From: Lance Yang <lance.yang@linux.dev>
To: Wei Yang <richard.weiyang@gmail.com>
Cc: akpm@linux-foundation.org, david@redhat.com,
lorenzo.stoakes@oracle.com, ziy@nvidia.com,
baolin.wang@linux.alibaba.com, Liam.Howlett@oracle.com,
npache@redhat.com, ryan.roberts@arm.com, dev.jain@arm.com,
baohua@kernel.org, ioworker0@gmail.com,
linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH mm-new 1/1] mm/khugepaged: guard is_zero_pfn() calls with pte_present()
Date: Thu, 16 Oct 2025 18:51:02 +0800 [thread overview]
Message-ID: <d288fc83-a1ee-4409-a457-83bc21356306@linux.dev> (raw)
In-Reply-To: <20251016093310.rilvenglgnr65ojq@master>
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@linux.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!
next prev parent reply other threads:[~2025-10-16 10:51 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2025-10-16 3:36 Lance Yang
2025-10-16 5:34 ` Dev Jain
2025-10-16 5:59 ` Lance Yang
2025-10-16 6:15 ` Dev Jain
2025-10-16 6:17 ` Dev Jain
2025-10-16 6:26 ` Lance Yang
2025-10-17 1:27 ` Wei Yang
2025-10-17 8:11 ` David Hildenbrand
2025-10-17 8:37 ` Lance Yang
2025-10-17 8:43 ` David Hildenbrand
2025-10-17 8:47 ` Lance Yang
2025-10-17 9:35 ` Lorenzo Stoakes
2025-10-16 9:33 ` Wei Yang
2025-10-16 10:51 ` Lance Yang [this message]
2025-10-17 8:10 ` Baolin Wang
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