Re: [PATCH 1/4] mm: migrate: use a folio in add_page_for_migration()

[Huang, Ying]

Mike Kravetz <mike.kravetz@xxxxxxxxxx> writes:

> On 08/10/23 09:49, Kefeng Wang wrote:
>> 
>> 
>> On 2023/8/10 6:44, Mike Kravetz wrote:
>> > On 08/09/23 13:53, Mike Kravetz wrote:
>> > > On 08/09/23 20:37, Kefeng Wang wrote:
>> > > > > 
>> > > > > Cc Mike to help us clarify the expected behavior of hugetlb.
>> > > > > 
>> > > > > Hi Mike, what is the expected behavior, if a user tries to use move_pages()
>> > > > > to migrate a non head page of a hugetlb page?
>> > > > 
>> > > > Could you give some advise, thanks
>> > > > 
>> > > 
>> > > Sorry, I was away for a while.
>> > > 
>> > > It seems unfortunate that move_pages says the passed user addresses
>> > > should be aligned to page boundaries.  However, IIUC this is not checked
>> > > or enforced.  Otherwise, passing a hugetlb page should return the same
>> > > error.
>> > > 
>> > > One thought would be that hugetlb mappings should behave the same
>> > > non-hugetlb mappings.  If passed the address of a hugetlb tail page, align
>> > > the address to a hugetlb boundary and migrate the page.  This changes the
>> > > existing behavior.  However, it would be hard to imagine anyone depending
>> > > on this.
>> > > 
>> > > After taking a closer look at the add_page_for_migration(), it seems to
>> > > just ignore passed tail pages and do nothing for such passed addresses.
>> > > Correct?  Or, am I missing something?  Perhaps that is behavior we want/
>> > > need to preserve?
>> > 
>> > My mistake, status -EACCES is returned when passing a tail page of a
>> > hugetlb page.
>> > 
>> 
>> As mentioned in previous mail， before e66f17ff7177 ("mm/hugetlb: take
>> page table lock in follow_huge_pmd()") in v4.0, follow_page() will
>> return NULL on tail page for Huagetlb page, so move_pages() will return
>> -ENOENT errno, but after that commit, -EACCES is returned.
>> 
>> Meanwhile, the behavior of THP/HUGETLB is different, the whole THP will be
>> migrated on a tail page, but HUGETLB will return -EACCES(after v4.0)
>> or -ENOENT(before v4.0) on tail page.
>> 
>> > Back to the question of 'What is the expected behavior if a tail page is
>> > passed?'.  I do not think we have defined an expected behavior.  If
>> > anything is 'expected' I would say it is -EACCES as returned today.
>> > 
>> 
>> My question is,
>> 
>> Should we keep seem behavior between HUGETLB and THP, or only change the
>> errno from -EACCES to -ENOENT/-EBUSY.
>
> Just to be clear.  When you say "keep seem behavior between HUGETLB and THP",
> are you saying that you would like hugetlb to perform migration of the entire
> hugetlb page if a tail page is passed?
>
> IMO, this would be ideal as it would mean that hugetlb and THP behave the same
> when passed the address of a tail page.  The fewer places where hugetlb
> behavior diverges, the better.  However, this does change behavior.

A separate patch will be needed for behavior change.

> As mentioned above, I have a hard time imagining someone depending on the
> behavior that passing the address of a hugetlb tail page returns error.  But,
> this is almost impossible to predict.
>
> Thoughts from others?  

--
Best Regards,
Huang, Ying