On 08/05/2024 13:45, David Hildenbrand wrote: > On 08.05.24 14:43, 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 don't know about "a lot of use cases", but for VMs that's certainly how it's > used. Gottya, thanks. And out of curiosity, what's the benefit of using tmpfs rather than private (or shared) anonymous memory for VMs?
