Re: [v2 PATCH 1/2] mm: mempolicy: make the behavior consistent when MPOL_MF_MOVE* and MPOL_MF_STRICT were specified

Vlastimil Babka
2019-07-16T08:12:18Z

On 6/22/19 2:20 AM, Yang Shi wrote:
> When both MPOL_MF_MOVE* and MPOL_MF_STRICT was specified, mbind() should
> try best to migrate misplaced pages, if some of the pages could not be
> migrated, then return -EIO.
>
> There are three different sub-cases:
> 1. vma is not migratable
> 2. vma is migratable, but there are unmovable pages
> 3. vma is migratable, pages are movable, but migrate_pages() fails
>
> If #1 happens, kernel would just abort immediately, then return -EIO,
> after the commit a7f40cfe3b7ada57af9b62fd28430eeb4a7cfcb7 ("mm:
> mempolicy: make mbind() return -EIO when MPOL_MF_STRICT is specified").
>
> If #3 happens, kernel would set policy and migrate pages with best-effort,
> but won't rollback the migrated pages and reset the policy back.
>
> Before that commit, they behaves in the same way. It'd better to keep
> their behavior consistent. But, rolling back the migrated pages and
> resetting the policy back sounds not feasible, so just make #1 behave as
> same as #3.
>
> Userspace will know that not everything was successfully migrated (via
> -EIO), and can take whatever steps it deems necessary - attempt rollback,
> determine which exact page(s) are violating the policy, etc.
>
> Make queue_pages_range() return 1 to indicate there are unmovable pages
> or vma is not migratable.
>
> The #2 is not handled correctly in the current kernel, the following
> patch will fix it.
>
> Cc: Vlastimil Babka <vbabka@suse.cz>
> Cc: Michal Hocko <mhocko@suse.com>
> Cc: Mel Gorman <mgorman@techsingularity.net>
> Signed-off-by: Yang Shi <yang.shi@linux.alibaba.com>

Agreed with the goal, but I think there's a bug, and room for improvement.

> ---
> mm/mempolicy.c | 86 +++++++++++++++++++++++++++++++++++++++++-----------------
> 1 file changed, 61 insertions(+), 25 deletions(-)
>
> diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> index 01600d8..b50039c 100644
> --- a/mm/mempolicy.c
> +++ b/mm/mempolicy.c
> @@ -429,11 +429,14 @@ static inline bool queue_pages_required(struct page *page,
> }
>
> /*
> - * queue_pages_pmd() has three possible return values:
> + * queue_pages_pmd() has four possible return values:
> + * 2 - there is unmovable page, and MPOL_MF_MOVE* & MPOL_MF_STRICT were
> + * specified.
> * 1 - pages are placed on the right node or queued successfully.
> * 0 - THP was split.

I think if you renumbered these, it would be more consistent with
queue_pages_pte_range() and simplify the code there.
0 - pages on right node/queued
1 - unmovable page with right flags specified
2 - THP split

> - * -EIO - is migration entry or MPOL_MF_STRICT was specified and an existing
> - * page was already on a node that does not follow the policy.
> + * -EIO - is migration entry or only MPOL_MF_STRICT was specified and an
> + * existing page was already on a node that does not follow the
> + * policy.
> */
> static int queue_pages_pmd(pmd_t *pmd, spinlock_t *ptl, unsigned long addr,
> unsigned long end, struct mm_walk *walk)
> @@ -463,7 +466,7 @@ static int queue_pages_pmd(pmd_t *pmd, spinlock_t *ptl, unsigned long addr,
> /* go to thp migration */
> if (flags & (MPOL_MF_MOVE | MPOL_MF_MOVE_ALL)) {
> if (!vma_migratable(walk->vma)) {
> - ret = -EIO;
> + ret = 2;
> goto unlock;
> }
>
> @@ -488,16 +491,29 @@ static int queue_pages_pte_range(pmd_t *pmd, unsigned long addr,

Perhaps this function now also deserves a list of possible return values.

