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@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). >> >> 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?
