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.
I would like to drop PageHead() check for Hugetlb to keep seem behavior,
which will keep seem error code if isolate fail or success on head/tail
page.
Thanks.
BTW - hugetlb pages not migrated due to passing a tail page does not
seem to contribute to a 'Positive return value' indicating the number of
non-migrated pages.
