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Thu, 16 Oct 2025 18:27:24 -0700 (PDT) Date: Fri, 17 Oct 2025 01:27:24 +0000 From: Wei Yang To: Dev Jain Cc: Lance Yang , 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, 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() Message-ID: <20251017012724.4bo5oj2g6tdmp2fv@master> Reply-To: Wei Yang References: <20251016033643.10848-1-lance.yang@linux.dev> <17c4c5f9-6ac8-4914-838f-f511dfbf948f@arm.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <17c4c5f9-6ac8-4914-838f-f511dfbf948f@arm.com> User-Agent: NeoMutt/20170113 (1.7.2) On Thu, Oct 16, 2025 at 11:47:06AM +0530, Dev Jain wrote: > >On 16/10/25 9:06 am, Lance Yang wrote: >> From: Lance Yang >> >> 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 >> Signed-off-by: Lance Yang >> --- >> mm/khugepaged.c | 13 ++++++++----- >> 1 file changed, 8 insertions(+), 5 deletions(-) >> >> diff --git a/mm/khugepaged.c b/mm/khugepaged.c >> index d635d821f611..0341c3d13e9e 100644 >> --- a/mm/khugepaged.c >> +++ b/mm/khugepaged.c >> @@ -516,7 +516,7 @@ static void release_pte_pages(pte_t *pte, pte_t *_pte, >> pte_t pteval = ptep_get(_pte); >> unsigned long pfn; >> - if (pte_none(pteval)) >> + if (!pte_present(pteval)) >> continue; >> pfn = pte_pfn(pteval); >> if (is_zero_pfn(pfn)) >> @@ -690,9 +690,10 @@ static void __collapse_huge_page_copy_succeeded(pte_t *pte, >> address += nr_ptes * PAGE_SIZE) { >> nr_ptes = 1; >> pteval = ptep_get(_pte); >> - if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) { >> + if (pte_none(pteval) || >> + (pte_present(pteval) && is_zero_pfn(pte_pfn(pteval)))) { >> add_mm_counter(vma->vm_mm, MM_ANONPAGES, 1); >> - if (is_zero_pfn(pte_pfn(pteval))) { >> + if (!pte_none(pteval)) { > >Could save a level of indentation by saying >if (pte_none(pteval)) > continue; > Vote for this :-) No other comment. Reviewed-by: Wei Yang -- Wei Yang Help you, Help me