Re: [PATCH v5 07/12] khugepaged: add mTHP support
From: David Hildenbrand
Date: Fri May 02 2025 - 11:19:11 EST
On 02.05.25 14:50, Jann Horn wrote:
On Fri, May 2, 2025 at 8:29 AM David Hildenbrand <david@xxxxxxxxxx> wrote:
On 02.05.25 00:29, Nico Pache wrote:
On Wed, Apr 30, 2025 at 2:53 PM Jann Horn <jannh@xxxxxxxxxx> wrote:
On Mon, Apr 28, 2025 at 8:12 PM Nico Pache <npache@xxxxxxxxxx> 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).
Why would you need to touch the PMD entry to prevent GUP-fast races for mTHP?
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 see it, you're not "keeping" the current locking behavior; you're
making a big implicit locking change by reusing a codepath designed
for PMD THP for mTHP, where the page table may not be exclusively
owned by one VMA.
That is not the intention. The intention in this series (at least as we
discussed) was to not do it across VMAs; that is considered the next
logical step (which will be especially relevant on arm64 IMHO).
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.
Right, the simplest approach on top of the current PMD collapse is to do
exactly what we do in the PMD case, including the locking: which
apparently is no completely the same yet :).
Instead of installing a PMD THP, we modify the page table and remap that.
Moving from the PMD lock to the PTE lock will not make a big change in
practice for most cases: we already must disable essentially all page
table walkers (vma lock, mmap lock in write, rmap lock in write).
The PMDP clear+flush is primarily to disable the last possible set of
page table walkers: (1) HW modifications and (2) GUP-fast.
So after the PMDP clear+flush we know that (A) HW can not modify the
pages concurrently and (B) GUP-fast cannot succeed anymore.
The issue with PTEP clear+flush is that we will have to remember all PTE
values, to reset them if anything goes wrong. Using a single PMD value
is arguably simpler. And then, the benefit vs. complexity is unclear.
Certainly something to look into later, but not a requirement for the
first support,
As I understand, one rule we currently have in MM is that an operation
that logically operates on one VMA (VMA 1) does not touch the page
tables of other VMAs (VMA 2) in any way, except that it may walk page
tables that cover address space that intersects with both VMA 1 and
VMA 2, and create such page tables if they are missing.
Yes, absolutely. That must not happen. And I think I raised it as a
problem in reply to one of Dev's series.
If this series does not rely on that it must be fixed.
This proposed patch changes that, without explicitly discussing this
locking change.
Yes, that must not happen. We must not zap a PMD to temporarily replace
it with a pmd_none() entry if any other sane page table walker could
stumble over it.
This includes another VMA that is not write-locked that could span the PMD.
--
Cheers,
David / dhildenb