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>
>> ---
>> 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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