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

[Minchan Kim]

On Wed, Mar 31, 2010 at 2:26 PM, KAMEZAWA Hiroyuki
<kamezawa.hiroyu@xxxxxxxxxxxxxx> wrote:
> On Tue, 30 Mar 2010 10:14:49 +0100
> Mel Gorman <mel@xxxxxxxxx> 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@xxxxxxxxx>
>> ---
>> Âmm/migrate.c | Â 15 ++++++++++-----
>> Âmm/rmap.c  Â|  Â6 ++++--
>> Â2 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,
>> Â Â Â void **pslot;
>>
>> Â Â Â if (!mapping) {
>> + Â Â Â Â Â Â if (PageSwapCache(page))
>> + Â Â Â Â Â Â Â Â Â Â SetPageSwapCache(newpage);
>> +
>
> Migration of SwapCache requires radix-tree replacement, IOW,
> Âmapping == NULL && PageSwapCache is BUG.
>
> So, this never happens.
>
>
>> Â Â Â Â Â Â Â /* Anonymous page without mapping */
>> Â Â Â Â Â Â Â if (page_count(page) != 1)
>> Â Â Â Â Â Â Â Â Â Â Â return -EAGAIN;
>> @@ -607,11 +610,13 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
>> Â Â Â Â Â Â Â Â* the page was isolated and when we reached here while
>> Â Â Â Â Â Â Â Â* the RCU lock was not held
>> Â Â Â Â Â Â Â Â*/
>> - Â Â Â Â Â Â if (!page_mapped(page))
>> - Â Â Â Â Â Â Â Â Â Â goto rcu_unlock;
>> -
>> - Â Â Â Â Â Â anon_vma = page_anon_vma(page);
>> - Â Â Â Â Â Â atomic_inc(&anon_vma->external_refcount);
>> + Â Â Â Â Â Â if (!page_mapped(page)) {
>> + Â Â Â Â Â Â Â Â Â Â if (!PageSwapCache(page))
>> + Â Â Â Â Â Â Â Â Â Â Â Â Â Â goto rcu_unlock;
>> + Â Â Â Â Â Â } else {
>> + Â Â Â Â Â Â Â Â Â Â anon_vma = page_anon_vma(page);
>> + Â Â Â Â Â Â Â Â Â Â atomic_inc(&anon_vma->external_refcount);
>> + Â Â Â Â Â Â }
>> Â Â Â }
>>
>> Â Â Â /*
>> 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 *,
>>
>> Â Â Â if (unlikely(PageKsm(page)))
>> Â Â Â Â Â Â Â return rmap_walk_ksm(page, rmap_one, arg);
>> - Â Â else if (PageAnon(page))
>> + Â Â else if (PageAnon(page)) {
>> + Â Â Â Â Â Â if (PageSwapCache(page))
>> + Â Â Â Â Â Â Â Â Â Â return SWAP_AGAIN;
>> Â Â Â Â Â Â Â 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()...
> Â Â Â swap_free(entry);
> Â Â Â Âif (vm_swap_full() || (vma->vm_flags & VM_LOCKED) || PageMlocked(page))
> Â Â Â Â Â Â Â Âtry_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
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/