On 08/05/2024 10:56, Baolin Wang wrote: > > > 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@xxxxxxxxxx/ >>>>>> >>>>>> 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? Yeah ok, makes sense to me.
