Date: Tue, 18 Jun 2024 11:31:55 +0800
From: yangge1116 <yangge1116@....com>
To: Barry Song <21cnbao@...il.com>,
 Baolin Wang <baolin.wang@...ux.alibaba.com>
Cc: akpm@...ux-foundation.org, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, liuzixing@...on.cn,
 Johannes Weiner <hannes@...xchg.org>, Vlastimil Babka <vbabka@...e.cz>,
 Zi Yan <ziy@...dia.com>
Subject: Re: [PATCH] mm/page_alloc: skip THP-sized PCP list when allocating
 non-CMA THP-sized page



在 2024/6/17 下午7:55, Barry Song 写道:
> On Mon, Jun 17, 2024 at 7:36 PM Baolin Wang
> <baolin.wang@...ux.alibaba.com> wrote:
>>
>>
>>
>> On 2024/6/17 18:43, Barry Song wrote:
>>> On Thu, Jun 6, 2024 at 3:07 PM Baolin Wang
>>> <baolin.wang@...ux.alibaba.com> wrote:
>>>>
>>>>
>>>>
>>>> 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@....com wrote:
>>>>>>> From: yangge <yangge1116@....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@....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
>>>
>>> This doesn't seem ideal either. It's possible that the PCP still has many
>>> non-CMA folios, but due to bad luck, the first entry is "always" CMA.
>>> In this case,
>>> allocations with is_migrate_movable(migratetype) == false will always lose the
>>> chance to use the PCP.   It also appears to incur a PCP spin lock/unlock.
>>
>> Yes, just some ideas to to mitigate the issue...
>>
>>>
>>> I don't see an ideal solution unless we bring back the CMA PCP :-)
>>
>> Tend to agree, and adding a CMA PCP seems the overhead can be acceptable?
> 
> yes. probably. Hi Ge,
> 
> Could we printk the size before and after adding 1 to NR_PCP_LISTS?
> Does it increase one cacheline?
> 
> struct per_cpu_pages {
> spinlock_t lock; /* Protects lists field */
> int count; /* number of pages in the list */
> int high; /* high watermark, emptying needed */
> int high_min; /* min high watermark */
> int high_max; /* max high watermark */
> int batch; /* chunk size for buddy add/remove */
> u8 flags; /* protected by pcp->lock */
> u8 alloc_factor; /* batch scaling factor during allocate */
> #ifdef CONFIG_NUMA
> u8 expire; /* When 0, remote pagesets are drained */
> #endif
> short free_count; /* consecutive free count */
> 
> /* Lists of pages, one per migrate type stored on the pcp-lists */
> struct list_head lists[NR_PCP_LISTS];
> } ____cacheline_aligned_in_smp;
> 

OK.

The size of struct per_cpu_pages is 256 bytes in current code containing 
commit 5d0a661d808f ("mm/page_alloc: use only one PCP list for THP-sized 
allocations").
crash> struct per_cpu_pages
struct per_cpu_pages {
     spinlock_t lock;
     int count;
     int high;
     int high_min;
     int high_max;
     int batch;
     u8 flags;
     u8 alloc_factor;
     u8 expire;
     short free_count;
     struct list_head lists[13];
}
SIZE: 256

After revert commit 5d0a661d808f ("mm/page_alloc: use only one PCP list 
for THP-sized allocations"), the size of struct per_cpu_pages is 272 bytes.
crash> struct per_cpu_pages
struct per_cpu_pages {
     spinlock_t lock;
     int count;
     int high;
     int high_min;
     int high_max;
     int batch;
     u8 flags;
     u8 alloc_factor;
     u8 expire;
     short free_count;
     struct list_head lists[15];
}
SIZE: 272

Seems commit 5d0a661d808f ("mm/page_alloc: use only one PCP list for 
THP-sized allocations") decrease one cacheline.

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