From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out30-131.freemail.mail.aliyun.com (out30-131.freemail.mail.aliyun.com [115.124.30.131]) (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 E19D418641 for ; Tue, 24 Jun 2025 01:44:18 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=115.124.30.131 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1750729463; cv=none; b=U9FAKF7Jq03pFeAIFPE40FDW8Lu5XqRIe7gijPEUlvi6JGI53H45skHD3VQ42PznVJsI1RCOVP9XMfxSv1K8lC+E7o6no8flEhEwRgnK5FM9osdsE0Cm82sq9G7fdaA8HfQ8lvnPD0WfCh2q0qE/+Rf+b1mJphCoMhiHdS3H95o= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1750729463; c=relaxed/simple; bh=FmI7Pvi2S/VWMt5surE3jm0RzIxZPGXcGqhhkIsUgK4=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=nVDiqYEMyE2OzTSuiQJRphnhXghZGBKaGu7F/K8CW2v4UmSKovisZ3tZQDpiCZVH7lZvINUjkkax0KlpJMbtz6e9mz2zAPHwqWKm221J9QlU31SBLzMSszPTpqK6ie3WW0TSHe1AM7gpJaQb3zGmuQBivZ+MOdA2HMODmMZu+AE= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.alibaba.com; spf=pass smtp.mailfrom=linux.alibaba.com; dkim=pass (1024-bit key) header.d=linux.alibaba.com header.i=@linux.alibaba.com header.b=SaXSMPnC; arc=none smtp.client-ip=115.124.30.131 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.alibaba.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.alibaba.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.alibaba.com header.i=@linux.alibaba.com header.b="SaXSMPnC" DKIM-Signature:v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.alibaba.com; s=default; t=1750729456; h=Message-ID:Date:MIME-Version:Subject:To:From:Content-Type; bh=bXzLG3qEfg6SUTPkxvvaO8P1W2oF5qK0rvfgE/cq0gA=; b=SaXSMPnCXUg7sAS38XMy501xCZHVqC5q2oKJEh831Hz/w+VpXyZnwZtWe+k2+Citjlr3rjN9AarE1EJxMVDY/iT+M6qhGEb/amO7sxlIiP99Oa81UImoTv0WrroqPXw+9h+qQoftlhCrN2DFv2drlXJNq/1Ji/6Gqc7aawlWPcI= Received: from 30.74.144.102(mailfrom:baolin.wang@linux.alibaba.com fp:SMTPD_---0WeeL9rz_1750729455 cluster:ay36) by smtp.aliyun-inc.com; Tue, 24 Jun 2025 09:44:15 +0800 Message-ID: Date: Tue, 24 Jun 2025 09:44:14 +0800 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 v3 1/2] mm: huge_memory: disallow hugepages if the system-wide THP sysfs settings are disabled To: Barry Song <21cnbao@gmail.com> Cc: akpm@linux-foundation.org, hughd@google.com, david@redhat.com, ziy@nvidia.com, lorenzo.stoakes@oracle.com, Liam.Howlett@oracle.com, npache@redhat.com, ryan.roberts@arm.com, dev.jain@arm.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: From: Baolin Wang In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 2025/6/23 19:08, Barry Song wrote: > On Mon, Jun 23, 2025 at 8:28 PM Baolin Wang > wrote: >> >> When invoking thp_vma_allowable_orders(), the TVA_ENFORCE_SYSFS flag is not >> specified, we will ignore the THP sysfs settings. Whilst it makes sense for the >> callers who do not specify this flag, it creates a odd and surprising situation >> where a sysadmin specifying 'never' for all THP sizes still observing THP pages >> being allocated and used on the system. >> >> The motivating case for this is MADV_COLLAPSE. The MADV_COLLAPSE will ignore >> the system-wide Anon THP sysfs settings, which means that even though we have >> disabled the Anon THP configuration, MADV_COLLAPSE will still attempt to collapse >> into a Anon THP. This violates the rule we have agreed upon: never means never. >> > > Should we update the man page for madv_collapse ? > https://man7.org/linux/man-pages/man2/madvise.2.html > > MADV_COLLAPSE is independent of any sysfs (see sysfs(5)) > setting under /sys/kernel/mm/transparent_hugepage, both in > terms of determining THP eligibility, and allocation > semantics. See Linux kernel source file > Documentation/admin-guide/mm/transhuge.rst for more > information. MADV_COLLAPSE also ignores huge= tmpfs mount > when operating on tmpfs files. Allocation for the new > hugepage may enter direct reclaim and/or compaction, > regardless of VMA flags (though VM_NOHUGEPAGE is still > respected). > > So this effectively changes the uABI, right? Good point. Will update the man page. >> Currently, besides MADV_COLLAPSE not setting TVA_ENFORCE_SYSFS, there is only >> one other instance where TVA_ENFORCE_SYSFS is not set, which is in the >> collapse_pte_mapped_thp() function, but I believe this is reasonable from its >> comments: >> >> " >> /* >> * If we are here, we've succeeded in replacing all the native pages >> * in the page cache with a single hugepage. If a mm were to fault-in >> * this memory (mapped by a suitably aligned VMA), we'd get the hugepage >> * and map it by a PMD, regardless of sysfs THP settings. As such, let's >> * analogously elide sysfs THP settings here. >> */ >> if (!thp_vma_allowable_order(vma, vma->vm_flags, 0, PMD_ORDER)) >> " >> >> Another rule for madvise, referring to David's suggestion: “allowing for >> collapsing in a VM without VM_HUGEPAGE in the "madvise" mode would be fine". >> >> To address this issue, the current strategy should be: >> >> If no hugepage modes are enabled for the desired orders, nor can we enable them >> by inheriting from a 'global' enabled setting - then it must be the case that >> all desired orders either specify or inherit 'NEVER' - and we must abort. >> >> Meanwhile, we should fix the khugepaged selftest for MADV_COLLAPSE by enabling >> THP. > > It’s a bit odd that the old test case expects collapsing to succeed > even when we’ve set it > to ‘never’. > Setting it to ‘always’ doesn’t seem to test anything as a counterpart. > > I assume the goal is to test that setting it to ‘never’ prevents collapsing? The original logic will prevent khugepaged by setting THP_NEVER, allowing only madvise_collapse() to perform THP collapse. And this is the logic this patchset tries to fix, which is to also prevent madvise_collapse() from performing THP collapse when system-wide THP sysfs settings are disabled. Therefore, it should be changed to THP_ALWAYS here to allow madvise_collapse() to perform THP collapse. Of course, the current logic cannot completely disable khugepaged, but I haven't found a better way to modify it. As David suggested, changing to MADVISE mode would cause some test cases to fail because some tests previously set MADV_NOHUGEPAGE, and now there is no other way to clear the MADV_NOHUGEPAGE flag except for setting MADV_HUGEPAGE. As a result, khugepaged cannot be completely disabled either. So I think we should introduce a new method to clear MADV_NOHUGEPAGE flag without setting MADV_HUGEPAGE in the future.