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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 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 X-Rspamd-Server: rspam01 X-Stat-Signature: o14kdga5spepoee8asu7zc1cq36uqcms X-Rspam-User: X-Rspamd-Queue-Id: 6DAA5C0017 X-HE-Tag: 1715162215-313464 X-HE-Meta: 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 8sIIDLA5 8VSg0WwB4Di8YtZmBM35P6NViB+zLZ3NcjG2MSaKZvu7SEzQJbzMx8CEyotzcadTMsVbXDB0Vv327rvDea7XKaN4yasjCiyUaW0lshA1eV/n5AXkT1U3+60RrIsyMakAPV24vM70BuspoN8G57ZT8HpwsFVFUzMwfqBbBtFBidmik6hYO8Lh/IXrz+z1FXsy0IOIrr05eOHFaxPV/XAJC15Wjuxy6/nZBzkN0lX0I7+cdKlQRoeb3Sk0fWP8N4fiuNtS65m7RZ1ugaxejjW2GzzhCbzh9q286teWvuRI4ShLbtsVMN4l6epEJKgBrYRI0/d8wx/+bKknnYxrZjzXoC+nbKCmZcAWhu+YGI2xeSfHWeJzftg/uto7ELoiWilv2xMGX X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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?