Re: [PATCH] mm: huge_memory: a new debugfs interface for splitting THP tests.

[David Hildenbrand]

On 08.03.21 20:11, Yang Shi wrote:
> On Mon, Mar 8, 2021 at 11:01 AM Zi Yan <ziy@xxxxxxxxxx> wrote:
> > On 8 Mar 2021, at 13:11, David Hildenbrand wrote:
> >
> > > On 08.03.21 18:49, Zi Yan wrote:
> > > > On 8 Mar 2021, at 11:17, David Hildenbrand wrote:
> > > >
> > > > > On 08.03.21 16:22, Zi Yan wrote:
> > > > > > From: Zi Yan <ziy@xxxxxxxxxx>
> > > > > >
> > > > > > By writing "<pid>,<vaddr_start>,<vaddr_end>" to
> > > > > > <debugfs>/split_huge_pages_in_range_pid, THPs in the process with the
> > > > > > given pid and virtual address range are split. It is used to test
> > > > > > split_huge_page function. In addition, a selftest program is added to
> > > > > > tools/testing/selftests/vm to utilize the interface by splitting
> > > > > > PMD THPs and PTE-mapped THPs.
> > > > >
> > > > > Won't something like
> > > > >
> > > > > 1. MADV_HUGEPAGE
> > > > >
> > > > > 2. Access memory
> > > > >
> > > > > 3. MADV_NOHUGEPAGE
> > > > >
> > > > > Have a similar effect? What's the benefit of this?
> > > >
> > > > Thanks for checking the patch.
> > > >
> > > > No, MADV_NOHUGEPAGE just replaces VM_HUGEPAGE with VM_NOHUGEPAGE,
> > > > nothing else will be done.
> > >
> > > Ah, okay - maybe my memory was tricking me. There is some s390x KVM code that forces MADV_NOHUGEPAGE and force-splits everything.
> > >
> > > I do wonder, though, if this functionality would be worth a proper user interface (e.g., madvise), though. There might be actual benefit in having this as a !debug interface.
> > >
> > > I think you aware of the discussion in https://lkml.kernel.org/r/d098c392-273a-36a4-1a29-59731cdf5d3d@xxxxxxxxxx
> >
> > Yes. Thanks for bringing this up.
> >
> > > If there will be an interface to collapse a THP -- "this memory area is worth extra performance now by collapsing a THP if possible" -- it might also be helpful to have the opposite functionality -- "this memory area is not worth a THP, rather use that somehwere else".
> > >
> > > MADV_HUGE_COLLAPSE vs. MADV_HUGE_SPLIT
> >
> > I agree that MADV_HUGE_SPLIT would be useful as the opposite of COLLAPSE when user might just want PAGESIZE mappings.
> > Right now, HUGE_SPLIT is implicit from mapping changes like mprotect or MADV_DONTNEED.
>
> IMHO, it sounds not very useful. MADV_DONTNEED would split PMD for any
> partial THP. If the range covers the whole THP, the whole THP is going
> to be freed anyway. All other places in kernel which need split THP
> have been covered. So I didn't realize any usecase from userspace for
> just splitting PMD to PTEs.

THP are a limited resource. So indicating which virtual memory regions 
are not performance sensitive right now (e.g., cold pages in a databse) 
and not worth a THP might be quite valuable, no?

--
Thanks,

David / dhildenb