On 08/05/2024 13:43, Ryan Roberts wrote: > 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? I know that's often the case for anon memory, but not sure if you would expect the same pattern with an explicit file?
