Re: [PATCH v8 02/14] mm/gup: check every subpage of a compound page during isolation

[Joao Martins]

On 2/3/21 2:51 PM, Pavel Tatashin wrote:
> On Wed, Feb 3, 2021 at 8:23 AM Joao Martins <joao.m.martins@xxxxxxxxxx> wrote:
>>
>> On 1/25/21 7:47 PM, Pavel Tatashin wrote:
>>> When pages are isolated in check_and_migrate_movable_pages() we skip
>>> compound number of pages at a time. However, as Jason noted, it is
>>> not necessary correct that pages[i] corresponds to the pages that
>>> we skipped. This is because it is possible that the addresses in
>>> this range had split_huge_pmd()/split_huge_pud(), and these functions
>>> do not update the compound page metadata.
>>>
>>> The problem can be reproduced if something like this occurs:
>>>
>>> 1. User faulted huge pages.
>>> 2. split_huge_pmd() was called for some reason
>>> 3. User has unmapped some sub-pages in the range
>>> 4. User tries to longterm pin the addresses.
>>>
>>> The resulting pages[i] might end-up having pages which are not compound
>>> size page aligned.
>>>
>>> Fixes: aa712399c1e8 ("mm/gup: speed up check_and_migrate_cma_pages() on huge page")
>>> Reported-by: Jason Gunthorpe <jgg@xxxxxxxxxx>
>>> Signed-off-by: Pavel Tatashin <pasha.tatashin@xxxxxxxxxx>
>>> Reviewed-by: Jason Gunthorpe <jgg@xxxxxxxxxx>
>>> ---
>>
>> [...]
>>
>>>               /*
>>>                * If we get a page from the CMA zone, since we are going to
>>>                * be pinning these entries, we might as well move them out
>>> @@ -1599,8 +1596,6 @@ static long check_and_migrate_cma_pages(struct mm_struct *mm,
>>>                               }
>>>                       }
>>>               }
>>> -
>>> -             i += step;
>>>       }
>>>
>>
> 
> Hi Joao,
> 
>> With this, longterm gup will 'regress' for hugetlbfs e.g. from ~6k -> ~32k usecs when
>> pinning a 16G hugetlb file.
> 
> Estimate or you actually measured?
> 
It's what I had measured before sending. The ~ is because there's error variance.

>>
> 
>> Splitting can only occur on THP right? If so, perhaps we could retain the @step increment
> 
> Yes, I do not think we can split HugePage, only THP.
> 
Right, that's my impression too.

>> for compound pages but when !is_transparent_hugepage(head) or just PageHuge(head) like:
>>
>> +               if (!is_transparent_hugepage(head) && PageCompound(page))
>> +                       i += (compound_nr(head) - (pages[i] - head));
>>
>> Or making specific to hugetlbfs:
>>
>> +               if (PageHuge(head))
>> +                       i += (compound_nr(head) - (pages[i] - head));
> 
> Yes, this is reasonable optimization. I will submit a follow up patch
> against linux-next.