Re: [PATCH bpf-next v4 1/4] bpf: Add a sleepable page allocator for map memory

On Mon 24-08-26 20:44:18, Jiayuan Chen wrote:
> 
> On 8/24/26 8:26 PM, Michal Hocko wrote:
> > On Fri 21-08-26 13:06:12, Jiayuan Chen wrote:
> > > bpf_map_alloc_pages() picks the allocator via can_alloc_pages(), a
> > > conservative guess for BPF program context that is always false under
> > > PREEMPT_RT. So even a caller that really is sleepable gets the
> > > non-blocking allocator, which never reclaims and never engages the OOM
> > > machinery.
> > I thought one of the main motivations was reentrancy. As you cannot
> > really assume the context bpf_map_alloc_pages is called from there is an
> > extra care needed so that this doesn't re-enter the allocator from bpf
> > program called from allocator path and deadlock.
> 
> 
> Agreed, bpf_map_alloc_pages() is designed to be safe to run in any context.
> 
> The commit message should be precise.

How do you achive any level of safety for the _sleepable version? Vast
majority of the kernel is sleepable but that doesn't mean this is safe
from the mm reentrancy POV.

-- 
Michal Hocko
SUSE Labs