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charset=UTF-8 Content-Transfer-Encoding: 8bit X-Rspamd-Queue-Id: 6B0441A0010 X-Rspam-User: X-Rspamd-Server: rspam04 X-Stat-Signature: w9ij18zucxdh3wt5s59xdgu5g7f1hxmr X-HE-Tag: 1715172875-681263 X-HE-Meta: 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 NODJHTFK pfSQJYTbxpQhBldQFTKdPOVEtwhNvjpOvbS5Cf7pStvbVjJweRvYOgMljOwDL9S3OjOOiMrCwTVOrt+Em16THCtQSMAmlNiq6nlqK5ZwMTydpWojh253/B6gw44aXhLPAeWpAhEBhsqJgKwou93NGSeq2Oq4YLbihY6TXudYkPQKz65/qpq+ZggATFNL33+PjsxMH/iJhSC5PvZ9np1h3l63WoU8lJu8mcfbQPFDGemXXuBJnQMjBPhSIpXsqhxwy/kwd X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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@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). >>>> >>>> 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?