Messages in this thread | | | Date | Wed, 8 May 2024 17:56:49 +0800 | Subject | Re: [PATCH 5/8] mm: shmem: add multi-size THP sysfs interface for anonymous shmem | From | Baolin Wang <> |
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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?
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