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 59AD851C5E for ; Wed, 8 May 2024 12:43:22 +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=1715172204; cv=none; b=V6iiaaK4JdtYkqRTjpDsBRzXQso5k+NXi/xPDllTTJI7RjycEyOCCqG8G6Z5pZhAZ3n7qpCySqvqdz02N1aEVIZ0N5Zdh8k6fHYdnr3cS1B2zfZooUcJWHCz3B7obYf0h4SbuUy4gm06cyamWWcR5fkU7shqzE72gO8GKqX3QKM= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1715172204; c=relaxed/simple; bh=siMa5/wUwvisBwU0iCVyBGFBjNUzxMzAbJQZJBk1IpQ=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=R4+IAyN5oD8amDasdipW7mJThWcJ3QouEbi4m7rkVA74M7uh7l/Hj9sVc2yDCRq8v5kQHa0iwcUXWTbiH5pEBVHwzcr/yXjFir3BV7ShnwZtP3xlT6oXXO3oToVPDUBhy8FXvdu90a9UKANCgk7/b7grNbmLyMQJXs2uJi+dRrA= 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 2A6401007; Wed, 8 May 2024 05:43:47 -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 B2D4E3F587; Wed, 8 May 2024 05:43:19 -0700 (PDT) Message-ID: <5c2c70fd-a291-4ca4-b229-dc54e92b3471@arm.com> Date: Wed, 8 May 2024 13:43:18 +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> From: Ryan Roberts In-Reply-To: <28f311ec-9b46-4f28-991c-ac74177acf32@redhat.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit 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?