From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out30-110.freemail.mail.aliyun.com (out30-110.freemail.mail.aliyun.com [115.124.30.110]) (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 8C3062264CF for ; Thu, 14 Aug 2025 10:03:38 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=115.124.30.110 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1755165822; cv=none; b=X+J4lcdB4a5WOfnzvrQ88e+dcidFOJcZgAF64ReMdV7c++APQCnyS7x6zlHtZhIPT/s6BsGDbApjq2KltCjghu4hwDuX6YqN4dNCTadJ+H2rUuQICdMgR5DAMeTdT3x+f4IFQ6XTp+gnqSN+9PHp7EktBq+QI/RygdJRPb0xw6g= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1755165822; c=relaxed/simple; bh=THF2iyildJN7JJ8MP+yNCExbHNYfFH5VwD1KwRNgbC8=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=hhp4BBBD2erTwo/CAyTEJ19HUoBgDrsFZbXXwpuYk/J+C9ivIrtOzd3L8t/cIv3MuFVTJfCxBJ3cDGPh+ay/eTp8MpxGTRaDe38FwSvTptoujoSDYmcAQORNHwcCoVCb+3qDBRUZXRLacKYBICgMoZCkcnGICxIX3XXAqoBE8P0= 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=VZmnqBoF; arc=none smtp.client-ip=115.124.30.110 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="VZmnqBoF" DKIM-Signature:v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.alibaba.com; s=default; t=1755165809; h=Message-ID:Date:MIME-Version:Subject:To:From:Content-Type; bh=ocqGFWwlEESKe0BZACXQhN4GorNLeklOvmcYh0YlYp8=; b=VZmnqBoFYHe4CKwy74cVt0WvDSxJj64VUEHM2a6e8VQHPHeb8FGHicpHWHNVi0thcl/cAzv7a8H4JiRBhM2k5T0r1MbSE4//FNu/ZCVtTVDQQzLOekx4mEKa5AyUJ8LShZTSS7bhL9nAc5VlEOZzEFN2gFQUGC+zeqgntYK6D1o= Received: from 30.74.144.115(mailfrom:baolin.wang@linux.alibaba.com fp:SMTPD_---0WljqW7d_1755165806 cluster:ay36) by smtp.aliyun-inc.com; Thu, 14 Aug 2025 18:03:27 +0800 Message-ID: <323ed726-fc69-4d80-a7e8-e3762c161ee1@linux.alibaba.com> Date: Thu, 14 Aug 2025 18:03:26 +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: [RFC PATCH] mm: shmem: fix the strategy for the tmpfs 'huge=' options To: Hugh Dickins Cc: akpm@linux-foundation.org, willy@infradead.org, david@redhat.com, lorenzo.stoakes@oracle.com, ziy@nvidia.com, Liam.Howlett@oracle.com, npache@redhat.com, ryan.roberts@arm.com, dev.jain@arm.com, baohua@kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <701271092af74c2d969b195321c2c22e15e3c694.1753863013.git.baolin.wang@linux.alibaba.com> <3705f034-808a-4afe-5dde-4b4e9815a8d0@google.com> From: Baolin Wang In-Reply-To: <3705f034-808a-4afe-5dde-4b4e9815a8d0@google.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 2025/8/13 14:59, Hugh Dickins wrote: > On Tue, 12 Aug 2025, Baolin Wang wrote: >> On 2025/7/30 16:14, Baolin Wang wrote: >>> After commit acd7ccb284b8 ("mm: shmem: add large folio support for tmpfs"), >>> we have extended tmpfs to allow any sized large folios, rather than just >>> PMD-sized large folios. >>> >>> The strategy discussed previously was: >>> >>> " >>> Considering that tmpfs already has the 'huge=' option to control the >>> PMD-sized large folios allocation, we can extend the 'huge=' option to >>> allow any sized large folios. The semantics of the 'huge=' mount option >>> are: >>> >>> huge=never: no any sized large folios >>> huge=always: any sized large folios >>> huge=within_size: like 'always' but respect the i_size >>> huge=advise: like 'always' if requested with madvise() >>> >>> Note: for tmpfs mmap() faults, due to the lack of a write size hint, still >>> allocate the PMD-sized huge folios if huge=always/within_size/advise is >>> set. >>> >>> Moreover, the 'deny' and 'force' testing options controlled by >>> '/sys/kernel/mm/transparent_hugepage/shmem_enabled', still retain the same >>> semantics. The 'deny' can disable any sized large folios for tmpfs, while >>> the 'force' can enable PMD sized large folios for tmpfs. >>> " >>> >>> This means that when tmpfs is mounted with 'huge=always' or >>> 'huge=within_size', >>> tmpfs will allow getting a highest order hint based on the size of write() >>> and >>> fallocate() paths. It will then try each allowable large order, rather than >>> continually attempting to allocate PMD-sized large folios as before. >>> >>> However, this might break some user scenarios for those who want to use >>> PMD-sized large folios, such as the i915 driver which did not supply a write >>> size hint when allocating shmem [1]. >>> >>> Moreover, Hugh also complained that this will cause a regression in >>> userspace >>> with 'huge=always' or 'huge=within_size'. >>> >>> So, let's revisit the strategy for tmpfs large page allocation. A simple fix >>> would be to always try PMD-sized large folios first, and if that fails, fall >>> back to smaller large folios. However, this approach differs from the >>> strategy >>> for large folio allocation used by other file systems. Is this acceptable? >>> >>> [1] >>> https://lore.kernel.org/lkml/0d734549d5ed073c80b11601da3abdd5223e1889.1753689802.git.baolin.wang@linux.alibaba.com/ >>> Fixes: acd7ccb284b8 ("mm: shmem: add large folio support for tmpfs") >>> Signed-off-by: Baolin Wang >>> --- >>> Note: this is just an RFC patch. I would like to hear others' opinions or >>> see if there is a better way to address Hugh's concern. > > Sorry, I am still evaluating this RFC patch. > > Certainly I observe it taking us in the right direction, giving PMD-sized > pages on tmpfs huge=always, as 6.13 and earlier releases did - thank you. > > But the explosion of combinations which mTHP and FS large folios bring, > the amount that needs checking, is close to defeating me; and I've had > to spend a lot of the time re-educating myself on the background - > not looking to see whether this particular patch is right or not. > Still working on it. OK. Thanks. >> If we use this approach to fix the PMD large folio regression, should we also >> change tmpfs mmap() to allow allocating any sized large folios, but always try >> to allocate PMD-sized large folios first? What do you think? Thanks. > > Probably: I would like the mmap allocations to follow the same rules. > > But finding it a bit odd how the current implementation limits tmpfs > large folios to when huge=notnever (is that a fair statement?), whereas Yes, this is mainly to ensure backward compatibility with the 'huge=' options. Moreover, in the future, we could set the default value of ‘tmpfs_huge’ to ‘always’ (controlled via the cmdline: transparent_hugepage_tmpfs=) to allow all tmpfs mounts to use large folios by default. > other filesystems are now being freely given large folios - using > different GFP flags from what MM uses (closest to defrag=always I think), > and with no limitation - whereas MM folks are off devising ever newer > ways to restrict access to huge pages. > > And (conversely) I am unhappy with the way write and fallocate (and split > and collapse? in flight I think) are following the FS approach of allowing > every fractal, when mTHP/shmem_enabled is (or can be) more limiting. I > think it less surprising (and more efficient when fragmented) for shmem > FS operations to be restricted to the same subset as "shared anon". Understood. We discussed this before, but it didn’t get support :(