Re: [PATCH 2/4] mm: compaction: convert to folio_isolate_movable()

[Kefeng Wang <wangkefeng.wang@huawei.com>]

On 2024/8/28 18:04, Baolin Wang wrote:
> Hi Kefeng,
> 
> On 2024/8/26 12:01, Kefeng Wang wrote:
>> The tail page will always fail to isolate for non-lru-movable and
>> LRU page in isolate_migratepages_block(), so move PageTail check
>> ahead, then convert to use folio_isolate_movable() helper and more
>> folios.
>>
>> Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
>> ---
>>   mm/compaction.c | 32 +++++++++++++++++---------------
>>   1 file changed, 17 insertions(+), 15 deletions(-)
>>
>> diff --git a/mm/compaction.c b/mm/compaction.c
>> index a2b16b08cbbf..aa2e8bb9fa58 100644
>> --- a/mm/compaction.c
>> +++ b/mm/compaction.c
>> @@ -1074,39 +1074,41 @@ isolate_migratepages_block(struct 
>> compact_control *cc, unsigned long low_pfn,
>>               }
>>           }
>> +        if (PageTail(page))
>> +            goto isolate_fail;
>> +
>> +        folio = page_folio(page);
> 
> I wonder if this is stable. Before page_folio(), it does not hold a 
> reference on the page, so seems we should re-check the folio still 
> contains this page after gaining a reference on the folio?

Oh, you are right, so two way to avoid this,

1) re-check 'page_folio(page)  == folio', but this need change a little 
more.

2)  directly use folio_get_nontail_page(page) here, and folio_put in the 
following path, this will try to get for any pages, but it should be 
accepted ?

I'd prefer 2) but any other suggestion?

Thanks.

> 
>>           /*
>>            * Check may be lockless but that's ok as we recheck later.
>> -         * It's possible to migrate LRU and non-lru movable pages.
>> -         * Skip any other type of page
>> +         * It's possible to migrate LRU and non-lru movable folioss.
>> +         * Skip any other type of folios.
>>            */
>> -        if (!PageLRU(page)) {
>> +        if (!folio_test_lru(folio)) {
>>               /*
>> -             * __PageMovable can return false positive so we need
>> -             * to verify it under page_lock.
>> +             * __folio_test_movable can return false positive so
>> +             * we need to verify it under page_lock.
>>                */
>> -            if (unlikely(__PageMovable(page)) &&
>> -                    !PageIsolated(page)) {
>> +            if (unlikely(__folio_test_movable(folio)) &&
>> +                    !folio_test_isolated(folio)) {
>>                   if (locked) {
>>                       unlock_page_lruvec_irqrestore(locked, flags);
>>                       locked = NULL;
>>                   }
>> -                if (isolate_movable_page(page, mode)) {
>> -                    folio = page_folio(page);
>> +                if (folio_isolate_movable(folio, mode))
>>                       goto isolate_success;
>> -                }
>>               }
>>               goto isolate_fail;
>>           }
>>           /*
>> -         * Be careful not to clear PageLRU until after we're
>> -         * sure the page is not being freed elsewhere -- the
>> -         * page release code relies on it.
>> +         * Be careful not to clear lru flag until after we're
>> +         * sure the folio is not being freed elsewhere -- the
>> +         * folio release code relies on it.
>>            */
>> -        folio = folio_get_nontail_page(page);
>> -        if (unlikely(!folio))
>> +        if (unlikely(folio_try_get(folio)))
>>               goto isolate_fail;
>>           /*