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

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Message-ID: <d288fc83-a1ee-4409-a457-83bc21356306@linux.dev>
Date: Thu, 16 Oct 2025 18:51:02 +0800
From: Lance Yang <lance.yang@...ux.dev>
To: Wei Yang <richard.weiyang@...il.com>
Cc: akpm@...ux-foundation.org, david@...hat.com, lorenzo.stoakes@...cle.com,
 ziy@...dia.com, baolin.wang@...ux.alibaba.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 17:33, Wei Yang wrote:
> On Thu, Oct 16, 2025 at 11:36:43AM +0800, 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.
>>
> 
> Does it more like to guard pte_pfn() with pte_present()?

Exactly! My thinking was that by guarding pte_pfn(), we're ultimately
protecting is_zero_pfn() from acting on a junk PFN.

So we're on the same page — I just described the end goal ;p

Thanks!