On Sat, Aug 31, 2024 at 10:29 PM David Hildenbrand <david@xxxxxxxxxx> wrote:
>
> On 31.08.24 12:21, Barry Song wrote:
> > On Sat, Aug 31, 2024 at 10:07 PM David Hildenbrand <david@xxxxxxxxxx> wrote:
> >>
> >> On 31.08.24 11:55, Barry Song wrote:
> >>> On Sat, Aug 31, 2024 at 9:44 PM David Hildenbrand <david@xxxxxxxxxx> wrote:
> >>>>
> >>>> On 31.08.24 11:23, Barry Song wrote:
> >>>>> From: Barry Song <v-songbaohua@xxxxxxxx>
> >>>>>
> >>>>> 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@xxxxxxxxxx
> >>>
> >>> 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.
>
I see. maybe we can have something like:
static bool pte_fault_around_estimate(int nr)
{
if (nr / arch_batched_ptes_nr() < 16)
return true;
return false;
}
if (pte_fault_around_estimate(folio_nr_pages(folio)))
set all ptes;
for arm64, arch_batched_ptes_nr() == 16. for
arch without cont-pte or similar things,
arch_batched_ptes_nr() == 1.
Just some rough ideas; all the naming might be quite messy.
at least, we won't lose the benefit of reduced TLB miss
before all nr_pages are written for aarch64 :-)
> >
> > 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.
>
i see, i assume it won't be worse than the current case where we have to
allocate small folios and copy? and folio allocation can even further incur
direct reclamation?
> --
> Cheers,
>
> David / dhildenb
>
Thanks
Barry
