Re: [PATCH 02/12] mm: hugetlb_vmemmap: unify VMEMMAP_{SPLIT,REMAP}_NO_TLB_FLUSH

[Muchun Song <muchun.song@linux.dev>]

> On Nov 1, 2026, at 14:22, Qi Zheng <qi.zheng@linux.dev> wrote:
> 
> 
> 
> On 10/20/26 7:58 PM, Muchun Song wrote:
>> VMEMMAP_SPLIT_NO_TLB_FLUSH and VMEMMAP_REMAP_NO_TLB_FLUSH do not
>> describe fully independent choices. A split-only walk can defer the
>> PMD-split flush to a later global flush, and a remap-only walk can
>> defer the final remap flush. However, when a PMD split is followed
>> by PTE remapping, the split flush must happen before the PTEs are
>> changed, because stale large-page translations must not overlap with
>> new small-page translations that map different physical pages.
>> Replace the two flags with a single VMEMMAP_REMAP_NO_TLB_FLUSH flag.
>> Keep split-only walks able to defer the flush, but force the split
>> flush whenever a remap_pte callback is present. Callers that batch
>> HVO work can still defer the final remap flush and issue their own
>> global flush.
>> Current callers keep the same flushing behavior.
>> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
>> ---
>>  mm/hugetlb_vmemmap.c | 15 +++++++++------
>>  1 file changed, 9 insertions(+), 6 deletions(-)
>> diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
>> index 1105aa925acc..8b3abb562599 100644
>> --- a/mm/hugetlb_vmemmap.c
>> +++ b/mm/hugetlb_vmemmap.c
>> @@ -42,10 +42,8 @@ struct vmemmap_remap_walk {
>>  	struct list_head	*vmemmap_pages;
>>    -/* Skip the TLB flush when we split the PMD */
>> -#define VMEMMAP_SPLIT_NO_TLB_FLUSH	BIT(0)
>> -/* Skip the TLB flush when we remap the PTE */
>> -#define VMEMMAP_REMAP_NO_TLB_FLUSH	BIT(1)
>> +/* Skip the TLB flush while updating the vmemmap page tables. */
>> +#define VMEMMAP_REMAP_NO_TLB_FLUSH	BIT(0)
> 
> Since this flag is now also used by the split-only path, the
> VMEMMAP_REMAP_NO_TLB_FLUSH name is somewhat misleading. Moreover, the
> flag does not suppress every TLB flush, as the PMD-split flush is still
> mandatory when remapping follows.

Yeah, good point.

> 
> Would VMEMMAP_DEFER_TLB_FLUSH better describe its semantics? It

I think so.

> indicates that flushes which can be safely batched are deferred, without
> implying that all TLB flushes are skipped.

I'll change it along side with the comment to:

  /* Defer safely batchable TLB flushes to the caller. */
  #define VMEMMAP_DEFER_TLB_FLUSH       BIT(0)

Thanks.

> 
> Thanks,
> Qi
> 
>>  	unsigned long		flags;
>>  };
>>  @@ -85,7 +83,12 @@ static int vmemmap_split_pmd(pmd_t *pmd, struct page *head, unsigned long start,
>>  		/* Make pte visible before pmd. See comment in pmd_install(). */
>>  		smp_wmb();
>>  		pmd_populate_kernel(&init_mm, pmd, pgtable);
>> -		if (!(walk->flags & VMEMMAP_SPLIT_NO_TLB_FLUSH))
>> +		/*
>> +		 * Modern MMUs allow overlapping TLB entries with different mapping
>> +		 * granularities only if they map the same physical address. So
>> +		 * remapping must not proceed without first flushing the TLB.
>> +		 */
>> +		if (walk->remap_pte || !(walk->flags & VMEMMAP_REMAP_NO_TLB_FLUSH))
>>  			flush_tlb_kernel_range(start, start + PMD_SIZE);
>>  	} else {
>>  		pte_free_kernel(&init_mm, pgtable);
>> @@ -272,7 +275,7 @@ static int vmemmap_remap_split(unsigned long start, unsigned long end)
>>  {
>>  	struct vmemmap_remap_walk walk = {
>>  		.remap_pte	= NULL,
>> -		.flags		= VMEMMAP_SPLIT_NO_TLB_FLUSH,
>> +		.flags		= VMEMMAP_REMAP_NO_TLB_FLUSH,
>>  	};
>>    	return vmemmap_remap_range(start, end, &walk);
>