[RFC PATCH 2/6] mm/gmem: add arch-independent abstraction to track address mapping status

[David Hildenbrand]

On 02.12.23 15:50, Pedro Falcato wrote:
> On Fri, Dec 1, 2023 at 9:23 AM David Hildenbrand <david at redhat.com> wrote:
>>
>> On 28.11.23 13:50, Weixi Zhu wrote:
>>> This patch adds an abstraction layer, struct vm_object, that maintains
>>> per-process virtual-to-physical mapping status stored in struct gm_mapping.
>>> For example, a virtual page may be mapped to a CPU physical page or to a
>>> device physical page. Struct vm_object effectively maintains an
>>> arch-independent page table, which is defined as a "logical page table".
>>> While arch-dependent page table used by a real MMU is named a "physical
>>> page table". The logical page table is useful if Linux core MM is extended
>>> to handle a unified virtual address space with external accelerators using
>>> customized MMUs.
>>
>> Which raises the question why we are dealing with anonymous memory at
>> all? Why not go for shmem if you are already only special-casing VMAs
>> with a MMAP flag right now?
>>
>> That would maybe avoid having to introduce controversial BSD design
>> concepts into Linux, that feel like going a step backwards in time to me
>> and adding *more* MM complexity.
>>
>>>
>>> In this patch, struct vm_object utilizes a radix
>>> tree (xarray) to track where a virtual page is mapped to. This adds extra
>>> memory consumption from xarray, but provides a nice abstraction to isolate
>>> mapping status from the machine-dependent layer (PTEs). Besides supporting
>>> accelerators with external MMUs, struct vm_object is planned to further
>>> union with i_pages in struct address_mapping for file-backed memory.
>>
>> A file already has a tree structure (pagecache) to manage the pages that
>> are theoretically mapped. It's easy to translate from a VMA to a page
>> inside that tree structure that is currently not present in page tables.
>>
>> Why the need for that tree structure if you can just remove anon memory
>> from the picture?
>>
>>>
>>> The idea of struct vm_object is originated from FreeBSD VM design, which
>>> provides a unified abstraction for anonymous memory, file-backed memory,
>>> page cache and etc[1].
>>
>> :/
>>
>>> Currently, Linux utilizes a set of hierarchical page walk functions to
>>> abstract page table manipulations of different CPU architecture. The
>>> problem happens when a device wants to reuse Linux MM code to manage its
>>> page table -- the device page table may not be accessible to the CPU.
>>> Existing solution like Linux HMM utilizes the MMU notifier mechanisms to
>>> invoke device-specific MMU functions, but relies on encoding the mapping
>>> status on the CPU page table entries. This entangles machine-independent
>>> code with machine-dependent code, and also brings unnecessary restrictions.
>>
>> Why? we have primitives to walk arch page tables in a non-arch specific
>> fashion and are using them all over the place.
>>
>> We even have various mechanisms to map something into the page tables
>> and get the CPU to fault on it, as if it is inaccessible (PROT_NONE as
>> used for NUMA balancing, fake swap entries).
>>
>>> The PTE size and format vary arch by arch, which harms the extensibility.
>>
>> Not really.
>>
>> We might have some features limited to some architectures because of the
>> lack of PTE bits. And usually the problem is that people don't care
>> enough about enabling these features on older architectures.
>>
>> If we ever *really* need more space for sw-defined data, it would be
>> possible to allocate auxiliary data for page tables only where required
>> (where the features apply), instead of crafting a completely new,
>> auxiliary datastructure with it's own locking.
>>
>> So far it was not required to enable the feature we need on the
>> architectures we care about.
>>
>>>
>>> [1] https://docs.freebsd.org/en/articles/vm-design/
>>
>> In the cover letter you have:
>>
>> "The future plan of logical page table is to provide a generic
>> abstraction layer that support common anonymous memory (I am looking at
>> you, transparent huge pages) and file-backed memory."
>>
>> Which I doubt will happen; there is little interest in making anonymous
>> memory management slower, more serialized, and wasting more memory on
>> metadata.
> 
> Also worth noting that:
> 
> 1) Mach VM (which FreeBSD inherited, from the old BSD) vm_objects
> aren't quite what's being stated here, rather they are somewhat
> replacements for both anon_vma and address_space[1]. Very similarly to
> Linux, they take pages from vm_objects and map them in page tables
> using pmap (the big difference is anon memory, which has its
> bookkeeping in page tables, on Linux)
> 
> 2) These vm_objects were a horrendous mistake (see CoW chaining) and
> FreeBSD has to go to horrendous lengths to make them tolerable. The
> UVM paper/dissertation (by Charles Cranor) talks about these issues at
> length, and 20 years later it's still true.
> 
> 3) Despite Linux MM having its warts, it's probably correct to
> consider it a solid improvement over FreeBSD MM or NetBSD UVM
> 
> And, finally, randomly tacking on core MM concepts from other systems
> is at best a *really weird* idea. Particularly when they aren't even
> what was stated!

Can you read my mind? :) thanks for noting all that, with which I 100% 
agree.

-- 
Cheers,

David / dhildenb