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

[David Hildenbrand]

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.

Note that you won't make many friends around here with statements like 
"To be honest, not using a logical page table for anonymous memory is 
why Linux THP fails compared with FreeBSD's superpage".

I read one paper that makes such claims (I'm curious how you define 
"winning"), and am aware of some shortcomings. But I am not convinced 
that a second datastructure "is why Linux THP fails". It just requires 
some more work to get it sorted under Linux (e.g., allocate THP, PTE-map 
it and map inaccessible parts PROT_NONE, later collapse it in-place into 
a PMD), and so far, there was not a lot of interest that I am ware of to 
even start working on that.

So if there is not enough pain for companies to even work on that in 
Linux, maybe FreeBSD superpages are "winning" "on paper" only? Remember 
that the target audience here are Linux developers.


But yeah, this here is all designed around the idea "core MM is extended 
to handle a unified virtual address space with external accelerators 
using customized MMUs." and then trying to find other arguments why it's 
a good idea, without going too much into detail why it's all unsolvable 
without that.

The first thing to sort out if we even want that, and some discussions 
here already went into the direction of "likely not". Let's see.

-- 
Cheers,

David / dhildenb