Re: [PATCH 2/3] bpf: Add sleepable arena page allocation path

["Alexei Starovoitov" <alexei.starovoitov@gmail.com>]

On Wed Sep 23, 2026 at 9:48 PM UTC, Emil Tsalapatis wrote:
> On Wed, Aug 26, 2026 at 1:25 AM Alexei Starovoitov
> <alexei.starovoitov@gmail.com> wrote:
>>
>> On Mon Aug 24, 2026 at 1:25 AM PDT, Emil Tsalapatis wrote:
>> > The bpf_arena_alloc_pages() function currently only allocates pages
>> > inside a spinlock critical section with IRQs off. This forces the use
>> > of alloc_pages_nolock() in the BPF allocator, even when the caller is
>> > a sleepable BPF function. This in turn causes allocation failures even
>> > in cases where falling into the allocator slow path and possibly
>> > sleeping would eventually succeed. This can be triggered consistently
>> > by heavy BPF arena users like scx.
>> >
>> > Add a separate arena page allocation path just for sleepable callers.
>> > The path preallocates the arena memory to be added to the tree before
>> > taking the critical section.
>> >
>> > Signed-off-by: Emil Tsalapatis <emil@etsalapatis.com>
>> > ---
>> >  include/linux/bpf.h  |  6 ++++
>> >  kernel/bpf/arena.c   | 76 +++++++++++++++++++++++++++++++++++++++++---
>> >  kernel/bpf/syscall.c |  8 +----
>> >  3 files changed, 79 insertions(+), 11 deletions(-)
>> >
>> > diff --git a/include/linux/bpf.h b/include/linux/bpf.h
>> > index ffa5626411ac..d15ed7a3879b 100644
>> > --- a/include/linux/bpf.h
>> > +++ b/include/linux/bpf.h
>> > @@ -710,6 +710,12 @@ void bpf_map_free_internal_structs(struct bpf_map *map, void *obj);
>> >  int bpf_dynptr_from_file_sleepable(struct file *file, u32 flags,
>> >                                  struct bpf_dynptr *ptr__uninit);
>> >
>> > +static inline bool is_bpf_alloc_nonsleepable(void)
>> > +{
>> > +     return preempt_count() > 0 || irqs_disabled() ||
>> > +             IS_ENABLED(CONFIG_PREEMPT_RT);
>> > +}
>> > +
>> >  #if defined(CONFIG_MMU) && defined(CONFIG_64BIT)
>> >  void *bpf_arena_alloc_pages_non_sleepable(void *p__map, void *addr__ign, u32 page_cnt, int node_id,
>> >                                         u64 flags);
>> > diff --git a/kernel/bpf/arena.c b/kernel/bpf/arena.c
>> > index da356989786a..126ddae763f9 100644
>> > --- a/kernel/bpf/arena.c
>> > +++ b/kernel/bpf/arena.c
>> > @@ -683,8 +683,73 @@ static int arena_adjust_tree(struct bpf_arena *arena, long uaddr, long page_cnt,
>> >       return range_tree_clear(&arena->rt, *pgoff, page_cnt);
>> >  }
>> >
>> > -static long arena_alloc_pages_internal(struct bpf_arena *arena, long page_cnt,
>> > -             long uaddr, long pgoff, int node_id, bool sleepable)
>> > +static long arena_alloc_pages_sleepable(struct bpf_arena *arena, long page_cnt,
>> > +             long uaddr, long pgoff, int node_id)
>> > +{
>> > +     u64 kern_vm_start = bpf_arena_get_kern_vm_start(arena);
>> > +     struct apply_range_data data;
>> > +     struct page **pages = NULL;
>> > +     unsigned long flags;
>> > +     u32 uaddr32;
>> > +     int ret, i;
>> > +
>> > +     pages = kvcalloc(page_cnt, sizeof(struct page *), GFP_KERNEL_ACCOUNT);
>> > +     if (!pages)
>> > +             return 0;
>> > +
>> > +     ret = bpf_map_alloc_pages(&arena->map, node_id, page_cnt, pages);
>> > +     if (ret) {
>> > +             kvfree(pages);
>> > +             return 0;
>> > +     }
>> > +
>> > +     data.i = 0;
>> > +     data.pages = pages;
>> > +     data.arena = arena;
>> > +
>>
>> Why have this split?
>> Always use an approach of allocating pages outside of the lock?
>>
>
> Mainly to avoid multiple kmalloc_nolock() allocations outside the lock
> for the sleepable
> path. That should matter for larger allocations. The two separate
> paths also stay simpler
> than if we had a single path (at the cost of a bit of extra
> duplication). I can change to a single
> code path during respin if you feel these are not worth having two
> internal calls.

Hmm. I would think one way to do things is easier. Less branches to worry about.
Give it a shot? If it looks worse, then keep it as-is.