Date: Tue, 18 Jun 2024 11:40:17 +0800
From: yangge1116 <yangge1116@....com>
To: Barry Song <21cnbao@...il.com>
Cc: akpm@...ux-foundation.org, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, baolin.wang@...ux.alibaba.com,
 liuzixing@...on.cn
Subject: Re: [PATCH] mm/page_alloc: skip THP-sized PCP list when allocating
 non-CMA THP-sized page



在 2024/6/18 上午9:55, Barry Song 写道:
> On Tue, Jun 18, 2024 at 9:36 AM yangge1116 <yangge1116@....com> wrote:
>>
>>
>>
>> 在 2024/6/17 下午8:47, yangge1116 写道:
>>>
>>>
>>> 在 2024/6/17 下午6:26, Barry Song 写道:
>>>> On Tue, Jun 4, 2024 at 9:15 PM <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.
>>>>
>>>> Could you please describe the impact on users in the commit log?
>>>
>>> If a large number of CMA memory are configured in the system (for
>>> example, the CMA memory accounts for 50% of the system memory), starting
>>> virtual machine with device passthrough will get stuck.
>>>
>>> During starting virtual machine, it will call pin_user_pages_remote(...,
>>> FOLL_LONGTERM, ...) to pin memory. If a page is in CMA area,
>>> pin_user_pages_remote() will migrate the page from CMA area to non-CMA
>>> area because of FOLL_LONGTERM flag. If non-movable allocation requests
>>> return CMA memory, pin_user_pages_remote() will enter endless loops.
>>>
>>> backtrace:
>>> pin_user_pages_remote
>>> ----__gup_longterm_locked //cause endless loops in this function
>>> --------__get_user_pages_locked
>>> --------check_and_migrate_movable_pages //always check fail and continue
>>> to migrate
>>> ------------migrate_longterm_unpinnable_pages
>>> ----------------alloc_migration_target // non-movable allocation
>>>
>>>> Is it possible that some CMA memory might be used by non-movable
>>>> allocation requests?
>>>
>>> Yes.
>>>
>>>
>>>> If so, will CMA somehow become unable to migrate, causing cma_alloc()
>>>> to fail?
>>>
>>>
>>> No, it will cause endless loops in __gup_longterm_locked(). If
>>> non-movable allocation requests return CMA memory,
>>> migrate_longterm_unpinnable_pages() will migrate a CMA page to another
>>> CMA page, which is useless and cause endless loops in
>>> __gup_longterm_locked().
> 
> This is only one perspective. We also need to consider the 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
> contiguous 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.
> 
>>>
>>> backtrace:
>>> pin_user_pages_remote
>>> ----__gup_longterm_locked //cause endless loops in this function
>>> --------__get_user_pages_locked
>>> --------check_and_migrate_movable_pages //always check fail and continue
>>> to migrate
>>> ------------migrate_longterm_unpinnable_pages
>>>
>>>
>>>
>>>
>>>
>>>>>
>>>>> 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) {
>>>>> +                       page = rmqueue_pcplist(preferred_zone, zone,
>>>>> order,
>>>>> +                                               migratetype,
>>>>> alloc_flags);
>>>>> +                       if (likely(page))
>>>>> +                               goto out;
>>>>> +               }
>>>>
>>>> This seems not ideal, because non-CMA THP gets no chance to use PCP.
>>>> But it
>>>> still seems better than causing the failure of CMA allocation.
>>>>
>>>> Is there a possible approach to avoiding adding CMA THP into pcp from
>>>> the first
>>>> beginning? Otherwise, we might need a separate PCP for CMA.
>>>>
>>
>> The vast majority of THP-sized allocations are GFP_MOVABLE, avoiding
>> adding CMA THP into pcp may incur a slight performance penalty.
>>
> 
> But the majority of movable pages aren't CMA, right?

Yes.

> Do we have an estimate for
> adding back a CMA THP PCP? Will per_cpu_pages introduce a new cacheline, which
> the original intention for THP was to avoid by having only one PCP[1]?
> 
> [1] https://patchwork.kernel.org/project/linux-mm/patch/20220624125423.6126-3-mgorman@techsingularity.net/
> 
> 
>> Commit 1d91df85f399 takes a similar approach to filter, and I mainly
>> refer to it.
>>
>>
>>>>> +#else
>>>>>                   page = rmqueue_pcplist(preferred_zone, zone, order,
>>>>>                                          migratetype, alloc_flags);
>>>>>                   if (likely(page))
>>>>>                           goto out;
>>>>> +#endif
>>>>>           }
>>>>>
>>>>>           page = rmqueue_buddy(preferred_zone, zone, order, alloc_flags,
>>>>> --
>>>>> 2.7.4
>>>>
>>>> Thanks
>>>> Barry
>>>>
>>
>>

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