linux-kernel - Re: [PATCH] mm/page_alloc: add one PCP list for THP

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Date: Thu, 20 Jun 2024 08:33:53 +0800
From: Ge Yang <yangge1116@....com>
To: Barry Song <21cnbao@...il.com>
Cc: akpm@...ux-foundation.org, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, stable@...r.kernel.org,
 baolin.wang@...ux.alibaba.com, mgorman@...hsingularity.net,
 liuzixing@...on.cn
Subject: Re: [PATCH] mm/page_alloc: add one PCP list for THP



在 2024/6/20 6:28, Barry Song 写道:
> On Thu, Jun 20, 2024 at 12:55 AM <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 that non-movable
>> allocation requests may get a CMA page from the list, in some cases,
>> it's not acceptable.
>>
>> If a large number of CMA memory are configured in system (for
>> example, the CMA memory accounts for 50% of the system memory),
>> starting a virtual machine with device passthrough will get stuck.
>> During starting the virtual machine, it will call
>> pin_user_pages_remote(..., FOLL_LONGTERM, ...) to pin memory. Normally
>> if a page is present and in CMA area, pin_user_pages_remote() will
>> migrate the page from CMA area to non-CMA area because of
>> FOLL_LONGTERM flag. But if non-movable allocation requests return
>> CMA memory, migrate_longterm_unpinnable_pages() will migrate a CMA
>> page to another CMA page, which will fail to pass the check in
>> check_and_migrate_movable_pages() and cause migration endless.
>> Call trace:
>> pin_user_pages_remote
>> --__gup_longterm_locked // endless loops in this function
>> ----_get_user_pages_locked
>> ----check_and_migrate_movable_pages
>> ------migrate_longterm_unpinnable_pages
>> --------alloc_migration_target
>>
>> This problem will also have a negative impact on CMA itself. For
>> example, when CMA is borrowed by THP, and we need to reclaim it
>> through cma_alloc() or dma_alloc_coherent(), we must move those
>> pages out to ensure CMA's users can retrieve that contigous memory.
>> Currently, CMA's memory is occupied by non-movable pages, meaning
>> we can't relocate them. As a result, cma_alloc() is more likely to
>> fail.
>>
>> To fix the problem above, we add one PCP list for THP, which will
>> not introduce a new cacheline for struct per_cpu_pages. THP will
>> have 2 PCP lists, one PCP list is used by MOVABLE allocation, and
>> the other PCP list is used by UNMOVABLE allocation. MOVABLE
>> allocation contains GPF_MOVABLE, and UNMOVABLE allocation contains
>> GFP_UNMOVABLE and GFP_RECLAIMABLE.
>>
>> Fixes: 5d0a661d808f ("mm/page_alloc: use only one PCP list for THP-sized allocations")
> 
> Please add the below tag
> 
> Cc: <stable@...r.kernel.org>
> 
> And I don't think 'mm/page_alloc: add one PCP list for THP' is a good
> title. Maybe:
> 
> 'mm/page_alloc: Separate THP PCP into movable and non-movable categories'
> 
> Whenever you send a new version, please add things like 'PATCH V2', 'PATCH V3'.
> You have already missed several version numbers, so we may have to start from V2
> though V2 is wrong.
> 

Ok, thanks.

>> Signed-off-by: yangge <yangge1116@....com>
>> ---
>>   include/linux/mmzone.h | 9 ++++-----
>>   mm/page_alloc.c        | 9 +++++++--
>>   2 files changed, 11 insertions(+), 7 deletions(-)
>>
>> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
>> index b7546dd..cb7f265 100644
>> --- a/include/linux/mmzone.h
>> +++ b/include/linux/mmzone.h
>> @@ -656,13 +656,12 @@ enum zone_watermarks {
>>   };
>>
>>   /*
>> - * One per migratetype for each PAGE_ALLOC_COSTLY_ORDER. One additional list
>> - * for THP which will usually be GFP_MOVABLE. Even if it is another type,
>> - * it should not contribute to serious fragmentation causing THP allocation
>> - * failures.
>> + * One per migratetype for each PAGE_ALLOC_COSTLY_ORDER. Two additional lists
>> + * are added for THP. One PCP list is used by GPF_MOVABLE, and the other PCP list
>> + * is used by GFP_UNMOVABLE and GFP_RECLAIMABLE.
>>    */
>>   #ifdef CONFIG_TRANSPARENT_HUGEPAGE
>> -#define NR_PCP_THP 1
>> +#define NR_PCP_THP 2
>>   #else
>>   #define NR_PCP_THP 0
>>   #endif
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index 8f416a0..0a837e6 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -504,10 +504,15 @@ static void bad_page(struct page *page, const char *reason)
>>
>>   static inline unsigned int order_to_pindex(int migratetype, int order)
>>   {
>> +       bool __maybe_unused movable;
>> +
>>   #ifdef CONFIG_TRANSPARENT_HUGEPAGE
>>          if (order > PAGE_ALLOC_COSTLY_ORDER) {
>>                  VM_BUG_ON(order != HPAGE_PMD_ORDER);
>> -               return NR_LOWORDER_PCP_LISTS;
>> +
>> +               movable = migratetype == MIGRATE_MOVABLE;
>> +
>> +               return NR_LOWORDER_PCP_LISTS + movable;
>>          }
>>   #else
>>          VM_BUG_ON(order > PAGE_ALLOC_COSTLY_ORDER);
>> @@ -521,7 +526,7 @@ static inline int pindex_to_order(unsigned int pindex)
>>          int order = pindex / MIGRATE_PCPTYPES;
>>
>>   #ifdef CONFIG_TRANSPARENT_HUGEPAGE
>> -       if (pindex == NR_LOWORDER_PCP_LISTS)
>> +       if (pindex >= NR_LOWORDER_PCP_LISTS)
>>                  order = HPAGE_PMD_ORDER;
>>   #else
>>          VM_BUG_ON(order > PAGE_ALLOC_COSTLY_ORDER);
>> --
>> 2.7.4
>>