On Thu, Nov 10, 2016 at 02:01:56PM +0530, Anshuman Khandual wrote:
> On 11/08/2016 05:01 AM, Naoya Horiguchi wrote:
> > TTU_MIGRATION is used to convert pte into migration entry until thp split
> > completes. This behavior conflicts with thp migration added later patches,
>
> Hmm, could you please explain why it conflicts with the PMD based
> migration without split ? Why TTU_MIGRATION cannot be used to
> freeze/hold on the PMD while it's being migrated ?
try_to_unmap() is used both for thp split (via freeze_page()) and page
migration (via __unmap_and_move()). In freeze_page(), ttu_flag given for
head page is like below (assuming anonymous thp):
(TTU_IGNORE_MLOCK | TTU_IGNORE_ACCESS | TTU_RMAP_LOCKED | \
TTU_MIGRATION | TTU_SPLIT_HUGE_PMD)
and ttu_flag given for tail pages is:
(TTU_IGNORE_MLOCK | TTU_IGNORE_ACCESS | TTU_RMAP_LOCKED | \
TTU_MIGRATION)
__unmap_and_move() calls try_to_unmap() with ttu_flag:
(TTU_MIGRATION | TTU_IGNORE_MLOCK | TTU_IGNORE_ACCESS)
Now I'm trying to insert a branch for thp migration at the top of
try_to_unmap_one() like below
static int try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
unsigned long address, void *arg)
{
...
if (flags & TTU_MIGRATION) {
if (!PageHuge(page) && PageTransCompound(page)) {
set_pmd_migration_entry(page, vma, address);
goto out;
}
}
, so try_to_unmap() for tail pages called by thp split can go into thp
migration code path (which converts *pmd* into migration entry), while
the expectation is to freeze thp (which converts *pte* into migration entry.)
I detected this failure as a "bad page state" error in a testcase where
split_huge_page() is called from queue_pages_pte_range().
Anyway, I'll add this explanation into the patch description in the next post.
Thanks,
Naoya Horiguchi
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