Re: [PATCH 2/2] mm/mempolicy: Skip walking HUGETLB vma if MPOL_MF_STRICT is specified alone

["Li Xinhai" <lixinhai.lxh@gmail.com>]

On 2020-01-15 at 13:23 Yang Shi wrote:
>
>
>On 1/14/20 8:28 PM, Mike Kravetz wrote:
>> On 1/14/20 5:24 PM, Yang Shi wrote:
>>>
>>> On 1/14/20 5:07 PM, Mike Kravetz wrote:
>>>> On 1/14/20 6:09 AM, Li Xinhai wrote:
>>>>> Add cc to
>>>>> Yang Shi <yang.shi@linux.alibaba.com>
>>>>> Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
>>>>> , who has been worked on this part
>>>>>
>>>>> On 2020-01-14 at 17:16 Li Xinhai wrote:
>>>>>> Checking MPOL_MF_STRICT is ignored for HUGETLB vma according to mbind man
>>>>>> page:
>>>>>>
>>>>>> Notes
>>>>>> MPOL_MF_STRICT is ignored on huge page mappings.
>>>>>>
>>>>>> If MPOL_MF_STRICT is specified alone without any MOVE flag, we should
>>>>>> indicate, from test_walk, that walking this vma should be skipped even if
>>>>>> there are misplaced pages.
>>>>>>
>>>>>> Signed-off-by: Li Xinhai <lixinhai.lxh@gmail.com>
>>>>>> Cc: Michal Hocko <mhocko@suse.com>
>>>>>> Cc: Mike Kravetz <mike.kravetz@oracle.com>
>>>> I do not necessarily disagree with the change.  However, this has made me
>>>> question a couple things:
>>>> 1) Why does the man page say MPOL_MF_STRICT is ignored on huge page mappings?
>>>>      - Is that leftover from the the days when huge page migration was not
>>>>        supported?
>>>>      - Is it just because huge page migration is more likely to fail than
>>>>        base page migration.
>>>> 2) Does the mbind code function properly when unable to migrate a huge page
>>>>      MPOL_MF_STRICT is set?  A quick look at the code looks like it returns
>>>>      EIO. 
for question (2),
look into queue_pages_hugetlb(), the misplaced page would not
cause -EIO reported, for both STRICT set alone and STRICT set with MOVE*;
that means STRICT been effectively ignored during isolation phase.

In unmap and move phase, -EIO is reported if failed to move page and
STRICT is set.

>>> I don't know the answer about question #1 I didn't dig into the history. The queue_pages_hugetlb() returns 0 unconditionally, I think this is what "MPOL_MF_STRICT is ignored on huge page mappings" means in code.
>>>
>>> It would return -EIO for base pages or THP as what the manpage describes.
>>>
>> I was thinking about a migration failure after isolation.  This block of
>> code in do_mbind() after queue_pages_range() and mbind_range().
>>
>>          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;
>
>Hmm.. I agree this part in man page does look ambiguous. We may assume
>"MPOL_MF_STRICT is ignored on huge page mappings." implies if
>MPOL_MF_STRICT is specified alone? If MOVE flag is specified it should
>return -EIO if some pages could not be moved as what the man page describes.
> 
It looks to me that current code has no feasible way to ignore STRICT
flag for hugetlb page when failure happen in  unmap&move phase,
because mbind is about to handle multiple vma(i.e., hugetlb vma mixed with
other vma) in one call.

>I don't know what the intention was at the first place. We may have to
>dig into the history.
>
>>
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