Re: [PATCH] mm/hugetlb: avoid recursive i_mmap_rwsem in PMD sharing

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

On 10/14/26 08:39, Li Zhe wrote:
> On 10/10/26 2:05 AM, David Hildenbrand (Arm) wrote:
>> On 10/9/26 13:52, Lorenzo Stoakes (ARM) wrote:
>>> Yeah I don't see why not! I don't think there's anything that needs to be
>>> private here.
>> Li, see below, can you work with the below?
>>
>>
>> ----8<----
>>  From 9e1c0676ea6c9cbcabf91cd8badf81a9d88fd3ef Mon Sep 17 00:00:00 2001
>> From: "Lorenzo Stoakes (ARM)" <ljs@kernel.org>
>> Date: Wed, 29 Jul 2026 18:33:08 +0100
>> Subject: [PATCH] fix
>>
>> ---
>>   mm/hugetlb.c | 103 +++++++++++++++++++++++++++++++++++++++------------
>>   1 file changed, 79 insertions(+), 24 deletions(-)
>>
>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
>> index e93c4d2456aa..88ff2c4ad483 100644
>> --- a/mm/hugetlb.c
>> +++ b/mm/hugetlb.c
>> @@ -5077,6 +5077,79 @@ static void move_huge_pte(struct vm_area_struct *vma, unsigned long old_addr,
>>   	spin_unlock(dst_ptl);
>>   }
>>
>> +/* Returns last address processed */
>> +static unsigned long prealloc_or_move_page_tables(struct vm_area_struct *vma,
>> +		struct vm_area_struct *new_vma, unsigned long old_addr,
>> +		unsigned long new_addr, unsigned long len, unsigned long sz,
>> +		unsigned long last_addr_mask, struct mmu_gather *tlb,
>> +		bool is_prealloc)
>> +{
>> +	unsigned long old_end = old_addr + len;
>> +	struct hstate *h = hstate_vma(vma);
>> +	struct mm_struct *mm = vma->vm_mm;
>> +	pte_t *src_pte, *dst_pte;
>> +
>> +	hugetlb_vma_assert_locked(vma);
>> +
>> +	for (; old_addr < old_end; old_addr += sz, new_addr += sz) {
>> +		src_pte = hugetlb_walk(vma, old_addr, sz);
>> +		if (!src_pte) {
>> +			old_addr |= last_addr_mask;
>> +			new_addr |= last_addr_mask;
>> +			continue;
>> +		}
>> +		if (huge_pte_none(huge_ptep_get(mm, old_addr, src_pte)))
>> +			continue;
>> +
>> +		if (is_prealloc) {
>> +			if (!huge_pte_alloc(mm, new_vma, new_addr, sz))
>> +				break;
>> +			continue;
>> +		}
>> +
>> +		if (huge_pmd_unshare(tlb, vma, old_addr, src_pte)) {
>> +			old_addr |= last_addr_mask;
>> +			new_addr |= last_addr_mask;
>> +			continue;
>> +		}
>> +
>> +		dst_pte = hugetlb_walk(new_vma, new_addr, sz);
>> +		if (!dst_pte)
>> +			break;
>> +
>> +		move_huge_pte(vma, old_addr, new_addr, src_pte, dst_pte, sz);
>> +		tlb_remove_huge_tlb_entry(h, tlb, src_pte, old_addr);
>> +	}
>> +
>> +	return old_addr;
>> +}
>> +
>> +/* Returns true if preallocation succeeded across range, otherwise false. */
>> +static bool prealloc_hugetlb_page_tables(struct vm_area_struct *vma,
>> +		struct vm_area_struct *new_vma, unsigned long old_addr,
>> +		unsigned long new_addr, unsigned long len, unsigned long sz,
>> +		unsigned long last_addr_mask)
>> +{
>> +	unsigned long old_end = old_addr + len;
>> +	unsigned long addr_end;
>> +
>> +	addr_end = prealloc_or_move_page_tables(vma, new_vma, old_addr, new_addr,
>> +			len, sz, last_addr_mask, NULL, /*is_prealloc=*/true);
>> +
>> +	return addr_end >= old_end;
>> +}
>> +
>> +/* Returns last processed address. */
>> +static unsigned long __move_hugetlb_page_tables(struct vm_area_struct *vma,
>> +		struct vm_area_struct *new_vma, unsigned long old_addr,
>> +		unsigned long new_addr, unsigned long len, unsigned long sz,
>> +		unsigned long last_addr_mask, struct mmu_gather *tlb)
