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@xxxxxxxxxxxxxxxxx>
>>>>> Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
>>>>> , 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@xxxxxxxxx>
>>>>>> Cc: Michal Hocko <mhocko@xxxxxxxx>
>>>>>> Cc: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
>>>> 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.
>
>>
>
>
