Re: [PATCH] mm/huge_memory: transfer the pmd dirty bit to the folio on zap

[Lance Yang]

On Wed, Aug 19, 2026 at 03:31:40PM +0100, Kiryl Shutsemau wrote:
>On Wed, Aug 19, 2026 at 03:12:22AM -0700, Usama Arif wrote:
[...]
>> ---
>>  mm/huge_memory.c | 2 ++
>>  1 file changed, 2 insertions(+)
>> 
>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>> index ced400f72d43a..afbb5974bd225 100644
>> --- a/mm/huge_memory.c
>> +++ b/mm/huge_memory.c
>> @@ -2449,6 +2449,8 @@ static void zap_huge_pmd_folio(struct mm_struct *mm, struct vm_area_struct *vma,
>>  		add_mm_counter(mm, mm_counter_file(folio),
>>  			       -HPAGE_PMD_NR);
>>  
>> +		if (is_present && pmd_dirty(pmdval))
>> +			folio_mark_dirty(folio);
>
>Unrelated to your patch, but noticed while looking at it: we drop the rmap
>here under the pmd lock, while the TLB flush is deferred to
>tlb_finish_mmu(). The pte path handles this with
>tlb_delay_rmap()/force_flush (5df397dec7c4), but there's no pmd equivalent:
>tlb_flush_rmap_batch() only knows folio_remove_rmap_ptes(), and
>zap_huge_pmd() uses tlb_remove_page_size(), which takes no delay_rmap.

Well spotted!

>Doesn't matter for shmem, but xfs & friends do get PMD-order folios, and

Right. pageout() cannot pass its refcount check while PMD mapping still
holds an extra folio ref, and mmu_gather drops that ref only after TLB
flush.

>do_set_pmd() makes the pmd dirty+writable once page_mkwrite() has run. So
>folio_mkclean() can clean the folio while another CPU still stores through a
>stale TLB entry -- silently lost write, no PG_dirty left behind.

Yep. Writeback can run folio_mkclean() while that ref is still held,
though, and with rmap already gone it misses the PMD ...

>I think we need to fix this too.

+1

>Wanna give it a try?

zap_huge_pmd() only handles one PMD under PTL anyway ... how about just
flushing before folio_remove_rmap_pmd()?

---8<---
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index afbb5974bd22..6fb34924ef66 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2531,6 +2531,14 @@ bool zap_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
 	is_present = pmd_present(orig_pmd);
 	folio = normal_or_softleaf_folio_pmd(vma, addr, orig_pmd, is_present);
 	has_deposit = has_deposited_pgtable(vma, orig_pmd, folio);
+	/*
+	 * folio_mkclean() relies on the rmap to find writable mappings.
+	 * Flush stale TLB entries before removing it below.
+	 */
+	if (folio && is_present && !folio_test_anon(folio) &&
+	    pmd_dirty(orig_pmd))
+		tlb_flush_mmu_tlbonly(tlb);
+
 	if (folio)
 		zap_huge_pmd_folio(mm, vma, orig_pmd, folio, is_present);
 	if (has_deposit)
---

Cheers, Lance

>
>>  		if (is_present && pmd_young(pmdval) &&
>>  		    likely(vma_has_recency(vma)))
>>  			folio_mark_accessed(folio);
>> -- 
>> 2.53.0-Meta
>> 
>
>-- 
>  Kiryl Shutsemau / Kirill A. Shutemov
>