Re: [PATCH v2] objpool: fix choosing allocation for percpu slots

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

On Tue, 22 Oct 2024 13:45:08 +0200
Viktor Malik <vmalik@redhat.com> wrote:

> On 10/22/24 07:17, Masami Hiramatsu (Google) wrote:
> > On Mon, 26 Aug 2024 08:07:18 +0200
> > Viktor Malik <vmalik@redhat.com> wrote:
> > 
> >> objpool intends to use vmalloc for default (non-atomic) allocations of
> >> percpu slots and objects. However, the condition checking if GFP flags
> >> are equal to GFP_ATOMIC is wrong b/c GFP_ATOMIC is a combination of bits
> > 
> > You meant "whether GFP flags sets any bit of GFP_ATOMIC is wrong"?
> 
> Well, I meant that the condition is wrong w.r.t. what is supposedly its
> original purpose. But feel free to rephrase as you seem fit or I can
> send v3 if you prefer.

No problem :) let me rephrase that part.

Thank you!

> 
> Thanks.
> Viktor
> 
> > 
> >> (__GFP_HIGH|__GFP_KSWAPD_RECLAIM) and so `pool->gfp & GFP_ATOMIC` will
> >> be true if either bit is set. Since GFP_ATOMIC and GFP_KERNEL share the
> >> ___GFP_KSWAPD_RECLAIM bit, kmalloc will be used in cases when GFP_KERNEL
> >> is specified, i.e. in all current usages of objpool.
> >>
> >> This may lead to unexpected OOM errors since kmalloc cannot allocate
> >> large amounts of memory.
> >>
> >> For instance, objpool is used by fprobe rethook which in turn is used by
> >> BPF kretprobe.multi and kprobe.session probe types. Trying to attach
> >> these to all kernel functions with libbpf using
> >>
> >>     SEC("kprobe.session/*")
> >>     int kprobe(struct pt_regs *ctx)
> >>     {
> >>         [...]
> >>     }
> >>
> >> fails on objpool slot allocation with ENOMEM.
> >>
> >> Fix the condition to truly use vmalloc by default.
> >>
> > 
> > Anyway, this looks good to me.
> > 
> > Thank you,
> > 
> >> Fixes: b4edb8d2d464 ("lib: objpool added: ring-array based lockless MPMC")
> >> Signed-off-by: Viktor Malik <vmalik@redhat.com>
> >> Acked-by: Andrii Nakryiko <andrii@kernel.org>
> >> Reviewed-by: Matt Wu <wuqiang.matt@bytedance.com>
> >> ---
> >>  lib/objpool.c | 2 +-
> >>  1 file changed, 1 insertion(+), 1 deletion(-)
> >>
> >> diff --git a/lib/objpool.c b/lib/objpool.c
> >> index 234f9d0bd081..fd108fe0d095 100644
> >> --- a/lib/objpool.c
> >> +++ b/lib/objpool.c
> >> @@ -76,7 +76,7 @@ objpool_init_percpu_slots(struct objpool_head *pool, int nr_objs,
> >>  		 * mimimal size of vmalloc is one page since vmalloc would
> >>  		 * always align the requested size to page size
> >>  		 */
> >> -		if (pool->gfp & GFP_ATOMIC)
> >> +		if ((pool->gfp & GFP_ATOMIC) == GFP_ATOMIC)
> >>  			slot = kmalloc_node(size, pool->gfp, cpu_to_node(i));
> >>  		else
> >>  			slot = __vmalloc_node(size, sizeof(void *), pool->gfp,
> >> -- 
> >> 2.46.0
> >>
> > 
> > 
> 


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>