>> +{
>> +	i_mmap_assert_write_locked(vma->vm_file->f_mapping);
>> +	return prealloc_or_move_page_tables(vma, new_vma, old_addr, new_addr,
>> +			len, sz, last_addr_mask, tlb, /*is_prealloc=*/false);
>> +}
>> +
>>   int move_hugetlb_page_tables(struct vm_area_struct *vma,
>>   			     struct vm_area_struct *new_vma,
>>   			     unsigned long old_addr, unsigned long new_addr,
>> @@ -5088,9 +5161,9 @@ int move_hugetlb_page_tables(struct vm_area_struct *vma,
>>   	struct mm_struct *mm = vma->vm_mm;
>>   	unsigned long old_end = old_addr + len;
>>   	unsigned long last_addr_mask;
>> -	pte_t *src_pte, *dst_pte;
>>   	struct mmu_notifier_range range;
>>   	struct mmu_gather tlb;
>> +	bool preallocated;
>>
>>   	mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, mm, old_addr,
>>   				old_end);
>> @@ -5106,30 +5179,12 @@ int move_hugetlb_page_tables(struct vm_area_struct *vma,
>>   	last_addr_mask = hugetlb_mask_last_page(h);
>>   	/* Prevent race with file truncation */
>>   	hugetlb_vma_lock_write(vma);
>> +	preallocated = prealloc_hugetlb_page_tables(vma, new_vma, old_addr,
>> +			new_addr, len, sz, last_addr_mask);
>>   	i_mmap_lock_write(mapping);
>> -	for (; old_addr < old_end; old_addr += sz, new_addr += sz) {
>> -		src_pte = hugetlb_walk(vma, old_addr, sz);
>> -		if (!src_pte) {
>> -			old_addr |= last_addr_mask;
>> -			new_addr |= last_addr_mask;
>> -			continue;
>> -		}
>> -		if (huge_pte_none(huge_ptep_get(mm, old_addr, src_pte)))
>> -			continue;
>> -
>> -		if (huge_pmd_unshare(&tlb, vma, old_addr, src_pte)) {
>> -			old_addr |= last_addr_mask;
>> -			new_addr |= last_addr_mask;
>> -			continue;
>> -		}
>> -
>> -		dst_pte = huge_pte_alloc(mm, new_vma, new_addr, sz);
>> -		if (!dst_pte)
>> -			break;
>> -
>> -		move_huge_pte(vma, old_addr, new_addr, src_pte, dst_pte, sz);
>> -		tlb_remove_huge_tlb_entry(h, &tlb, src_pte, old_addr);
>> -	}
>> +	if (preallocated)
>> +		old_addr = __move_hugetlb_page_tables(vma, new_vma, old_addr,
>> +			new_addr, len, sz, last_addr_mask, &tlb);
>>
>>   	tlb_flush_mmu_tlbonly(&tlb);
>>   	huge_pmd_unshare_flush(&tlb, vma);
>> --
>> 2.55.0
> 
> Hi David,
> 
> I thought more about the case where the destination page table was
> preallocated as a shared PMD.
> 
> I do not think the current move_huge_pte() handles that case very well. The
> reason is that it assumes the destination PTE is not part of a shared PMD
> page table: it clears the source PTE and then installs that PTE into the
> destination. That works when the destination PMD page table is not shared,
> because the source removal and destination install have a net-zero effect on
> rmap and hugetlb accounting.
> 
> If the destination PTE belongs to a shared PMD, that assumption does not
> hold. In that case, I think we should leave the shared destination page
> table untouched and treat the operation as a source-side unmap. Otherwise
> move_huge_pte() either writes through a shared page table or skips the
> source-side unmap accounting. That can make the rmap, hugetlb count and
> folio release handling inconsistent with the actual PTE update.
> 
> So I think the change should be roughly:

IIUC, what you are saying is:

A) If the source table is shared, unshare it.

b) If the destination table is shared, there is nothing to do.

?

-- 
Cheers,

David