Re: [PATCH] mm/page_alloc: skip THP-sized PCP list when allocating non-CMA THP-sized page

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 2024/6/4 20:36, yangge1116 wrote:
> 
> 
> 在 2024/6/4 下午8:01, Baolin Wang 写道:
>> Cc Johannes, Zi and Vlastimil.
>>
>> On 2024/6/4 17:14, yangge1116@126.com wrote:
>>> From: yangge <yangge1116@126.com>
>>>
>>> Since commit 5d0a661d808f ("mm/page_alloc: use only one PCP list for
>>> THP-sized allocations") no longer differentiates the migration type
>>> of pages in THP-sized PCP list, it's possible to get a CMA page from
>>> the list, in some cases, it's not acceptable, for example, allocating
>>> a non-CMA page with PF_MEMALLOC_PIN flag returns a CMA page.
>>>
>>> The patch forbids allocating non-CMA THP-sized page from THP-sized
>>> PCP list to avoid the issue above.
>>>
>>> Fixes: 5d0a661d808f ("mm/page_alloc: use only one PCP list for 
>>> THP-sized allocations")
>>> Signed-off-by: yangge <yangge1116@126.com>
>>> ---
>>>   mm/page_alloc.c | 10 ++++++++++
>>>   1 file changed, 10 insertions(+)
>>>
>>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>>> index 2e22ce5..0bdf471 100644
>>> --- a/mm/page_alloc.c
>>> +++ b/mm/page_alloc.c
>>> @@ -2987,10 +2987,20 @@ struct page *rmqueue(struct zone 
>>> *preferred_zone,
>>>       WARN_ON_ONCE((gfp_flags & __GFP_NOFAIL) && (order > 1));
>>>       if (likely(pcp_allowed_order(order))) {
>>> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
>>> +        if (!IS_ENABLED(CONFIG_CMA) || alloc_flags & ALLOC_CMA ||
>>> +                        order != HPAGE_PMD_ORDER) {
>>
>> Seems you will also miss the non-CMA THP from the PCP, so I wonder if 
>> we can add a migratetype comparison in __rmqueue_pcplist(), and if 
>> it's not suitable, then fallback to buddy?
> 
> Yes, we may miss some non-CMA THPs in the PCP. But, if add a migratetype 
> comparison in __rmqueue_pcplist(), we may need to compare many times 
> because of pcp batch.

I mean we can only compare once, focusing on CMA pages.

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 3734fe7e67c0..960a3b5744d8 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -2973,6 +2973,11 @@ struct page *__rmqueue_pcplist(struct zone *zone, 
unsigned int order,
                 }

                 page = list_first_entry(list, struct page, pcp_list);
+#ifdef CONFIG_TRANSPARENT_HUGEPAGE
+               if (order == HPAGE_PMD_ORDER && 
!is_migrate_movable(migratetype) &&
+                   is_migrate_cma(get_pageblock_migratetype(page)))
+                       return NULL;
+#endif
                 list_del(&page->pcp_list);
                 pcp->count -= 1 << order;
         } while (check_new_pages(page, order));