Re: [PATCH v2 4/4] mm/hugetlb: fix excessive IPI broadcasts when unsharing PMD tables using mmu_gather

["David Hildenbrand (Red Hat)" <david@kernel.org>]

On 12/19/25 14:52, David Hildenbrand (Red Hat) wrote:
> On 12/19/25 13:37, Harry Yoo wrote:
>> On Fri, Dec 12, 2025 at 08:10:19AM +0100, David Hildenbrand (Red Hat) wrote:
>>> As reported, ever since commit 1013af4f585f ("mm/hugetlb: fix
>>> huge_pmd_unshare() vs GUP-fast race") we can end up in some situations
>>> where we perform so many IPI broadcasts when unsharing hugetlb PMD page
>>> tables that it severely regresses some workloads.
>>>
>>> In particular, when we fork()+exit(), or when we munmap() a large
>>> area backed by many shared PMD tables, we perform one IPI broadcast per
>>> unshared PMD table.
>>>
>>
>> [...snip...]
>>
>>> Fixes: 1013af4f585f ("mm/hugetlb: fix huge_pmd_unshare() vs GUP-fast race")
>>> Reported-by: Uschakow, Stanislav" <suschako@amazon.de>
>>> Closes: https://lore.kernel.org/all/4d3878531c76479d9f8ca9789dc6485d@amazon.de/
>>> Tested-by: Laurence Oberman <loberman@redhat.com>
>>> Cc: <stable@vger.kernel.org>
>>> Signed-off-by: David Hildenbrand (Red Hat) <david@kernel.org>
>>> ---
>>>    include/asm-generic/tlb.h |  74 ++++++++++++++++++++++-
>>>    include/linux/hugetlb.h   |  19 +++---
>>>    mm/hugetlb.c              | 121 ++++++++++++++++++++++----------------
>>>    mm/mmu_gather.c           |   7 +++
>>>    mm/mprotect.c             |   2 +-
>>>    mm/rmap.c                 |  25 +++++---
>>>    6 files changed, 179 insertions(+), 69 deletions(-)
>>>
>>> @@ -6522,22 +6511,16 @@ long hugetlb_change_protection(struct vm_area_struct *vma,
>>>    				pte = huge_pte_clear_uffd_wp(pte);
>>>    			huge_ptep_modify_prot_commit(vma, address, ptep, old_pte, pte);
>>>    			pages++;
>>> +			tlb_remove_huge_tlb_entry(h, tlb, ptep, address);
>>>    		}
>>>    
>>>    next:
>>>    		spin_unlock(ptl);
>>>    		cond_resched();
>>>    	}
>>> -	/*
>>> -	 * There is nothing protecting a previously-shared page table that we
>>> -	 * unshared through huge_pmd_unshare() from getting freed after we
>>> -	 * release i_mmap_rwsem, so flush the TLB now. If huge_pmd_unshare()
>>> -	 * succeeded, flush the range corresponding to the pud.
>>> -	 */
>>> -	if (shared_pmd)
>>> -		flush_hugetlb_tlb_range(vma, range.start, range.end);
>>> -	else
>>> -		flush_hugetlb_tlb_range(vma, start, end);
>>> +
>>> +	tlb_flush_mmu_tlbonly(tlb);
>>> +	huge_pmd_unshare_flush(tlb, vma);
>>
>> Shouldn't we teach mmu_gather that it has to call
> 
> I hope not :) In the worst case we could keep the
> flush_hugetlb_tlb_range() in the !shared case in. Suboptimal but I am
> sick and tired of dealing with this hugetlb mess.
> 
> 
> Let me CC Ryan and Catalin for the arm64 pieces and Christophe on the
> ppc pieces: See [1] where we convert away from some
> flush_hugetlb_tlb_range() users to operate on mmu_gather using
> * tlb_remove_huge_tlb_entry() for mremap() and mprotect(). Before we
>     would only use it in __unmap_hugepage_range().
> * tlb_flush_pmd_range() for unsharing of shared PMD tables. We already
>     used that in one call path.

To clarify, powerpc does not select ARCH_WANT_HUGE_PMD_SHARE, so the 
second change does not apply to ppc.

-- 
Cheers

David