Subject: Re: [PATCH] arm64: tlb: Flush walk cache when unsharing PMD tables

From: Catalin Marinas <[email-masked]>
Date: Thu, 21 May 2026 16:15:37 +0100

On Thu, May 21, 2026 at 04:05:07PM +0100, Catalin Marinas wrote:
> + David H.
> 
> On Thu, May 21, 2026 at 03:30:11PM +0800, Zeng Heng wrote:
> > From: Zeng Heng <[email-masked]>
> > 
> > When huge_pmd_unshare() is called to unshare a PMD table, the
> > tlb_unshare_pmd_ptdesc() function sets tlb->unshared_tables=true
> > but the aarch64 tlb_flush() only checked tlb->freed_tables to
> > determine whether to use TLBF_NONE (vae1is, invalidates walk
> > cache) or TLBF_NOWALKCACHE (vale1is, leaf-only).
> > 
> > This caused the stale PMD page table entry to remain in the walk cache
> > after unshare, potentially leading to incorrect page table walks.
> > 
> > Fix by including unshared_tables in the check, so that when
> > unsharing tables, TLBF_NONE is used and the walk cache is properly
> > invalidated.
> > 
> > Here is the detailed distinction between vae1is and vale1is:
> > 
> > | Instruction Combination  | Actual Invalidation Scope                         |
> > | ------------------------ | --------------------------------------------------|
> > | `VAE1IS`  + TTL=`0`      | All entries at all levels (full invalidation)     |
> > | `VAE1IS`  + TTL=`2` (L2) | Non-leaf at Level 0/1 + leaf at Level 2           |
> > | `VALE1IS` + TTL=`0`      | Leaf entries at all levels (non-leaf not cleared) |
> > | `VALE1IS` + TTL=`2` (L2) | Leaf entry at Level 2 only                        |
> > 
> > Signed-off-by: Zeng Heng <[email-masked]>
> 
> The fix looks fine but does it need:
> 
> Fixes: 8ce720d5bd91 ("mm/hugetlb: fix excessive IPI broadcasts when unsharing PMD tables using mmu_gather")
> Cc: <[email-masked]>
> 
> > ---
> >  arch/arm64/include/asm/tlb.h | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/arm64/include/asm/tlb.h b/arch/arm64/include/asm/tlb.h
> > index 10869d7731b8..751bd57bc3ba 100644
> > --- a/arch/arm64/include/asm/tlb.h
> > +++ b/arch/arm64/include/asm/tlb.h
> > @@ -53,7 +53,8 @@ static inline int tlb_get_level(struct mmu_gather *tlb)
> >  static inline void tlb_flush(struct mmu_gather *tlb)
> >  {
> >  	struct vm_area_struct vma = TLB_FLUSH_VMA(tlb->mm, 0);
> > -	tlbf_t flags = tlb->freed_tables ? TLBF_NONE : TLBF_NOWALKCACHE;
> > +	tlbf_t flags = (tlb->freed_tables || tlb->unshared_tables) ?
> > +			TLBF_NONE : TLBF_NOWALKCACHE;
> >  	unsigned long stride = tlb_get_unmap_size(tlb);
> >  	int tlb_level = tlb_get_level(tlb);

Do we need this as well?

diff --git a/arch/arm64/include/asm/tlb.h b/arch/arm64/include/asm/tlb.h
index 10869d7731b8..3f4ab38cfd6e 100644
--- a/arch/arm64/include/asm/tlb.h
+++ b/arch/arm64/include/asm/tlb.h
@@ -24,7 +24,7 @@ static void tlb_flush(struct mmu_gather *tlb);
 static inline int tlb_get_level(struct mmu_gather *tlb)
 {
 	/* The TTL field is only valid for the leaf entry. */
-	if (tlb->freed_tables)
+	if (tlb->freed_tables || tlb->unshared_tables)
 		return TLBI_TTL_UNKNOWN;
 
 	if (tlb->cleared_ptes && !(tlb->cleared_pmds ||