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

[Barry Song <21cnbao@gmail.com>]

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’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?

>
> --
> Cheers,
>
> David / dhildenb
>

Thanks
Barry