linux-kernel - Re: [PATCH mm-new 1/1] mm/khugepaged: guard is_zero_pfn() calls with pte

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Message-ID: <cdd47a33-d0c5-4e7d-a210-fb3236e6749c@linux.alibaba.com>
Date: Fri, 17 Oct 2025 16:10:06 +0800
From: Baolin Wang <baolin.wang@...ux.alibaba.com>
To: Lance Yang <lance.yang@...ux.dev>, akpm@...ux-foundation.org,
 david@...hat.com, lorenzo.stoakes@...cle.com
Cc: ziy@...dia.com, Liam.Howlett@...cle.com, npache@...hat.com,
 ryan.roberts@....com, dev.jain@....com, baohua@...nel.org,
 ioworker0@...il.com, linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH mm-new 1/1] mm/khugepaged: guard is_zero_pfn() calls with
 pte_present()



On 2025/10/16 11:36, Lance Yang wrote:
> From: Lance Yang <lance.yang@...ux.dev>
> 
> A non-present entry, like a swap PTE, contains completely different data
> (swap type and offset). pte_pfn() doesn't know this, so if we feed it a
> non-present entry, it will spit out a junk PFN.
> 
> What if that junk PFN happens to match the zeropage's PFN by sheer
> chance? While really unlikely, this would be really bad if it did.
> 
> So, let's fix this potential bug by ensuring all calls to is_zero_pfn()
> in khugepaged.c are properly guarded by a pte_present() check.
> 
> Suggested-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> Signed-off-by: Lance Yang <lance.yang@...ux.dev>
> ---

LGTM.
Reviewed-by: Baolin Wang <baolin.wang@...ux.alibaba.com>