On Thu, Nov 14, 2019 at 08:02:23PM -0800, Andrii Nakryiko wrote:
> Add ability to memory-map contents of BPF array map. This is extremely useful
> for working with BPF global data from userspace programs. It allows to avoid
> typical bpf_map_{lookup,update}_elem operations, improving both performance
> and usability.
>
> There had to be special considerations for map freezing, to avoid having
> writable memory view into a frozen map. To solve this issue, map freezing and
> mmap-ing is happening under mutex now:
> - if map is already frozen, no writable mapping is allowed;
> - if map has writable memory mappings active (accounted in map->writecnt),
> map freezing will keep failing with -EBUSY;
> - once number of writable memory mappings drops to zero, map freezing can be
> performed again.
>
> Only non-per-CPU plain arrays are supported right now. Maps with spinlocks
> can't be memory mapped either.
>
> For BPF_F_MMAPABLE array, memory allocation has to be done through vmalloc()
> to be mmap()'able. We also need to make sure that array data memory is
> page-sized and page-aligned, so we over-allocate memory in such a way that
> struct bpf_array is at the end of a single page of memory with array->value
> being aligned with the start of the second page. On deallocation we need to
> accomodate this memory arrangement to free vmalloc()'ed memory correctly.
>
> One important consideration regarding how memory-mapping subsystem functions.
> Memory-mapping subsystem provides few optional callbacks, among them open()
> and close(). close() is called for each memory region that is unmapped, so
> that users can decrease their reference counters and free up resources, if
> necessary. open() is *almost* symmetrical: it's called for each memory region
> that is being mapped, **except** the very first one. So bpf_map_mmap does
> initial refcnt bump, while open() will do any extra ones after that. Thus
> number of close() calls is equal to number of open() calls plus one more.
>
> Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> Cc: Rik van Riel <riel@xxxxxxxxxxx>
> Acked-by: Song Liu <songliubraving@xxxxxx>
> Acked-by: John Fastabend <john.fastabend@xxxxxxxxx>
> Signed-off-by: Andrii Nakryiko <andriin@xxxxxx>
> ---
> include/linux/bpf.h | 11 ++--
> include/linux/vmalloc.h | 1 +
> include/uapi/linux/bpf.h | 3 ++
> kernel/bpf/arraymap.c | 59 +++++++++++++++++---
> kernel/bpf/syscall.c | 99 ++++++++++++++++++++++++++++++++--
> mm/vmalloc.c | 20 +++++++
> tools/include/uapi/linux/bpf.h | 3 ++
> 7 files changed, 184 insertions(+), 12 deletions(-)
>
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index 6fbe599fb977..8021fce98868 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -12,6 +12,7 @@
> #include <linux/err.h>
> #include <linux/rbtree_latch.h>
> #include <linux/numa.h>
> +#include <linux/mm_types.h>
> #include <linux/wait.h>
> #include <linux/u64_stats_sync.h>
>
> @@ -66,6 +67,7 @@ struct bpf_map_ops {
> u64 *imm, u32 off);
> int (*map_direct_value_meta)(const struct bpf_map *map,
> u64 imm, u32 *off);
> + int (*map_mmap)(struct bpf_map *map, struct vm_area_struct *vma);
> };
>
> struct bpf_map_memory {
> @@ -94,9 +96,10 @@ struct bpf_map {
> u32 btf_value_type_id;
> struct btf *btf;
> struct bpf_map_memory memory;
> + char name[BPF_OBJ_NAME_LEN];
> bool unpriv_array;
> - bool frozen; /* write-once */
> - /* 48 bytes hole */
> + bool frozen; /* write-once; write-protected by freeze_mutex */
> + /* 22 bytes hole */
>
> /* The 3rd and 4th cacheline with misc members to avoid false sharing
> * particularly with refcounting.
> @@ -104,7 +107,8 @@ struct bpf_map {
> atomic64_t refcnt ____cacheline_aligned;
> atomic64_t usercnt;
> struct work_struct work;
> - char name[BPF_OBJ_NAME_LEN];
> + struct mutex freeze_mutex;
> + u64 writecnt; /* writable mmap cnt; protected by freeze_mutex */
> };
Can the mutex be moved into bpf_array instead of being in bpf_map that is
shared across all map types?
If so then can you reuse the mutex that Daniel is adding in his patch 6/8
of tail_call series? Not sure what would the right name for such mutex.
It will be used for your map_freeze logic and for Daniel's text_poke.
