Re: [PATCH bpf-next v3 2/6] mm, bpf: Introduce free_pages_nolock()

Michal Hocko
2024-12-19T07:03:10Z

On Wed 18-12-24 17:45:20, Alexei Starovoitov wrote:
> On Wed, Dec 18, 2024 at 3:32 AM Michal Hocko <mhocko@suse.com> wrote:
> >
> > On Tue 17-12-24 19:07:15, alexei.starovoitov@gmail.com wrote:
> > > From: Alexei Starovoitov <ast@kernel.org>
> > >
> > > Introduce free_pages_nolock() that can free pages without taking locks.
> > > It relies on trylock and can be called from any context.
> > > Since spin_trylock() cannot be used in RT from hard IRQ or NMI
> > > it uses lockless link list to stash the pages which will be freed
> > > by subsequent free_pages() from good context.
> >
> > Yes, this makes sense. Have you tried a simpler implementation that
> > would just queue on the lockless link list unconditionally? That would
> > certainly reduce the complexity. Essentially something similar that we
> > do in vfree_atomic (well, except the queue_work which is likely too
> > heavy for the usecase and potentialy not reentrant).
>
> We cannot use llist approach unconditionally.
> One of the ways bpf maps are used is non-stop alloc/free.
> We cannot delay the free part. When memory is free it's better to
> be available for kernel and bpf uses right away.
> llist is the last resort.

This is an important detail that should be mentioned in the changelog.

--
Michal Hocko
SUSE Labs