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 97EEA79949 for ; Wed, 8 May 2024 09:56:55 +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=1715162218; cv=none; b=h+1u5tLdlm6yZ3UIE2HEjzrKdkYpcw06Pca7pbfrrVEXTRssUfgFOn6EBkwmLtR7OScFA9F5dB8OVontuHa5hM8JfsS/eLO3A9m6MC51DZyVz0ZqaG+0qITc5Yb/EI1DcEnGP9dRb75Su94coIXDHipZYa/9aehv66rOqqnoQIw= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1715162218; c=relaxed/simple; bh=9xNyjQkipGmx1mN5SWcI9+/ajbVZSr+JzYwwStd2HpA=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=PA3rtM6uY9292MdUCMUVqqRqNSGdYHQo8DC4BhwwclgHdh0LGhcYZnmhY7O2UQ/zoypa0WndVoIjVWG8VX4n2kZJuuKqzmrTdDwobaAQz3Gm/v/WvaVGdW6k6iu1GUYbqXlSjo5NZALvVnphRQ7sMPCK7cXut9jZuBFduvlawI8= 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=mBNYTKZH; 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="mBNYTKZH" DKIM-Signature:v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.alibaba.com; s=default; t=1715162213; h=Message-ID:Date:MIME-Version:Subject:To:From:Content-Type; bh=9mDT2UAsElMka1cWBGabDh1k3Fz10LnTvOvoW1OfKOU=; b=mBNYTKZH+bmd49LqRhJXZACvWk+Ac3Ad55rLLtxeXIeVz/1B9vSkBXNLCVXXEZV/3r/g9tqwOLgIJTmh27+QyEYG7UTKS1EXTej7xZBb1I3pp1gqqfwktqnr2jASbkbBgFTXy+QT+0LZJM42sLgOfDu2rAFAfRkrWx3hynuyzrE= X-Alimail-AntiSpam:AC=PASS;BC=-1|-1;BR=01201311R891e4;CH=green;DM=||false|;DS=||;FP=0|-1|-1|-1|0|-1|-1|-1;HT=maildocker-contentspam033037067113;MF=baolin.wang@linux.alibaba.com;NM=1;PH=DS;RN=13;SR=0;TI=SMTPD_---0W63AqH0_1715162209; Received: from 30.97.56.69(mailfrom:baolin.wang@linux.alibaba.com fp:SMTPD_---0W63AqH0_1715162209) by smtp.aliyun-inc.com; Wed, 08 May 2024 17:56:51 +0800 Message-ID: <5b23a706-5fb3-41b4-ba2e-d38a29b51820@linux.alibaba.com> Date: Wed, 8 May 2024 17:56:49 +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 5/8] mm: shmem: add multi-size THP sysfs interface for anonymous shmem To: Ryan Roberts , David Hildenbrand , akpm@linux-foundation.org, hughd@google.com Cc: willy@infradead.org, ioworker0@gmail.com, wangkefeng.wang@huawei.com, ying.huang@intel.com, 21cnbao@gmail.com, shy828301@gmail.com, ziy@nvidia.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <6b4afed1ef26dbd08ae9ec58449b329564dcef3e.1714978902.git.baolin.wang@linux.alibaba.com> <30329a82-45b9-4e78-8c48-bd56af113786@arm.com> <0b3735bc-2ad7-44f8-808b-37fc90d57199@linux.alibaba.com> From: Baolin Wang In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 2024/5/8 17:02, Ryan Roberts wrote: > On 08/05/2024 08:12, David Hildenbrand wrote: >> On 08.05.24 09:08, David Hildenbrand wrote: >>> On 08.05.24 06:45, Baolin Wang wrote: >>>> >>>> >>>> On 2024/5/7 18:52, Ryan Roberts wrote: >>>>> On 06/05/2024 09:46, Baolin Wang wrote: >>>>>> To support the use of mTHP with anonymous shmem, add a new sysfs interface >>>>>> 'shmem_enabled' in the '/sys/kernel/mm/transparent_hugepage/hugepages-kB/' >>>>>> directory for each mTHP to control whether shmem is enabled for that mTHP, >>>>>> with a value similar to the top level 'shmem_enabled', which can be set to: >>>>>> "always", "inherit (to inherit the top level setting)", "within_size", >>>>>> "advise", >>>>>> "never", "deny", "force". These values follow the same semantics as the top >>>>>> level, except the 'deny' is equivalent to 'never', and 'force' is equivalent >>>>>> to 'always' to keep compatibility. >>>>> >>>>> We decided at [1] to not allow 'force' for non-PMD-sizes. >>>>> >>>>> [1] >>>>> https://lore.kernel.org/linux-mm/533f37e9-81bf-4fa2-9b72-12cdcb1edb3f@redhat.com/ >>>>> >>>>> However, thinking about this a bit more, I wonder if the decision we made to >>>>> allow all hugepages-xxkB/enabled controls to take "inherit" was the wrong one. >>>>> Perhaps we should have only allowed the PMD-sized enable=inherit (this is just >>>>> for legacy back compat after all, I don't think there is any use case where >>>>> changing multiple mTHP size controls atomically is actually useful). Applying >>>> >>>> Agree. This is also our usage of 'inherit'. >> >> Missed that one: there might be use cases in the future once we would start >> defaulting to "inherit" for all knobs (a distro might default to that) and >> default-enable THP in the global knob. Then, it would be easy to disable any THP >> by disabling the global knob. (I think that's the future we're heading to, where >> we'd have an "auto" mode that can be set on the global toggle). >> >> But I am just making up use cases ;) I think it will be valuable and just doing >> it consistently now might be cleaner. > > I agree that consistency between enabled and shmem_enabled is top priority. And > yes, I had forgotten about the glorious "auto" future. So probably continuing > all sizes to select "inherit" is best. > > But for shmem_enabled, that means we need the following error checking: > > - It is an error to set "force" for any size except PMD-size > > - It is an error to set "force" for the global control if any size except PMD- > size is set to "inherit" > > - It is an error to set "inherit" for any size except PMD-size if the global > control is set to "force". > > Certainly not too difficult to code and prove to be correct, but not the nicest > UX from the user's point of view when they start seeing errors. > > I think we previously said this would likely be temporary, and if/when tmpfs > gets mTHP support, we could simplify and allow all sizes to be set to "force". > But I wonder if tmpfs would ever need explicit mTHP control? Maybe it would be > more suited to the approach the page cache takes to transparently ramp up the > folio size as it faults more in. (Just saying there is a chance that this error > checking becomes permanent). The strategy for tmpfs supporting mTHP will require more discussions and evaluations in the future. However, regardless of the strategy (explicit mTHP control or page cache control), I think it would be possible to use 'force' to override previous strategies for some testing purposes. This appears to be permissible according to the explanation in the current documentation: "force the huge option on for all - very useful for testing". So it seems not permanent?