Re: [PATCH 14/14] mm,migration: Allow the migration of PageSwapCache pages

[Minchan Kim <minchan.kim@gmail.com>]

On Thu, Apr 1, 2010 at 6:30 PM, Mel Gorman <mel@csn.ul.ie> wrote:
> On Thu, Apr 01, 2010 at 01:44:29PM +0900, Minchan Kim wrote:
>> On Thu, Apr 1, 2010 at 12:01 PM, KAMEZAWA Hiroyuki
>> <kamezawa.hiroyu@jp.fujitsu.com> wrote:
>> > On Thu, 1 Apr 2010 11:43:18 +0900
>> > Minchan Kim <minchan.kim@gmail.com> wrote:
>> >
>> >> On Wed, Mar 31, 2010 at 2:26 PM, KAMEZAWA Hiroyuki       /*
>> >> >> diff --git a/mm/rmap.c b/mm/rmap.c
>> >> >> index af35b75..d5ea1f2 100644
>> >> >> --- a/mm/rmap.c
>> >> >> +++ b/mm/rmap.c
>> >> >> @@ -1394,9 +1394,11 @@ int rmap_walk(struct page *page, int (*rmap_one)(struct page *,
>> >> >>
>> >> >>       if (unlikely(PageKsm(page)))
>> >> >>               return rmap_walk_ksm(page, rmap_one, arg);
>> >> >> -     else if (PageAnon(page))
>> >> >> +     else if (PageAnon(page)) {
>> >> >> +             if (PageSwapCache(page))
>> >> >> +                     return SWAP_AGAIN;
>> >> >>               return rmap_walk_anon(page, rmap_one, arg);
>> >> >
>> >> > SwapCache has a condition as (PageSwapCache(page) && page_mapped(page) == true.
>> >> >
>> >>
>> >> In case of tmpfs, page has swapcache but not mapped.
>> >>
>> >> > Please see do_swap_page(), PageSwapCache bit is cleared only when
>> >> >
>> >> > do_swap_page()...
>> >> >       swap_free(entry);
>> >> >        if (vm_swap_full() || (vma->vm_flags & VM_LOCKED) || PageMlocked(page))
>> >> >                try_to_free_swap(page);
>> >> >
>> >> > Then, PageSwapCache is cleared only when swap is freeable even if mapped.
>> >> >
>> >> > rmap_walk_anon() should be called and the check is not necessary.
>> >>
>> >> Frankly speaking, I don't understand what is Mel's problem, why he added
>> >> Swapcache check in rmap_walk, and why do you said we don't need it.
>> >>
>> >> Could you explain more detail if you don't mind?
>> >>
>> > I may miss something.
>> >
>> > unmap_and_move()
>> >  1. try_to_unmap(TTU_MIGRATION)
>> >  2. move_to_newpage
>> >  3. remove_migration_ptes
>> >        -> rmap_walk()
>> >
>> > Then, to map a page back we unmapped we call rmap_walk().
>> >
>> > Assume a SwapCache which is mapped, then, PageAnon(page) == true.
>> >
>> >  At 1. try_to_unmap() will rewrite pte with swp_entry of SwapCache.
>> >       mapcount goes to 0.
>> >  At 2. SwapCache is copied to a new page.
>> >  At 3. The new page is mapped back to the place. Now, newpage's mapcount is 0.
>> >       Before patch, the new page is mapped back to all ptes.
>> >       After patch, the new page is not mapped back because its mapcount is 0.
>> >
>> > I don't think shared SwapCache of anon is not an usual behavior, so, the logic
>> > before patch is more attractive.
>> >
>> > If SwapCache is not mapped before "1", we skip "1" and rmap_walk will do nothing
>> > because page->mapping is NULL.
>> >
>>
>> Thanks. I agree. We don't need the check.
>> Then, my question is why Mel added the check in rmap_walk.
>> He mentioned some BUG trigger and fixed things after this patch.
>> What's it?
>
> If I remove the check for (PageSwapCache(page) && !page_mapped(page))
> in rmap_walk(), then the bug below occurs. The first one is lockdep going
> bad because it's accessing a bad lock implying that anon_vma->lock is
> already invalid. The bug that triggers after it is the list walk.

Thanks. I think it's possible. It's subtle problem.
Assume !page_mapped  && PageAnon(page)  && PageSwapCache

0. PageAnon check
1. race window <---- anon_vma free!!!!
2. rcu_read_lock()
3. skip_unmap
4. move_to_new_page
5. newpage->mapping = page->mapping <--- !!!! It's invalid
6.     mapping->a_ops->migratepage
7.         radix tree change, copy page (still new page anon is NULL)
8.     remove_migrate_ptes
9.     rmap_walk
10.         PageAnon is true --> we are deceived.
11.         rmap_walk_anon -> go bomb!

Does it make sense?
-- 
Kind regards,
Minchan Kim

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