From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id 1BCFB2E7186 for ; Thu, 18 Sep 2025 07:37:54 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=217.140.110.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1758181076; cv=none; b=cW9RJxYjrM0kbEozv2l8MgQ5d9QQ9u8v+4ATEsokGlLBEIgbflzn+5fkI+BTTvho4obwtXSy7ESkqaueJnTqyRJ1M21DRtL6HZnXm0UQaM4L5AqY+wSuH3XkqRHH6P4wA5bFHhh+QpAt/2mCVkfSwcVN/i5hrd074VQEvALM3Bc= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1758181076; c=relaxed/simple; bh=zQLf+BaYOhlij8LvcdFDaViqL1SNEov0oWdvXy9Q8qQ=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=uftuOjO9fHJaPyKmldQffB0Bng5V3WutMIJelWTo0IHxBF/Pf6MYhfNKQIXKlr/r2Y+net2iNLS/8cyNEj1dEPmGzEnR5TmdW/jmuZR9JYxwNxhUp5rd4JAGRTvyh3Emf1AIgRUQDFbr1Y64a07UOx17fuWxgkhJhtd+uLlhw4k= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com; spf=pass smtp.mailfrom=arm.com; arc=none smtp.client-ip=217.140.110.172 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=arm.com Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id E00DD1A25; Thu, 18 Sep 2025 00:37:45 -0700 (PDT) Received: from [10.164.18.52] (MacBook-Pro.blr.arm.com [10.164.18.52]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 71EAE3F66E; Thu, 18 Sep 2025 00:37:49 -0700 (PDT) Message-ID: Date: Thu, 18 Sep 2025 13:07:46 +0530 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH mm-new v2 2/2] mm/khugepaged: abort collapse scan on guard PTEs To: Lance Yang , 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> Content-Language: en-US From: Dev Jain In-Reply-To: <20250918050431.36855-3-lance.yang@linux.dev> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit 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.