Re: [PATCH RFC] mm: entirely reuse the whole anon mTHP in do_wp_page

[David Hildenbrand <david@redhat.com>]

On 31.08.24 11:55, Barry Song wrote:
> On Sat, Aug 31, 2024 at 9:44 PM David Hildenbrand <david@redhat.com> wrote:
>>
>> On 31.08.24 11:23, Barry Song wrote:
>>> From: Barry Song <v-songbaohua@oppo.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.

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.

-- 
Cheers,

David / dhildenb