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

[Pasha Tatashin <pasha.tatashin@soleen.com>]

On Mon, Mar 4, 2024 at 10:05 PM Alexei Starovoitov
<alexei.starovoitov@gmail.com> wrote:
>
> From: Alexei Starovoitov <ast@kernel.org>
>
> 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@kernel.org>
> ---
>  include/linux/vmalloc.h |  5 ++++
>  mm/vmalloc.c            | 59 +++++++++++++++++++++++++++++++++++++++--
>  2 files changed, 62 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/vmalloc.h b/include/linux/vmalloc.h
> index c720be70c8dd..0f72c85a377b 100644
> --- a/include/linux/vmalloc.h
> +++ b/include/linux/vmalloc.h
> @@ -35,6 +35,7 @@ struct iov_iter;              /* in uio.h */
>  #else
>  #define VM_DEFER_KMEMLEAK      0
>  #endif
> +#define VM_SPARSE              0x00001000      /* sparse vm_area. not all pages are present. */
>
>  /* bits [20..32] reserved for arch specific ioremap internals */
>
> @@ -232,6 +233,10 @@ static inline bool is_vm_area_hugepages(const void *addr)
>  }
>
>  #ifdef CONFIG_MMU
> +int vm_area_map_pages(struct vm_struct *area, unsigned long start,
> +                     unsigned long end, struct page **pages);
> +void vm_area_unmap_pages(struct vm_struct *area, unsigned long start,
> +                        unsigned long end);
>  void vunmap_range(unsigned long addr, unsigned long end);
>  static inline void set_vm_flush_reset_perms(void *addr)
>  {
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index f42f98a127d5..e5b8c70950bc 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -648,6 +648,58 @@ static int vmap_pages_range(unsigned long addr, unsigned long end,
>         return err;
>  }
>
> +static int check_sparse_vm_area(struct vm_struct *area, unsigned long start,
> +                               unsigned long end)
> +{
> +       might_sleep();

This interface and in general VM_SPARSE would be useful for
dynamically grown kernel stacks [1]. However, the might_sleep() here
would be a problem. We would need to be able to handle
vm_area_map_pages() from interrupt disabled context therefore no
sleeping. The caller would need to guarantee that the page tables are
pre-allocated before the mapping.

Pasha

[1] https://lore.kernel.org/all/CA+CK2bBYt9RAVqASB2eLyRQxYT5aiL0fGhUu3TumQCyJCNTWvw@mail.gmail.com