From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out30-119.freemail.mail.aliyun.com (out30-119.freemail.mail.aliyun.com [115.124.30.119]) (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 45CE72580D2 for ; Tue, 24 Jun 2025 09:57:37 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=115.124.30.119 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1750759061; cv=none; b=YBZkLhS5zORPEUB9ykRRRb/nkoFcmn3/R7Ag8SZnsGNi70S1CfoRKA0IrTShkygsfiQ6UAl/B356Rqj+Co5z++3o43J9KfNryOHnGpHMYD+VlkhsbtvM7VaZZ0W702/gAHcVn8CYbBQEXxxlXD0Txh6aEzgoOzfZUcw5ZmNPfu0= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1750759061; c=relaxed/simple; bh=1XBVB3eb1tiQqyGdoFEHDqt5hVaBqmfIEf5oWaUPC38=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=EYM+sd0lPZHAqi27UfmD0Bklj0vgM/Ir4tEhmjhJIo62fK56PRRdVHoPrjT2B+JzW2CLvjIlkmrDV9QjGN15qpCm+GstnfCYoQYA7pYmat39B51I878hZhE9DfCN2Cy8YOqqO9lmYPOlbNLEK1Bp/Jvd6tAnL35QZ6sdP1N0uYs= 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=pXHkkyRn; arc=none smtp.client-ip=115.124.30.119 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="pXHkkyRn" DKIM-Signature:v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.alibaba.com; s=default; t=1750759054; h=Message-ID:Date:MIME-Version:Subject:To:From:Content-Type; bh=3geTY9Lg0SUAa3yFGCcB0qvuVBnVvAoChOzavuhTgtQ=; b=pXHkkyRnnbmpHkRCJvxo0hPIY3CBAPK+wWI9ZLtr/bEXNOjjdisuMg+XkOFV2qgXT9v11o2URy6IS+WeTB96GCZP5g7KaT+/pW+maxerM05fS4emecunbkOGC4B3713NxKo9n7XQZEQ+jPFpqEfzWkkrvRIasYJ2RPAnb/24Lo4= Received: from 30.74.144.102(mailfrom:baolin.wang@linux.alibaba.com fp:SMTPD_---0WegcnpY_1750759053 cluster:ay36) by smtp.aliyun-inc.com; Tue, 24 Jun 2025 17:57:34 +0800 Message-ID: Date: Tue, 24 Jun 2025 17:57:33 +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: Dev Jain , akpm@linux-foundation.org, hughd@google.com, david@redhat.com Cc: ziy@nvidia.com, lorenzo.stoakes@oracle.com, Liam.Howlett@oracle.com, npache@redhat.com, ryan.roberts@arm.com, baohua@kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <8912e179-601a-4677-b2f6-14f40d488d98@arm.com> From: Baolin Wang In-Reply-To: <8912e179-601a-4677-b2f6-14f40d488d98@arm.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 2025/6/24 16:41, Dev Jain wrote: > > On 23/06/25 1:58 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. >> >> 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)) > > So the behaviour now is: First check whether THP settings converge to > never. > Then, if enforce_sysfs is not set, return immediately. So in this > khugepaged > code will it be better to call __thp_vma_allowable_orders()? If the sysfs > settings are changed to never before hitting collapse_pte_mapped_thp(), > then right now we will return SCAN_VMA_CHECK from here, whereas, the > comment > says "regardless of sysfs THP settings", which should include "regardless > of whether the sysfs settings say never". Sounds reasonable to me. Thanks. I will change thp_vma_allowable_order() to __thp_vma_allowable_orders() in the collapse_pte_mapped_thp() function to maintain consistency with the original logic. Lorenzo and David, how do you think? Thanks.