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

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

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?
Is it really related to this logic?
I don't think so or we are missing something.

-- 
Kind regards,
Minchan Kim

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