Re: [LSF/MM/BPF TOPIC] Guaranteed CMA

[David Hildenbrand <david@redhat.com>]

On 10.10.25 03:30, Suren Baghdasaryan wrote:
> On Mon, Sep 1, 2025 at 9:01 AM David Hildenbrand <david@redhat.com> wrote:
>>
>> On 27.08.25 02:17, Suren Baghdasaryan wrote:
>>> On Tue, Aug 26, 2025 at 1:58 AM David Hildenbrand <david@redhat.com> wrote:
>>>>
>>>> On 23.08.25 00:14, Suren Baghdasaryan wrote:
>>>>> On Wed, Apr 2, 2025 at 9:35 AM Suren Baghdasaryan <surenb@google.com> wrote:
>>>>>>
>>>>>> On Thu, Mar 20, 2025 at 11:06 AM Suren Baghdasaryan <surenb@google.com> wrote:
>>>>>>>
>>>>>>> On Tue, Feb 4, 2025 at 8:33 AM Suren Baghdasaryan <surenb@google.com> wrote:
>>>>>>>>
>>>>>>>> On Tue, Feb 4, 2025 at 3:23 AM Alexandru Elisei
>>>>>>>> <alexandru.elisei@arm.com> wrote:
>>>>>>>>>
>>>>>>>>> Hi,
>>>>>>>>>
>>>>>>>>> On Tue, Feb 04, 2025 at 09:18:20AM +0100, David Hildenbrand wrote:
>>>>>>>>>> On 02.02.25 01:19, Suren Baghdasaryan wrote:
>>>>>>>>>>> Hi,
>>>>>>>>>>
>>>>>>>>>> Hi,
>>>>>>>>>>
>>>>>>>>>>> I would like to discuss the Guaranteed Contiguous Memory Allocator
>>>>>>>>>>> (GCMA) mechanism that is being used by many Android vendors as an
>>>>>>>>>>> out-of-tree feature, collect input on its possible usefulness for
>>>>>>>>>>> others, feasibility to upstream and suggestions for possible better
>>>>>>>>>>> alternatives.
>>>>>>>>>>>
>>>>>>>>>>> Problem statement: Some workloads/hardware require physically
>>>>>>>>>>> contiguous memory and carving out reserved memory areas for such
>>>>>>>>>>> allocations often lead to inefficient usage of those carveouts. CMA
>>>>>>>>>>> was designed to solve this inefficiency by allowing movable memory
>>>>>>>>>>> allocations to use this reserved memory when it’s otherwise unused.
>>>>>>>>>>> When a contiguous memory allocation is requested, CMA finds the
>>>>>>>>>>> requested contiguous area, possibly migrating some of the movable
>>>>>>>>>>> pages out of that area.
>>>>>>>>>>> In latency-sensitive use cases, like face unlock on phones, we need to
>>>>>>>>>>> allocate contiguous memory quickly and page migration in CMA takes
>>>>>>>>>>> enough time to cause user-perceptible lag. Such allocations can also
>>>>>>>>>>> fail if page migration is not possible.
>>>>>>>>>>>
>>>>>>>>>>> GCMA (Guaranteed CMA) is a mechanism previously proposed in [1] which
>>>>>>>>>>> was not upstreamed but got adopted later by many Android vendors as an
>>>>>>>>>>> out-of-tree feature. It is similar to CMA but backing memory is
>>>>>>>>>>> cleancache backend, containing only clean file-backed pages. Most
>>>>>>>>>>> importantly, the kernel can’t take a reference to pages from the
>>>>>>>>>>> cleancache, therefore can’t prevent GCMA from quickly dropping them
>>>>>>>>>>> when required. This guarantees GCMA low allocation latency and
>>>>>>>>>>> improves allocation success rate.
>>>>>>>>>>>
>>>>>>>>>>> We would like to standardize GCMA implementation and upstream it since
>>>>>>>>>>> many Android vendors are asking to include it as a generic feature.
>>>>>>>>>>>
>>>>>>>>>>> Note: removal of cleancache in 5.17 kernel due to no users (sorry, we
>>>>>>>>>>> didn’t know at the time about this use case) might complicate
>>>>>>>>>>> upstreaming.
>>>>>>>>>>
>>>>>>>>>> we discussed another possible user last year: using MTE tag storage memory
>>>>>>>>>> while the storage is not getting used to store MTE tags [1].
>>>>>>>>>>
>>>>>>>>>> As long as the "ordinary RAM" that maps to a given MTE tag storage area does
>>>>>>>>>> not use MTE tagging, we can reuse the MTE tag storage ("almost ordinary RAM,
>>>>>>>>>> just that it doesn't support MTE itself") for different purposes.
>>>>>>>>>>
>>>>>>>>>> We need a guarantee that that memory can be freed up / migrated once the tag
>>>>>>>>>> storage gets activated.
>>>>>>>>>
>>>>>>>>> If I remember correctly, one of the issues with the MTE project that might be
>>>>>>>>> relevant to GCMA, was that userspace, once it gets a hold of a page, it can pin
>>>>>>>>> it for a very long time without specifying FOLL_LONGTERM.
>>>>>>>>>
>>>>>>>>> If I remember things correctly, there were two examples given for this; there
>>>>>>>>> might be more, or they might have been eliminated since then:
>>>>>>>>>
>>>>>>>>> * The page is used as a buffer for accesses to a file opened with
>>>>>>>>>      O_DIRECT.
>>>>>>>>>
>>>>>>>>> * 'vmsplice() can pin pages forever and doesn't use FOLL_LONGTERM yet' - that's
