Re: [PATCH 3/6] mm: free zapped tail pages when splitting isolated thp

Usama Arif
2024-08-04T19:02:53Z

On 30/07/2024 16:14, David Hildenbrand wrote:
> On 30.07.24 14:46, Usama Arif wrote:
>> From: Yu Zhao <yuzhao@google.com>
>>
>> If a tail page has only two references left, one inherited from the
>> isolation of its head and the other from lru_add_page_tail() which we
>> are about to drop, it means this tail page was concurrently zapped.
>> Then we can safely free it and save page reclaim or migration the
>> trouble of trying it.
>>
>> Signed-off-by: Yu Zhao <yuzhao@google.com>
>> Tested-by: Shuang Zhai <zhais@google.com>
>> Signed-off-by: Usama Arif <usamaarif642@gmail.com>
>> ---
>>   mm/huge_memory.c | 26 ++++++++++++++++++++++++++
>>   1 file changed, 26 insertions(+)
>>
>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>> index 0167dc27e365..76a3b6a2b796 100644
>> --- a/mm/huge_memory.c
>> +++ b/mm/huge_memory.c
>> @@ -2923,6 +2923,8 @@ static void __split_huge_page(struct page *page, struct list_head *list,
>>       unsigned int new_nr = 1 << new_order;
>>       int order = folio_order(folio);
>>       unsigned int nr = 1 << order;
>> +    LIST_HEAD(pages_to_free);
>> +    int nr_pages_to_free = 0;
>>         /* complete memcg works before add pages to LRU */
>>       split_page_memcg(head, order, new_order);
>> @@ -3007,6 +3009,24 @@ static void __split_huge_page(struct page *page, struct list_head *list,
>>           if (subpage == page)
>>               continue;
>>           folio_unlock(new_folio);
>> +        /*
>> +         * If a tail page has only two references left, one inherited
>> +         * from the isolation of its head and the other from
>> +         * lru_add_page_tail() which we are about to drop, it means this
>> +         * tail page was concurrently zapped. Then we can safely free it
>> +         * and save page reclaim or migration the trouble of trying it.
>> +         */
>> +        if (list && page_ref_freeze(subpage, 2)) {
>> +            VM_BUG_ON_PAGE(PageLRU(subpage), subpage);
>> +            VM_BUG_ON_PAGE(PageCompound(subpage), subpage);
>> +            VM_BUG_ON_PAGE(page_mapped(subpage), subpage);
>> +
>
> No VM_BUG_*, VM_WARN is good enough.
>
>> +            ClearPageActive(subpage);
>> +            ClearPageUnevictable(subpage);
>> +            list_move(&subpage->lru, &pages_to_free);
>
> Most checks here should operate on new_folio instead of subpage.
>
>
Do you mean instead of doing the PageLRU, PageCompound and page_mapped check on the subpage, there should be checks on new_folio?
If new_folio is a large folio, then it could be that only some of the subpages were zapped?

Could do below if subpage makes sense

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 3305e6d0b90e..abfcd4b7cbba 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -3041,9 +3041,9 @@ static void __split_huge_page(struct page *page, struct list_head *list,
* and save page reclaim or migration the trouble of trying it.
*/
if (list && page_ref_freeze(subpage, 2)) {
- VM_BUG_ON_PAGE(PageLRU(subpage), subpage);
- VM_BUG_ON_PAGE(PageCompound(subpage), subpage);
- VM_BUG_ON_PAGE(page_mapped(subpage), subpage);
+ VM_WARN_ON_ONCE_PAGE(PageLRU(subpage), subpage);
+ VM_WARN_ON_ONCE_PAGE(PageCompound(subpage), subpage);
+ VM_WARN_ON_ONCE_PAGE(page_mapped(subpage), subpage);

ClearPageActive(subpage);
ClearPageUnevictable(subpage);