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

Yang Shi
2019-07-16T17:18:37Z

On 7/16/19 1:12 AM, Vlastimil Babka wrote:
> 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.

Sure, will add some comments to elaborate the 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()
> }

Yes, the old code had putback_movable_pages called when !err. But, I
think that is for error handling of mbind_range() if I understand it
correctly since if queue_pages_range() returns -EIO (only MPOL_MF_STRICT
was specified and there was misplaced page) that page list should be
empty . The old code should checked whether that list is empty or not.

So, in the new code I just removed that.

>
> The rest can have reduced identation now.

Yes, the goto does eliminate the extra indentation.

>
>> + 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.

Really? The old code doesn't have '{ }' for else, and checkpatch doesn't
report any error or warning.

Thanks,
Yang

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