>>>>>>>>>      a direct quote from David [1].
>>>>>>>>>
>>>>>>>>> Depending on your usecases, failing the allocation might be acceptable, but for
>>>>>>>>> MTE that wasn't the case.
>>>>>>>>>
>>>>>>>>> Hope some of this is useful.
>>>>>>>>>
>>>>>>>>> [1] https://lore.kernel.org/linux-arm-kernel/4e7a4054-092c-4e34-ae00-0105d7c9343c@redhat.com/
>>>>>>>>
>>>>>>>> Thanks for the references! I'll read through these discussions to see
>>>>>>>> how much useful information for GCMA I can extract.
>>>>>>>
>>>>>>> I wanted to get an RFC code ahead of LSF/MM and just finished putting
>>>>>>> it together. Sorry for the last minute posting. You can find it here:
>>>>>>> https://lore.kernel.org/all/20250320173931.1583800-1-surenb@google.com/
>>>>>>
>>>>>> Sorry about the delay. Attached are the slides from my GCMA
>>>>>> presentation at the conference.
>>>>>
>>>>> Hi Folks,
>>>>
>>>> Hi,
>>>>
>>>>> As I'm getting close to finalizing the GCMA patchset, one question
>>>>> keeps bugging me. How do we account the memory that is allocated from
>>>>> GCMA... In case of CMA allocations, they are backed by the system
>>>>> memory, so accounting is straightforward, allocations contribute to
>>>>> RSS, counted towards memcg limits, etc. In case of GCMA, the backing
>>>>> memory is reserved memory (a carveout) not directly accessible by the
>>>>> rest of the system and not part of the total_memory. So, if a process
>>>>> allocates a buffer from GCMA, should it be accounted as a normal
>>>>> allocation from system memory or as something else entirely? Any
>>>>> thoughts?
>>>>
>>>> You mean, an application allocates the memory and maps it into its page
>>>> tables?
>>>
>>> Allocation will happen via cma_alloc() or a similar interface, so
>>> applications would have to use some driver to allocate from GCMA. Once
>>> allocated, an application can map that memory if the driver supports
>>> mapping.
>>
>> Right, and that might happen either through a VM_PFNMAP or !VM_PFNMAP
>> (ordinarily ref- and currently map-counted).
>>
>> In the insert_page() case we do an inc_mm_counter, which increases the RSS.
>>
>> That could happen with pages from carevouts (memblock allocations)
>> already, but we don't run into that in general I assume.
>>
>>>
>>>>
>>>> Can that memory get reclaimed somehow?
>>>
>>> Hmm. I assume that once a driver allocates pages from GCMA it won't
>>> put them into system-managed LRU or free them into buddy allocator for
>>> kernel to use. If it does then at the time of cma_release() it can't
>>> guarantee there are no more users for such pages.
>>>
>>>>
>>>> How would we be mapping these pages into processes (VM_PFNMAP or
>>>> "normal" mappings)?
>>>
>>> They would be normal mappings as the pages do have `struct page` but I
>>> expect these pages to be managed by the driver that allocated them
>>> rather than the core kernel itself.
>>>
>>> I was trying to design GCMA to be used as close to CMA as possible so
>>> that we can use the same cma_alloc/cma_release API and reuse CMA's
>>> page management code but the fact that CMA is backed by the system
>>> memory and GCMA is backed by a carveout makes it a bit difficult.
>>
>> Makes sense. So I assume memcg does not apply here already -- memcg does
>> not apply on the CMA layer IIRC.
>>
>> The RSS is a bit tricky. We would have to modify things like
>> inc_mm_counter() to special-case on these things.
>>
>> But then, smaps output would still count these pages towards the rss/pss
>> (e.g., mss->resident). So that needs care as well ...
> 
> In the end I decided to follow CMA as closely as possible, including
> accounting. GCMA and CMA both use reserved area and the difference is
> that CMA donates its memory to kernel to use for movable allocations
> while GCMA donates it to the cleancache. But once that donation is
> taken back by CMA/GCMA to satisfy cma_alloc() request, the memory
> usage is pretty much the same and therefore accounting should probably
> be the same. Anyway, that was the reasoning I eventually arrived at. I
> posted the GCMA patchset at [1] and included this reasoning in the
> cover letter. Happy to discuss this further in that patchset.

Right, probably best to keep it simple. Will these GCMA pages be 
accounted towards MemTotal like CMA pages would?

-- 
Cheers

David / dhildenb