> struct queue_pages *qp = walk->private;
> unsigned long flags = qp->flags;
> int ret;
> + bool has_unmovable = false;
> pte_t *pte;
> spinlock_t *ptl;
>
> ptl = pmd_trans_huge_lock(pmd, vma);
> if (ptl) {
> ret = queue_pages_pmd(pmd, ptl, addr, end, walk);
> - if (ret > 0)
> + switch (ret) {

With renumbering suggested above, this could be:
if (ret != 2)
return ret;

> + /* THP was split, fall through to pte walk */
> + case 0:
> + break;
> + /* Pages are placed on the right node or queued successfully */
> + case 1:
> return 0;
> - else if (ret < 0)
> + /*
> + * Met unmovable pages, MPOL_MF_MOVE* & MPOL_MF_STRICT
> + * were specified.
> + */
> + case 2:
> + return 1;
> + case -EIO:
> return ret;
> + }
> }
>
> if (pmd_trans_unstable(pmd))
> @@ -519,14 +535,21 @@ static int queue_pages_pte_range(pmd_t *pmd, unsigned long addr,
> if (!queue_pages_required(page, qp))
> continue;
> if (flags & (MPOL_MF_MOVE | MPOL_MF_MOVE_ALL)) {
> - if (!vma_migratable(vma))
> + /* MPOL_MF_STRICT must be specified if we get here */
> + if (!vma_migratable(vma)) {
> + has_unmovable |= true;
> break;
> + }
> migrate_page_add(page, qp->pagelist, flags);
> } else
> break;
> }
> pte_unmap_unlock(pte - 1, ptl);
> cond_resched();
> +
> + if (has_unmovable)
> + return 1;
> +
> return addr != end ? -EIO : 0;
> }
>
> @@ -639,7 +662,13 @@ static int queue_pages_test_walk(unsigned long start, unsigned long end,
> *
> * If pages found in a given range are on a set of nodes (determined by
> * @nodes and @flags,) it's isolated and queued to the pagelist which is
> - * passed via @private.)
> + * passed via @private.
> + *
> + * queue_pages_range() has three possible return values:
> + * 1 - there is unmovable page, but MPOL_MF_MOVE* & MPOL_MF_STRICT were
> + * specified.
> + * 0 - queue pages successfully or no misplaced page.
> + * -EIO - there is misplaced page and only MPOL_MF_STRICT was specified.
> */
> static int
> queue_pages_range(struct mm_struct *mm, unsigned long start, unsigned long end,
> @@ -1182,6 +1211,7 @@ static long do_mbind(unsigned long start, unsigned long len,
> struct mempolicy *new;
> unsigned long end;
> int err;
> + int ret;
> LIST_HEAD(pagelist);
>
> if (flags & ~(unsigned long)MPOL_MF_VALID)
> @@ -1243,26 +1273,32 @@ static long do_mbind(unsigned long start, unsigned long len,
> if (err)
> goto mpol_out;
>
> - err = queue_pages_range(mm, start, end, nmask,
> + ret = queue_pages_range(mm, start, end, nmask,
> flags | MPOL_MF_INVERT, &pagelist);
> - if (!err)
> - 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_page, NULL,
> - start, MIGRATE_SYNC, MR_MEMPOLICY_MBIND);
> - if (nr_failed)
> - putback_movable_pages(&pagelist);
> - }
> + if (ret < 0)
> + err = -EIO;

I think after your patch, you miss putback_movable_pages() in cases
where some were queued, and later the walk returned -EIO. The previous
code doesn't miss it, but it's also not obvious due to the multiple if
(!err) checks. I would rewrite it some thing like this:

if (ret < 0) {
putback_movable_pages(&pagelist);
err = ret;
goto mmap_out; // a new label above up_write()
}

The rest can have reduced identation now.

> + else {
> + err = mbind_range(mm, start, end, new);
>
> - if (nr_failed && (flags & MPOL_MF_STRICT))
> - err = -EIO;
> - } else
> - putback_movable_pages(&pagelist);
> + if (!err) {
> + int nr_failed = 0;
> +
> + if (!list_empty(&pagelist)) {
> + WARN_ON_ONCE(flags & MPOL_MF_LAZY);
> + nr_failed = migrate_pages(&pagelist, new_page,
> + NULL, start, MIGRATE_SYNC,
> + MR_MEMPOLICY_MBIND);
> + if (nr_failed)
> + putback_movable_pages(&pagelist);
> + }
> +
> + if ((ret > 0) ||
> + (nr_failed && (flags & MPOL_MF_STRICT)))
> + err = -EIO;
> + } else
> + putback_movable_pages(&pagelist);

While at it, IIRC the kernel style says that when the 'if' part uses
'{ }' then the 'else' part should as well, and it shouldn't be mixed.

Thanks,
Vlastimil

> + }
>
> up_write(&mm->mmap_sem);
> mpol_out:
>