Re: [PATCH RFC 3/3] mm/khugepaged: skip redundant IPI in collapse_huge_page()

[Lance Yang <lance.yang@linux.dev>]

On 2025/12/19 16:25, David Hildenbrand (Red Hat) wrote:
> On 12/18/25 15:35, Lance Yang wrote:
>>
>>
>> On 2025/12/18 21:13, David Hildenbrand (Red Hat) wrote:
>>> On 12/13/25 09:00, Lance Yang wrote:
>>>> From: Lance Yang <lance.yang@linux.dev>
>>>>
>>>> Similar to the hugetlb PMD unsharing optimization, skip the second IPI
>>>> in collapse_huge_page() when the TLB flush already provides necessary
>>>> synchronization.
>>>>
>>>> Before commit a37259732a7d ("x86/mm: Make MMU_GATHER_RCU_TABLE_FREE
>>>> unconditional"), bare metal x86 didn't enable 
>>>> MMU_GATHER_RCU_TABLE_FREE.
>>>> In that configuration, tlb_remove_table_sync_one() was a NOP. GUP-fast
>>>> synchronization relied on IRQ disabling, which blocks TLB flush IPIs.
>>>>
>>>> When Rik made MMU_GATHER_RCU_TABLE_FREE unconditional to support AMD's
>>>> INVLPGB, all x86 systems started sending the second IPI. However, on
>>>> native x86 this is redundant:
>>>>
>>>>     - pmdp_collapse_flush() calls flush_tlb_range(), sending IPIs to 
>>>> all
>>>>       CPUs to invalidate TLB entries
>>>>
>>>>     - GUP-fast runs with IRQs disabled, so when the flush IPI 
>>>> completes,
>>>>       any concurrent GUP-fast must have finished
>>>>
>>>>     - tlb_remove_table_sync_one() provides no additional 
>>>> synchronization
>>>>
>>>> On x86, skip the second IPI when running native (without paravirt) and
>>>> without INVLPGB. For paravirt with non-native flush_tlb_multi and for
>>>> INVLPGB, conservatively keep both IPIs.
>>>>
>>>> Use tlb_table_flush_implies_ipi_broadcast(), consistent with the 
>>>> hugetlb
>>>> optimization.
>>>>
>>>> Suggested-by: David Hildenbrand (Red Hat) <david@kernel.org>
>>>> Signed-off-by: Lance Yang <lance.yang@linux.dev>
>>>> ---
>>>>    mm/khugepaged.c | 7 ++++++-
>>>>    1 file changed, 6 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>>>> index 97d1b2824386..06ea793a8190 100644
>>>> --- a/mm/khugepaged.c
>>>> +++ b/mm/khugepaged.c
>>>> @@ -1178,7 +1178,12 @@ static int collapse_huge_page(struct mm_struct
>>>> *mm, unsigned long address,
>>>>        _pmd = pmdp_collapse_flush(vma, address, pmd);
>>>>        spin_unlock(pmd_ptl);
>>>>        mmu_notifier_invalidate_range_end(&range);
>>>> -    tlb_remove_table_sync_one();
>>>> +    /*
>>>> +     * Skip the second IPI if the TLB flush above already synchronized
>>>> +     * with concurrent GUP-fast via broadcast IPIs.
>>>> +     */
>>>> +    if (!tlb_table_flush_implies_ipi_broadcast())
>>>> +        tlb_remove_table_sync_one();
>>>
>>> We end up calling
>>>
>>>       flush_tlb_range(vma, address, address + HPAGE_PMD_SIZE);
>>>
>>>       -> flush_tlb_mm_range(freed_tables = true)
>>>
>>>       -> flush_tlb_multi(mm_cpumask(mm), info);
>>>
>>> So freed_tables=true and we should be doing the right thing.
>>
>> Yep ;)
>>
>>> BTW, I was wondering whether we should embed that
>>> tlb_table_flush_implies_ipi_broadcast() check in
>>> tlb_remove_table_sync_one() instead.
>>> It then relies on the caller to do the right thing (flush with
>>> freed_tables=true or unshared_tables = true).
>>>
>>> Thoughts?
>>
>> Good point! Let me check the other callers to ensure they
>> are all preceded by a flush with freed_tables=true (or unshared_tables).
>>
>> Will get back to you with what I find :)
> 
> The use case in tlb_table_flush() is a bit confusing. But I would assume 
> that we have a TLB flush with remove_tables=true beforehand. Otherwise 
> we cannot possibly free the page table.

Right! I assume you meant freed_tables=true (not remove_tables) ;)

Verified all callers have proper TLB flushes *beforehand*:

-> 1. mm/khugepaged.c:1188 (collapse_huge_page)

	pmdp_collapse_flush(vma, address, pmd)
	-> flush_tlb_range(vma, address, address + HPAGE_PMD_SIZE)
		-> flush_tlb_mm_range(mm, ..., freed_tables = true)
			-> flush_tlb_multi(mm_cpumask(mm), info)

So freed_tables=true and we should be doing the right thing :)

-> 2. include/asm-generic/tlb.h:861 (tlb_flush_unshared_tables)

	tlb_flush_mmu_tlbonly(tlb)
	-> tlb_flush(tlb)
		-> flush_tlb_mm_range(mm, ..., unshared_tables = true)
			-> flush_tlb_multi(mm_cpumask(mm), info)

unshared_tables=true (equivalent to freed_tables for sending IPIs).

-> 3. mm/mmu_gather.c:341 (__tlb_remove_table_one)

When we can't allocate a batch page in tlb_remove_table(), we do:

	tlb_table_invalidate(tlb)
	-> tlb_flush_mmu_tlbonly(tlb)
		-> flush_tlb_mm_range(mm, ..., freed_tables = true)
			-> flush_tlb_multi(mm_cpumask(mm), info)

	Then:
	tlb_remove_table_one(table)
	-> __tlb_remove_table_one(table) // if !CONFIG_PT_RECLAIM
		-> tlb_remove_table_sync_one()

freed_tables=true, and this should work too.

Why is tlb->freed_tables guaranteed? Because callers like pte_free_tlb()
(via free_pte_range) set freed_tables=true before calling __pte_free_tlb(),
which then calls tlb_remove_table(). As you mentioned, we cannot free page
tables without freed_tables=true.

Note that tlb_remove_table_sync_one() was a NOP on bare metal x86
(CONFIG_MMU_GATHER_RCU_TABLE_FREE=n) before commit a37259732a7d.

-> 4-5. mm/khugepaged.c:1683,1819 (pmdp_get_lockless_sync macro)

Same as #1.

So all callers satisfy the requirement! Will embed the check in v2.

Hopefully I didn't miss any callers ;)

Cheers,
Lance