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 7D71E3FE3F for ; Wed, 8 May 2024 12:54:35 +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=1715172877; cv=none; b=mQUSGi/17HETZcg3jJLb30lg6YWSVeDjb6Jfi4Uxr0MotPcU9akyG3U8lHHfISNOF0hLOU3Jov3kxzkOsOM3MlCkqzf0a8n4sckyCGZ5XRIym0FMuFOHAz2TXE/nKX9abCXkMmr3nxLySR5JR7x1mlhmnPhj7gI6qpxg1MqcOiQ= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1715172877; c=relaxed/simple; bh=cD/8PKb9NB8l6lxsvNCcYp6Oj0siztPfOupqTFXX8zE=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=QKbL1KrT0/nMOrOadxeERM7enzuwCDnDrzAUcyVGXY5KfvRIRYP4OGsp3EO0T8Qi1L+kFsyVjiBMahtdQZaUG2J28eB9l+2Y8NpHL46hEVhGJGgFrQZL3eirjC6jmD3JaxUWTSLlt+lVMxNxk7kgGsVd/S6xd0UMqpOPBw1UKkg= 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 485B41007; Wed, 8 May 2024 05:55:00 -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 D86E73F6A8; Wed, 8 May 2024 05:54:32 -0700 (PDT) Message-ID: <578e878b-259e-4944-99c2-9caf578e9642@arm.com> Date: Wed, 8 May 2024 13:54:31 +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 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> From: Ryan Roberts In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On 08/05/2024 13:45, David Hildenbrand wrote: > On 08.05.24 14:43, 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 don't know about "a lot of use cases", but for VMs that's certainly how it's > used. Gottya, thanks. And out of curiosity, what's the benefit of using tmpfs rather than private (or shared) anonymous memory for VMs?