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

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

On Wed, Mar 31, 2010 at 2:26 PM, KAMEZAWA Hiroyuki
<kamezawa.hiroyu@jp.fujitsu.com> wrote:
> On Tue, 30 Mar 2010 10:14:49 +0100
> Mel Gorman <mel@csn.ul.ie> wrote:
>
>> PageAnon pages that are unmapped may or may not have an anon_vma so
>> are not currently migrated. However, a swap cache page can be migrated
>> and fits this description. This patch identifies page swap caches and
>> allows them to be migrated.
>>
>
> Some comments.
>
>> Signed-off-by: Mel Gorman <mel@csn.ul.ie>
>> ---
>>  mm/migrate.c |   15 ++++++++++-----
>>  mm/rmap.c    |    6 ++++--
>>  2 files changed, 14 insertions(+), 7 deletions(-)
>>
>> diff --git a/mm/migrate.c b/mm/migrate.c
>> index 35aad2a..f9bf37e 100644
>> --- a/mm/migrate.c
>> +++ b/mm/migrate.c
>> @@ -203,6 +203,9 @@ static int migrate_page_move_mapping(struct address_space *mapping,
>>       void **pslot;
>>
>>       if (!mapping) {
>> +             if (PageSwapCache(page))
>> +                     SetPageSwapCache(newpage);
>> +
>
> Migration of SwapCache requires radix-tree replacement, IOW,
>  mapping == NULL && PageSwapCache is BUG.
>
> So, this never happens.
>
>
>>               /* Anonymous page without mapping */
>>               if (page_count(page) != 1)
>>                       return -EAGAIN;
>> @@ -607,11 +610,13 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
>>                * the page was isolated and when we reached here while
>>                * the RCU lock was not held
>>                */
>> -             if (!page_mapped(page))
>> -                     goto rcu_unlock;
>> -
>> -             anon_vma = page_anon_vma(page);
>> -             atomic_inc(&anon_vma->external_refcount);
>> +             if (!page_mapped(page)) {
>> +                     if (!PageSwapCache(page))
>> +                             goto rcu_unlock;
>> +             } else {
>> +                     anon_vma = page_anon_vma(page);
>> +                     atomic_inc(&anon_vma->external_refcount);
>> +             }
>>       }
>>
>>       /*
>> 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?

>
> Thanks,
> -Kame
>
>



-- 
Kind regards,
Minchan Kim

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