Re: [PATCH v4 bpf-next 2/2] mm: Introduce VM_SPARSE kind and vm_area_[un]map_pages().

[Pasha Tatashin <pasha.tatashin@xxxxxxxxxx>]

On Mon, Mar 4, 2024 at 10:05 PM Alexei Starovoitov
<alexei.starovoitov@xxxxxxxxx> wrote:
>
> From: Alexei Starovoitov <ast@xxxxxxxxxx>
>
> vmap/vmalloc APIs are used to map a set of pages into contiguous kernel
> virtual space.
>
> get_vm_area() with appropriate flag is used to request an area of kernel
> address range. It's used for vmalloc, vmap, ioremap, xen use cases.
> - vmalloc use case dominates the usage. Such vm areas have VM_ALLOC flag.
> - the areas created by vmap() function should be tagged with VM_MAP.
> - ioremap areas are tagged with VM_IOREMAP.
>
> BPF would like to extend the vmap API to implement a lazily-populated
> sparse, yet contiguous kernel virtual space. Introduce VM_SPARSE flag
> and vm_area_map_pages(area, start_addr, count, pages) API to map a set
> of pages within a given area.
> It has the same sanity checks as vmap() does.
> It also checks that get_vm_area() was created with VM_SPARSE flag
> which identifies such areas in /proc/vmallocinfo
> and returns zero pages on read through /proc/kcore.
>
> The next commits will introduce bpf_arena which is a sparsely populated
> shared memory region between bpf program and user space process. It will
> map privately-managed pages into a sparse vm area with the following steps:
>
>   // request virtual memory region during bpf prog verification
>   area = get_vm_area(area_size, VM_SPARSE);
>
>   // on demand
>   vm_area_map_pages(area, kaddr, kend, pages);
>   vm_area_unmap_pages(area, kaddr, kend);
>
>   // after bpf program is detached and unloaded
>   free_vm_area(area);
>
> Signed-off-by: Alexei Starovoitov <ast@xxxxxxxxxx>

Reviewed-by: Pasha Tatashin <pasha.tatashin@xxxxxxxxxx>