Re: [PATCH bpf-next v12 1/2] mm/bpf: Add bpf_proactive_reclaim kfunc

[Shakeel Butt]

On Fri, Sep 18, 2026 at 12:45:01PM -0700, JP Kobryn wrote:
> On 9/17/26 11:58 PM, Hui Zhu wrote:
> > From: Hui Zhu <zhuhui@xxxxxxxxxx>
> > 
> > BPF programs can observe memory pressure on a cgroup, e.g. refault
> > stats via bpf_mem_cgroup_page_state(), but cannot act on it:
> > triggering reclaim requires writing to memory.reclaim, which BPF
> > cannot do.
> > 
> > Add bpf_proactive_reclaim(), a sleepable kfunc performing one
> > proactive reclaim pass on a memcg, like a write to memory.reclaim
> > but without retrying until the target is reached, so that when and
> > how hard to reclaim is BPF policy rather than hard-coded thresholds.
> > The reclaim target of a single call is capped at MEMCG_CHARGE_BATCH,
> > as high_work_func() does for memory.high; reclaiming more is left to
> > the program, which can call the kfunc once per bpf_wq callback and
> > stop at any point. It is limited to BPF_PROG_TYPE_SYSCALL, because
> > other sleepable programs may run with filesystem locks held, on
> > which the reclaim path could deadlock via filesystem shrinkers.
> > 
> > Convert MIN_SWAPPINESS, MAX_SWAPPINESS and SWAPPINESS_ANON_ONLY from
> > macros to an enum so that they are emitted into BTF and usable from
> > BPF programs via vmlinux.h.
> > 
> > Signed-off-by: Hui Zhu <zhuhui@xxxxxxxxxx>
> > Acked-by: Shakeel Butt <shakeel.butt@xxxxxxxxx>
> > ---
> >   mm/bpf_memcontrol.c | 62 ++++++++++++++++++++++++++++++++++++++++++++-
> >   mm/internal.h       | 10 +++++---
> >   2 files changed, 67 insertions(+), 5 deletions(-)
> > 
> > diff --git a/mm/bpf_memcontrol.c b/mm/bpf_memcontrol.c
> > index 716df49d7647..c5d7f29ade85 100644
> > --- a/mm/bpf_memcontrol.c
> > +++ b/mm/bpf_memcontrol.c
> > @@ -8,6 +8,8 @@
> >   #include <linux/memcontrol.h>
> >   #include <linux/bpf.h>
> > +#include "internal.h"
> > +
> >   __bpf_kfunc_start_defs();
> >   /**
> > @@ -159,6 +161,48 @@ __bpf_kfunc void bpf_mem_cgroup_flush_stats(struct mem_cgroup *memcg)
> >   	mem_cgroup_flush_stats(memcg);
> >   }
> > +/**
> > + * bpf_proactive_reclaim - proactively reclaim memory from a memory cgroup
> > + * @memcg: the target memory cgroup to reclaim from.
> > + * @size: the amount of memory to reclaim, in bytes, clamped to
> > + *        MEMCG_CHARGE_BATCH.
> > + * @swappiness: the reclaim swappiness, in the range [MIN_SWAPPINESS,
> > + *              SWAPPINESS_ANON_ONLY], or -1 to use the memcg's own
> > + *              swappiness.
> 
> The ANON_ONLY enumerator shouldn't be included as part of the range. I
> would change this to [MIN_SWAPPINESS, MAX_SWAPPINESS] and then specify
> that ANON_ONLY is a special mode like -1 is.
> 
> > + *
> > + * Performs one proactive reclaim pass on @memcg, like a write to
> > + * memory.reclaim but without retrying until @size is reached. Call it
> > + * repeatedly to reclaim more than one batch.
> > + *
> > + * Only available to BPF_PROG_TYPE_SYSCALL, because other sleepable programs
> > + * may run with filesystem locks held, which the reclaim path can deadlock
> > + * on via filesystem shrinkers.
> > + *
> > + * Return: The amount of memory reclaimed, in bytes, or a negative error.
> > + */
> > +__bpf_kfunc long bpf_proactive_reclaim(struct mem_cgroup *memcg,
> > +				       unsigned long size,
> > +				       int swappiness)
> > +{
> > +	unsigned long nr_reclaimed;
> > +	unsigned long nr_pages;
> > +
> > +	if (swappiness < -1 || swappiness > SWAPPINESS_ANON_ONLY)
> > +		return -EINVAL;
> 
> Related to the previous comment, you treat the special values as part of
> the range. It works currently, but creates a layout dependency on the
> enum. I think it would be more future-proof if you did:
> 
> if (swappiness != -1 && swappiness != SWAPPINESS_ANON_ONLY) {
> 	if (swappiness < MIN_SWAPPINESS || swappiness > MAX_SWAPPINESS)
> 		return -EINVAL;
> }
> 
> The previous comments I brought up are now resolved, so assuming you'll
> make the changes above you can include:
> 
> Reviewed-by: JP Kobryn <jp.kobryn@xxxxxxxxx>

Hui, please make these changes and just send this patch in next version.