From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-183.mta1.migadu.com (out-183.mta1.migadu.com [95.215.58.183]) (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 0150731C565 for ; Mon, 15 Sep 2025 14:42:10 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=95.215.58.183 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1757947333; cv=none; b=UM9K4+ZjfFlbJio3aMNEekqjZ180K0eT6FBlDcJgukVZGvhd0PnfD6bQ6IQ0VFP4gwrJcaNXU2pJWusVX/uLyJRnIlbkzCbRyoT+8KSewi6cGw/0SQpSdefcBS7Zxl+x8dhAJ4jEQYgiFIARe3FBZ6dEs5ttNzR5ZUCMPN2VXcI= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1757947333; c=relaxed/simple; bh=MsgQ+XEybDBFnv1Sn7YCQ6b2cPbqNlmkdHgNFFRWULQ=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=juJ9HCSFWH3QMo0jBsBMzlGJThIjPOy1qiUKTmiFQtAmkg7nosjrTjyBDRWwjHGtmW/rUgllJBeKwZbPqNmlmBiMc9jjR5PyClJ4R/bKCOyHRuRFMfUQ8YtJ8grUOTmt4F3HBMY8pqIHTq6/BM+xjo0kER8H1dQNZFSvUA0WfEU= 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=m8NJI9pn; arc=none smtp.client-ip=95.215.58.183 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="m8NJI9pn" Message-ID: <121a0947-88cd-4515-bf14-a2af9b69eb0b@linux.dev> DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1757947329; 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=otD0lF88c/W1mhXnZZlmqwcx36asc6J+ruH2BHy82vs=; b=m8NJI9pn7LA1CV/E/15NfiRNHl0491YASPDwDn4YMfZP3tGltyzJLFHg+HiEf4XsPtEI1s h0OVkE2EOubpGiDYXx9Ycwl2n+i0QGGB2Bw2l++MvS+vszzPFGSj1ni4Q7I1hioL12BmpY X88TdsxVe7494tG2OsZwjvuzm/deTls= Date: Mon, 15 Sep 2025 22:42:00 +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 3/3] mm/khugepaged: abort collapse scan on guard PTEs Content-Language: en-US To: Lorenzo Stoakes Cc: akpm@linux-foundation.org, david@redhat.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 References: <20250914143547.27687-1-lance.yang@linux.dev> <20250914143547.27687-4-lance.yang@linux.dev> <59519b0f-af14-4486-9aca-4e721842d45b@lucifer.local> X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Lance Yang In-Reply-To: <59519b0f-af14-4486-9aca-4e721842d45b@lucifer.local> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Migadu-Flow: FLOW_OUT On 2025/9/15 22:08, Lorenzo Stoakes wrote: > On Sun, Sep 14, 2025 at 10:35:47PM +0800, 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 ;) > > No smileys in commit messages please... :) Got it. Apparently, I'm a bit too fond of them ... Will remove it in v2. > >> >> This patch adds a check directly inside the PTE scan loop. If a guard >> marker is found, the scan is aborted immediately with a new SCAN_PTE_GUARD >> status, avoiding wasted work. >> >> Signed-off-by: Lance Yang >> --- >> mm/khugepaged.c | 12 ++++++++++++ >> 1 file changed, 12 insertions(+) >> >> diff --git a/mm/khugepaged.c b/mm/khugepaged.c >> index e54f99bb0b57..910a6f2ec8a9 100644 >> --- a/mm/khugepaged.c >> +++ b/mm/khugepaged.c >> @@ -59,6 +59,7 @@ enum scan_result { >> SCAN_STORE_FAILED, >> SCAN_COPY_MC, >> SCAN_PAGE_FILLED, >> + SCAN_PTE_GUARD, > > I wonder if we really need to have it literally called out though, can we just > use: > > SCAN_PTE_NON_PRESENT > > Instead? > > As it is, indeed, non-present :) Makes sense to me. A guard PTE is indeed a special non-present case. So, let's reuse SCAN_PTE_NON_PRESENT for that case ;) Cheers, Lance > >> }; >> >> #define CREATE_TRACE_POINTS >> @@ -1317,6 +1318,16 @@ 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_GUARD; >> + goto out_unmap; >> + } >> continue; >> } else { >> result = SCAN_EXCEED_SWAP_PTE; >> @@ -2860,6 +2871,7 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned long start, >> case SCAN_PAGE_COMPOUND: >> case SCAN_PAGE_LRU: >> case SCAN_DEL_PAGE_LRU: >> + case SCAN_PTE_GUARD: >> last_fail = result; >> break; >> default: >> -- >> 2.49.0 >> > > Cheers, Lorenzo