From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-173.mta0.migadu.com (out-173.mta0.migadu.com [91.218.175.173]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id BA3D92D3A86 for ; Thu, 16 Oct 2025 06:27:01 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.173 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1760596025; cv=none; b=mKs6yofuNvd9DbBC+y1h7W66d+MYaaVvSHjFtb24Q4tO+L9vzCB5VLDykIm3vdW27K14dbowksXUXAzIIGHpG05TxVshZc9e0QiF6ux41wu8cLnZ4diaObSYL6+S7glUAOJf1hbvsgiLtylrXBJZrzTnYaA3paMcSAJg1dFAKQk= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1760596025; c=relaxed/simple; bh=Qqp0kiIWND20xDyYJe74c11eBzT4sy+ggYL2fspdYEo=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=mBvl57l5HN3Us3UFdEPPUICwbyUQjhXcWUajgvPzNfIAKqvgk0BkGh2DJHRMWp95qfOqTG3nbOQhjqNqIi612osvqenIEKR4H8ZuqdWamebyT19bjeQLi+am/QiKe9Eo5QVxm5UiUgtlGIqbpNyBYfvl0qKiXt0ATgfqeHSjd7A= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=YqhTURIH; arc=none smtp.client-ip=91.218.175.173 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="YqhTURIH" Message-ID: <8d88fb47-5318-433c-bb07-aeea4a025db8@linux.dev> DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1760596019; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=gNG5Ju61aC+No7m/Kz/oSUIjtFO+hsFQlNAY/X9LidA=; b=YqhTURIHrRbmBDr00b7O/r+QOfL1IC659ssjuugIVUHtGQNorJNmYWoZgzwPIOMeG0K9QQ Ev8Lylb2963TmGHQU9HxEsU8r2RZxDkZFM1xjN+0D81fkx9o+1DIUImQ/m9U3L8aBanIXG r40hajjDxdeZgFEQ33VXvxFbWIbWz/c= Date: Thu, 16 Oct 2025 14:26:52 +0800 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Subject: Re: [PATCH mm-new 1/1] mm/khugepaged: guard is_zero_pfn() calls with pte_present() To: Dev Jain Cc: 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, akpm@linux-foundation.org, lorenzo.stoakes@oracle.com, david@redhat.com References: <20251016033643.10848-1-lance.yang@linux.dev> <17c4c5f9-6ac8-4914-838f-f511dfbf948f@arm.com> Content-Language: en-US X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Lance Yang In-Reply-To: <17c4c5f9-6ac8-4914-838f-f511dfbf948f@arm.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT On 2025/10/16 14:17, 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; > > That would make it crystal clear that we do nothing when pte is none, > and we do something when pte is pointing to zero pfn. Yes! That does look clearer ;) I'll pick that up in the next spin, if one is needed. > > Reviewed-by: Dev Jain Cheers! > >>                   /* >>                    * ptl mostly unnecessary. >>                    */ >> @@ -794,7 +795,8 @@ static int __collapse_huge_page_copy(pte_t *pte, >> struct folio *folio, >>           unsigned long src_addr = address + i * PAGE_SIZE; >>           struct page *src_page; >> -        if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) { >> +        if (pte_none(pteval) || >> +            (pte_present(pteval) && is_zero_pfn(pte_pfn(pteval)))) { >>               clear_user_highpage(page, src_addr); >>               continue; >>           } >> @@ -1294,7 +1296,8 @@ static int hpage_collapse_scan_pmd(struct >> mm_struct *mm, >>                   goto out_unmap; >>               } >>           } >> -        if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) { >> +        if (pte_none(pteval) || >> +            (pte_present(pteval) && is_zero_pfn(pte_pfn(pteval)))) { >>               ++none_or_zero; >>               if (!userfaultfd_armed(vma) && >>                   (!cc->is_khugepaged ||