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

[Qi Zheng]

On 11/1/26 2:56 PM, Muchun Song wrote:
> > On Nov 1, 2026, at 14:22, Qi Zheng <qi.zheng@xxxxxxxxx> 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@xxxxxxxxxxxxx>
> > > ---
> > >   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@xxxxxxxxx>

Thanks,
Qi