd_range(vma, start, endvma, nodes,
> flags, private);
> if (err) {
> @@ -606,6 +616,7 @@ check_range(struct mm_struct *mm, unsign
> break;
> }
> }
> +next:
> prev = vma;
> }
> return first;
> @@ -1137,8 +1148,7 @@ static long do_mbind(unsigned long start
> int err;
> LIST_HEAD(pagelist);
>
> - if (flags & ~(unsigned long)(MPOL_MF_STRICT |
> - MPOL_MF_MOVE | MPOL_MF_MOVE_ALL))
> + if (flags & ~(unsigned long)MPOL_MF_VALID)
> return -EINVAL;
> if ((flags & MPOL_MF_MOVE_ALL) && !capable(CAP_SYS_NICE))
> return -EPERM;
> @@ -1161,6 +1171,9 @@ static long do_mbind(unsigned long start
> 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
> @@ -1197,21 +1210,23 @@ static long do_mbind(unsigned long start
> vma = check_range(mm, start, end, nmask,
> flags | MPOL_MF_INVERT, &pagelist);
>
> - err = PTR_ERR(vma);
> - if (!IS_ERR(vma)) {
> - int nr_failed = 0;
> -
> + err = PTR_ERR(vma); /* maybe ... */
> + if (!IS_ERR(vma) && mode != MPOL_NOOP)
> err = mbind_range(mm, start, end, new);
>
> + if (!err) {
> + int nr_failed = 0;
> +
> if (!list_empty(&pagelist)) {
> + WARN_ON_ONCE(flags & MPOL_MF_LAZY);
> nr_failed = migrate_pages(&pagelist, new_vma_page,
> - (unsigned long)vma,
> - false, MIGRATE_SYNC);
> + (unsigned long)vma,
> + false, MIGRATE_SYNC);
> if (nr_failed)
> putback_lru_pages(&pagelist);
> }
>
> - if (!err && nr_failed && (flags & MPOL_MF_STRICT))
> + if (nr_failed && (flags & MPOL_MF_STRICT))
> err = -EIO;
> } else
> putback_lru_pages(&pagelist);
>
>

--
Mel Gorman
SUSE Labs


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