linux-kernel - Re: [PATCH 1/1] mm/rmap: make folio unmap batching safe and support partial batches

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Message-ID: <CAGsJ_4yjvHzeUTDoALafLddTCTOSzYiFQNvAmSQORLJV1HPhPQ@mail.gmail.com>
Date: Fri, 27 Jun 2025 17:02:20 +1200
From: Barry Song <21cnbao@...il.com>
To: Lance Yang <ioworker0@...il.com>
Cc: akpm@...ux-foundation.org, david@...hat.com, baolin.wang@...ux.alibaba.com, 
	chrisl@...nel.org, kasong@...cent.com, linux-arm-kernel@...ts.infradead.org, 
	linux-kernel@...r.kernel.org, linux-mm@...ck.org, 
	linux-riscv@...ts.infradead.org, lorenzo.stoakes@...cle.com, 
	ryan.roberts@....com, v-songbaohua@...o.com, x86@...nel.org, 
	huang.ying.caritas@...il.com, zhengtangquan@...o.com, riel@...riel.com, 
	Liam.Howlett@...cle.com, vbabka@...e.cz, harry.yoo@...cle.com, 
	mingzhe.yang@...com, Lance Yang <lance.yang@...ux.dev>
Subject: Re: [PATCH 1/1] mm/rmap: make folio unmap batching safe and support
 partial batches

On Fri, Jun 27, 2025 at 2:53 PM Lance Yang <ioworker0@...il.com> wrote:
>
> From: Lance Yang <lance.yang@...ux.dev>
>
> As pointed out by David[1], the batched unmap logic in try_to_unmap_one()
> can read past the end of a PTE table if a large folio is mapped starting at
> the last entry of that table.
>
> So let's fix the out-of-bounds read by refactoring the logic into a new
> helper, folio_unmap_pte_batch().
>
> The new helper now correctly calculates the safe number of pages to scan by
> limiting the operation to the boundaries of the current VMA and the PTE
> table.
>
> In addition, the "all-or-nothing" batching restriction is removed to
> support partial batches. The reference counting is also cleaned up to use
> folio_put_refs().
>
> [1] https://lore.kernel.org/linux-mm/a694398c-9f03-4737-81b9-7e49c857fcbe@redhat.com
>
> Fixes: 354dffd29575 ("mm: support batched unmap for lazyfree large folios during reclamation")
> Suggested-by: David Hildenbrand <david@...hat.com>
> Suggested-by: Barry Song <baohua@...nel.org>
> Signed-off-by: Lance Yang <lance.yang@...ux.dev>

I'd prefer changing the subject to something like
"Fix potential out-of-bounds page table access during batched unmap"

Supporting partial batching is a cleanup-related benefit of this fix.
It's worth mentioning that the affected cases are quite rare,
since MADV_FREE typically performs split_folio().

Also, we need to Cc stable.

Thanks
Barry