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

[Qi Zheng <qi.zheng@linux.dev>]

On 11/1/26 2:56 PM, Muchun Song wrote:
> 
> 
>> 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)

LGTM, with this change:

Reviewed-by: Qi Zheng <qi.zheng@linux.dev>

Thanks,
Qi