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

[Mike Kravetz <mike.kravetz@oracle.com>]

On 08/15/23 11:58, Huang, Ying wrote:
> Mike Kravetz <mike.kravetz@oracle.com> 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.
> 

Correct.

Since the goal of this series is to convert to folios, we should maintain the
existing behavior and errno (-EACCES).  In a subsequent patch, we can
change behavior.

That would be my suggestion.
-- 
Mike Kravetz