linux-kernel - Re: [PATCH RFC] mm: entirely reuse the whole anon mTHP in do_wp

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Message-ID: <36933711-ae0f-468c-93bd-d6a67d974c9d@redhat.com>
Date: Sat, 31 Aug 2024 12:29:11 +0200
From: David Hildenbrand <david@...hat.com>
To: Barry Song <21cnbao@...il.com>
Cc: akpm@...ux-foundation.org, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, Barry Song <v-songbaohua@...o.com>,
 Chuanhua Han <hanchuanhua@...o.com>,
 Baolin Wang <baolin.wang@...ux.alibaba.com>,
 Ryan Roberts <ryan.roberts@....com>, Zi Yan <ziy@...dia.com>,
 Chris Li <chrisl@...nel.org>, Kairui Song <kasong@...cent.com>,
 Kalesh Singh <kaleshsingh@...gle.com>, Suren Baghdasaryan <surenb@...gle.com>
Subject: Re: [PATCH RFC] mm: entirely reuse the whole anon mTHP in do_wp_page

On 31.08.24 12:21, Barry Song wrote:
> On Sat, Aug 31, 2024 at 10:07 PM David Hildenbrand <david@...hat.com> wrote:
>>
>> On 31.08.24 11:55, Barry Song wrote:
>>> On Sat, Aug 31, 2024 at 9:44 PM David Hildenbrand <david@...hat.com> wrote:
>>>>
>>>> On 31.08.24 11:23, Barry Song wrote:
>>>>> From: Barry Song <v-songbaohua@...o.com>
>>>>>
>>>>> On a physical phone, it's sometimes observed that deferred_split
>>>>> mTHPs account for over 15% of the total mTHPs. Profiling by Chuanhua
>>>>> indicates that the majority of these originate from the typical fork
>>>>> scenario.
>>>>> When the child process either execs or exits, the parent process should
>>>>> ideally be able to reuse the entire mTHP. However, the current kernel
>>>>> lacks this capability and instead places the mTHP into split_deferred,
>>>>> performing a CoW (Copy-on-Write) on just a single subpage of the mTHP.
>>>>>
>>>>>     main()
>>>>>     {
>>>>>     #define SIZE 1024 * 1024UL
>>>>>             void *p = malloc(SIZE);
>>>>>             memset(p, 0x11, SIZE);
>>>>>             if (fork() == 0)
>>>>>                     exec(....);
>>>>>            /*
>>>>>          * this will trigger cow one subpage from
>>>>>          * mTHP and put mTHP into split_deferred
>>>>>          * list
>>>>>          */
>>>>>         *(int *)(p + 10) = 10;
>>>>>         printf("done\n");
>>>>>         while(1);
>>>>>     }
>>>>>
>>>>> This leads to two significant issues:
>>>>>
>>>>> * Memory Waste: Before the mTHP is fully split by the shrinker,
>>>>> it wastes memory. In extreme cases, such as with a 64KB mTHP,
>>>>> the memory usage could be 64KB + 60KB until the last subpage
>>>>> is written, at which point the mTHP is freed.
>>>>>
>>>>> * Fragmentation and Performance Loss: It destroys large folios
>>>>> (negating the performance benefits of CONT-PTE) and fragments memory.
>>>>>
>>>>> To address this, we should aim to reuse the entire mTHP in such cases.
>>>>>
>>>>> Hi David,
>>>>>
>>>>> I’ve renamed wp_page_reuse() to wp_folio_reuse() and added an
>>>>> entirely_reuse argument because I’m not sure if there are still cases
>>>>> where we reuse a subpage within an mTHP. For now, I’m setting
>>>>> entirely_reuse to true only for the newly supported case, while all
>>>>> other cases still get false. Please let me know if this is incorrect—if
>>>>> we don’t reuse subpages at all, we could remove the argument.
>>>>
>>>> See [1] I sent out this week, that is able to reuse even without
>>>> scanning page tables. If we find the the folio is exclusive we could try
>>>> processing surrounding PTEs that map the same folio.
>>>>
>>>> [1] https://lkml.kernel.org/r/20240829165627.2256514-1-david@redhat.com
>>>
>>> Great! It looks like I missed your patch again. Since you've implemented this
>>> in a better way, I’d prefer to use your patchset.
>>
>> I wouldn't say better, just more universally. And while taking care of
>> properly sync'ing the mapcount vs. refcount :P
>>
>>>
>>> I’m curious about how you're handling ptep_set_access_flags_nr() or similar
>>> things because I couldn’t find the related code in your patch 10/17:
>>>
>>> [PATCH v1 10/17] mm: COW reuse support for PTE-mapped THP with CONFIG_MM_ID
>>>
>>> Am I missing something?
>>
>> The idea is to keep individual write faults as fast as possible. So the
>> patch set keeps it simple and only reuses a single PTE at a time,
>> setting that one PAE and mapping it writable.
> 
> I got your point, thanks! as anyway the mTHP has been exclusive,
> so the following nr-1 minor page faults will set their particular PTE
> to writable one by one.

Yes, assuming you would get these page faults, and assuming you would 
get them in the near future.

> 
>>
>> As the patch states, it might be reasonable to optimize some cases,
>> maybe also only on some architectures. For example to fault-around and
>> map the other ones writable as well. It might not always be desirable
>> though, especially not for larger folios.
> 
> as anyway, the mTHP has been entirely exclusive, setting all PTEs
> directly to writable should help reduce nr - 1 minor page faults and
> ideally help reduce CONTPTE unfold and fold?

Yes, doing that on CONTPTE granularity would very likely make sense. For 
anything bigger than that, I am not sure.

Assuming we have a 1M folio mapped by PTEs. Trying to fault-around in 
aligned CONTPTE granularity likely makes sense. Bigger than that, I am 
not convinced.

> 
> What is the downside to doing that? I also don't think mapping them
> all together will waste memory?

No, it's all about increasing the latency of individual write faults.

-- 
Cheers,

David / dhildenb