Re: [PATCH v2 02/12] mm: hugetlb_vmemmap: unify VMEMMAP_{SPLIT,REMAP}_NO_TLB_FLUSH

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

> On Nov 2, 2026, at 18:18, Mike Rapoport <rppt@kernel.org> wrote:
> 
> Hi Muchun,

Hi,

I didn't realized that I'm not using a plain text in my last mail.
It’s my email client's fault. Let me email it again.

> 
>> 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_DEFER_TLB_FLUSH flag. The
>> new name reflects that only safely batchable flushes are deferred. 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>
>> Reviewed-by: Lance Yang <lance.yang@linux.dev>
>> Reviewed-by: Qi Zheng <qi.zheng@linux.dev>
>> ---
>> v2:
>> - Rename the unified flag to VMEMMAP_DEFER_TLB_FLUSH and update its
>>  comment (suggested by Qi Zheng)
>> - Update the commit message for the new flag name
>> - Collect Reviewed-by from Lance Yang and Qi Zheng
>> ---
>> mm/hugetlb_vmemmap.c | 23 +++++++++++++----------
>> 1 file changed, 13 insertions(+), 10 deletions(-)
>> 
>> diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
>> index 4b73d6ec8dcea..63e899cfaac3a 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)
>> +/* Defer safely batchable TLB flushes to the caller. */
>> +#define VMEMMAP_DEFER_TLB_FLUSH BIT(0)
>> 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.
> 
> Are we certain that *all* modern MMUs for *all* architecures that 
> support hugetlb are fine with skipping TLB flush after a split?

I think this is the case on architectures that currently enable
ARCH_WANT_OPTIMIZE_HUGETLB_VMEMMAP. Of course, the point I want to
further clarify is that the "skip" action here is not really a skip,
but rather a "defer"—the TLB is subsequently flushed in a single batch.

Thanks for your review.

Muchun.

> 
> AFAIU, x86::set_memory always flushes TLBs after a large leaf is split.
> 
> -- 
> Sincerely yours,
> Mike.