Re: [PATCH v5 07/12] khugepaged: add mTHP support

[Nico Pache <npache@redhat.com>]

On Wed, Apr 30, 2025 at 2:53 PM Jann Horn <jannh@google.com> wrote:
>
> On Mon, Apr 28, 2025 at 8:12 PM Nico Pache <npache@redhat.com> wrote:
> > Introduce the ability for khugepaged to collapse to different mTHP sizes.
> > While scanning PMD ranges for potential collapse candidates, keep track
> > of pages in KHUGEPAGED_MIN_MTHP_ORDER chunks via a bitmap. Each bit
> > represents a utilized region of order KHUGEPAGED_MIN_MTHP_ORDER ptes. If
> > mTHPs are enabled we remove the restriction of max_ptes_none during the
> > scan phase so we dont bailout early and miss potential mTHP candidates.
> >
> > After the scan is complete we will perform binary recursion on the
> > bitmap to determine which mTHP size would be most efficient to collapse
> > to. max_ptes_none will be scaled by the attempted collapse order to
> > determine how full a THP must be to be eligible.
> >
> > If a mTHP collapse is attempted, but contains swapped out, or shared
> > pages, we dont perform the collapse.
> [...]
> > @@ -1208,11 +1211,12 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
> >         vma_start_write(vma);
> >         anon_vma_lock_write(vma->anon_vma);
> >
> > -       mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, mm, address,
> > -                               address + HPAGE_PMD_SIZE);
> > +       mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, mm, _address,
> > +                               _address + (PAGE_SIZE << order));
> >         mmu_notifier_invalidate_range_start(&range);
> >
> >         pmd_ptl = pmd_lock(mm, pmd); /* probably unnecessary */
> > +
> >         /*
> >          * This removes any huge TLB entry from the CPU so we won't allow
> >          * huge and small TLB entries for the same virtual address to
>
> It's not visible in this diff, but we're about to do a
> pmdp_collapse_flush() here. pmdp_collapse_flush() tears down the
> entire page table, meaning it tears down 2MiB of address space; and it
> assumes that the entire page table exclusively corresponds to the
> current VMA.
>
> I think you'll need to ensure that the pmdp_collapse_flush() only
> happens for full-size THP, and that mTHP only tears down individual
> PTEs in the relevant range. (That code might get a bit messy, since
> the existing THP code tears down PTEs in a detached page table, while
> mTHP would have to do it in a still-attached page table.)
Hi Jann!

I was under the impression that this is needed to prevent GUP-fast
races (and potentially others).
As you state here, conceptually the PMD case is, detach the PMD, do
the collapse, then reinstall the PMD (similarly to how the system
recovers from a failed PMD collapse). I tried to keep the current
locking behavior as it seemed the easiest way to get it right (and not
break anything). So I keep the PMD detaching and reinstalling for the
mTHP case too. As Hugh points out I am releasing the anon lock too
early. I will comment further on his response.

As I familiarize myself with the code more, I do see potential code
improvements/cleanups and locking improvements, but I was going to
leave those to a later series.

Thanks
-- Nico
>