From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id 1A89753389 for ; Wed, 8 May 2024 12:45:00 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=217.140.110.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1715172303; cv=none; b=NvXaFUUx6oU52hqBZla2805r1cL6eUp327loIyMm9bsm8Uu5no6BQv28T191BZNFlY9UTlleoxt36tjAW95Mb4bIMyfDcDmjfDAyftK5W1qa4rNRjtDBqFAD67uC+iduXcGG+HxEir22lmfZFAB/QICvGlUPcyqMKGJFrXfoMuw= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1715172303; c=relaxed/simple; bh=G6mMO8Hw203WkjN6OLfUWlZeQIa0p1RukkdeLtcAqmg=; h=Message-ID:Date:MIME-Version:Subject:From:To:Cc:References: In-Reply-To:Content-Type; b=sTKj3RUV9rJL2nqyrwjmhp95K7QQ6itu8/HT5TzXNmDBUCbzAkCvgdHoI9zlDbjzkEwoSA4DnVlh4SPlxEwxbnjOYbs0uFQvR4xUPrlfrRTwA0osbuz2Gko9CL9rq2UXMXdqTrkp5iCOq9H8AAUthUZwDmK7eb3wgpw5vEgIGtc= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com; spf=pass smtp.mailfrom=arm.com; arc=none smtp.client-ip=217.140.110.172 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=arm.com Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 27F421007; Wed, 8 May 2024 05:45:26 -0700 (PDT) Received: from [10.57.67.194] (unknown [10.57.67.194]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id B61523F587; Wed, 8 May 2024 05:44:58 -0700 (PDT) Message-ID: Date: Wed, 8 May 2024 13:44:57 +0100 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 Content-Language: en-GB From: Ryan Roberts To: David Hildenbrand , Baolin Wang , 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> <928c73de-76b0-40d6-a0c3-23d72270ac5c@redhat.com> <28f311ec-9b46-4f28-991c-ac74177acf32@redhat.com> <5c2c70fd-a291-4ca4-b229-dc54e92b3471@arm.com> In-Reply-To: <5c2c70fd-a291-4ca4-b229-dc54e92b3471@arm.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On 08/05/2024 13:43, Ryan Roberts wrote: > On 08/05/2024 13:10, David Hildenbrand wrote: >> On 08.05.24 14:02, David Hildenbrand wrote: >>> On 08.05.24 11:02, 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). >>> >>> Note that with shmem you're inherently facing the same memory waste >>> issues etc as you would with anonymous memory. (sometimes even worse, if >>> you're running shmem that's configured to be unswappable!). >> >> Also noting that memory waste is not really a problem when a write to a shmem >> file allocates a large folio that stays within boundaries of that write; issues >> only pop up if you end up over-allocating, especially, during page faults where >> you have not that much clue about what to do (single address, no real range >> provided). >> >> There is the other issue that wasting large chunks of contiguous memory on stuff >> that barely benefits from it. With memory that maybe never gets evicted, there >> is no automatic "handing back" of that memory to the system to be used by >> something else. With ordinary files, that's a bit different. But I did not look >> closer into that issue yet, it's one of the reasons MADV_HUGEPAGE was added IIRC. > > OK understood. Although, with tmpfs you're not going to mmap it then randomly > extend the file through page faults - mmap doesn't permit that, I don't think? > So presumably the user must explicitly set the size of the file first? Are you > suggesting there are a lot of use cases where a large tmpfs file is created, > mmaped then only accessed sparsely? I know that's often the case for anon memory, but not sure if you would expect the same pattern with an explicit file?