Re: [PATCH v2 03/12] mm: thp: introduce separate TTU flag for thp freezing

[Anshuman Khandual]

On 11/10/2016 02:39 PM, Naoya Horiguchi wrote:
> 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)

Right.

> 
> and ttu_flag given for tail pages is:
> 
>     (TTU_IGNORE_MLOCK | TTU_IGNORE_ACCESS | TTU_RMAP_LOCKED | \
>      TTU_MIGRATION)

Right.

> 
> __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.)

Got it.

> 
> 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.

Sure, thanks for the explanation.