From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-173.mta1.migadu.com (out-173.mta1.migadu.com [95.215.58.173]) (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 D72412EFD88 for ; Thu, 18 Sep 2025 08:11:35 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=95.215.58.173 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1758183097; cv=none; b=rEVKKzUUglSdhQ5yMTu6YILr2BO6tYm1vv0yFMPR5WgvBOXrkWi0PgwqcmsYGogTXr9gaKGwYqwL0+eY4H4KEj0B6Wjxd6gmMzqHXUp40NyU6HLn4yCeOFzk7S4MBYbiQXzIhAr/pw+FU2Eaf5EkhmW6JxDxcKTbYPN3qYLj2No= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1758183097; c=relaxed/simple; bh=SdhwhcNlJ1nV8CH4uyU08FVWAsLK/OB1V48VnTIvZLY=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=OLkepIIum6k3ndvtalQZ5o3OM3zjBM0ZrcHnHRS8Ihx+V1DrInauPHtYju5kVZMaiYX7WCAu5hRuV7JDiU4RFZszy3FwyWWSoi2rXwPchqzB6ZGygjm0faywGF1/X0ns40Q8LtA7ylxUaQVjo+ri5jLuEqH2++UfrLxbHcfx5Bw= 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=M5YnV4AA; arc=none smtp.client-ip=95.215.58.173 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="M5YnV4AA" Message-ID: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1758183093; 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=0hchChWtwLQOxGyljAivB4hYgL0w8MxSX4qQYtSpmXc=; b=M5YnV4AAlwAIyMsfjkSwpmcD1EeIFgGdofp5XRQmaOtLY8h87Q8IE+iWJg5O4r7HWnqzNP R4Di5InjHFUxI1aeV20Iox2vG1nLWpu66cztX6dN0du3p7c/mD/infBxiQAnTnAlkPMZrd DNc9R+TQ1TST6OUdp3N7fP6+9nVWynI= Date: Thu, 18 Sep 2025 16:11:21 +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 v2 2/2] mm/khugepaged: abort collapse scan on guard PTEs Content-Language: en-US To: Dev Jain , akpm@linux-foundation.org, david@redhat.com, lorenzo.stoakes@oracle.com 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, kirill@shutemov.name, hughd@google.com, mpenttil@redhat.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org References: <20250918050431.36855-1-lance.yang@linux.dev> <20250918050431.36855-3-lance.yang@linux.dev> X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Lance Yang In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT On 2025/9/18 15:37, Dev Jain wrote: > > On 18/09/25 10:34 am, Lance Yang wrote: >> From: Lance Yang >> >> Guard PTE markers are installed via MADV_GUARD_INSTALL to create >> lightweight guard regions. >> >> Currently, any collapse path (khugepaged or MADV_COLLAPSE) will fail when >> encountering such a range. >> >> MADV_COLLAPSE fails deep inside the collapse logic when trying to swap-in >> the special marker in __collapse_huge_page_swapin(). >> >> hpage_collapse_scan_pmd() >>   `- collapse_huge_page() >>       `- __collapse_huge_page_swapin() -> fails! >> >> khugepaged's behavior is slightly different due to its max_ptes_swap >> limit >> (default 64). It won't fail as deep, but it will still needlessly scan up >> to 64 swap entries before bailing out. >> >> IMHO, we can and should detect this much earlier. >> >> This patch adds a check directly inside the PTE scan loop. If a guard >> marker is found, the scan is aborted immediately with >> SCAN_PTE_NON_PRESENT, >> avoiding wasted work. >> >> Suggested-by: Lorenzo Stoakes >> Signed-off-by: Lance Yang >> --- >>   mm/khugepaged.c | 10 ++++++++++ >>   1 file changed, 10 insertions(+) >> >> diff --git a/mm/khugepaged.c b/mm/khugepaged.c >> index 9ed1af2b5c38..70ebfc7c1f3e 100644 >> --- a/mm/khugepaged.c >> +++ b/mm/khugepaged.c >> @@ -1306,6 +1306,16 @@ static int hpage_collapse_scan_pmd(struct >> mm_struct *mm, >>                       result = SCAN_PTE_UFFD_WP; >>                       goto out_unmap; >>                   } >> +                /* >> +                 * Guard PTE markers are installed by >> +                 * MADV_GUARD_INSTALL. Any collapse path must >> +                 * not touch them, so abort the scan immediately >> +                 * if one is found. >> +                 */ >> +                if (is_guard_pte_marker(pteval)) { >> +                    result = SCAN_PTE_NON_PRESENT; >> +                    goto out_unmap; >> +                } >>                   continue; >>               } else { >>                   result = SCAN_EXCEED_SWAP_PTE; >> >> > > I would like to hear everyone else's thoughts on > https://lore.kernel.org/linux-mm/750a06dc-db3d-43c6- > b234-95efb393a9df@arm.com/ > wherein I suggest that we should not continue to try collapsing other > regions > but immediately exit. The SCAN_PTE_NON_PRESENT case does not exit. Yes! Let's hear what other folks think on that[1]. [1] https://lore.kernel.org/linux-mm/c9d4d761-202f-48ce-8e3d-fb9075671ff3@linux.dev Cheers, Lance