Message-ID: <CAJuCfpGyKK7rw_KMicLwGcJx3gYam4HLVRqkc7y163p2H5ykiA@mail.gmail.com>
Date: Wed, 20 Aug 2025 13:01:03 -0700
From: Suren Baghdasaryan <surenb@...gle.com>
To: Roman Gushchin <roman.gushchin@...ux.dev>
Cc: linux-mm@...ck.org, bpf@...r.kernel.org, 
	Johannes Weiner <hannes@...xchg.org>, Michal Hocko <mhocko@...e.com>, 
	David Rientjes <rientjes@...gle.com>, Matt Bobrowski <mattbobrowski@...gle.com>, 
	Song Liu <song@...nel.org>, Kumar Kartikeya Dwivedi <memxor@...il.com>, Alexei Starovoitov <ast@...nel.org>, 
	Andrew Morton <akpm@...ux-foundation.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 01/14] mm: introduce bpf struct ops for OOM handling

On Wed, Aug 20, 2025 at 12:53 PM Roman Gushchin
<roman.gushchin@...ux.dev> wrote:
>
> Suren Baghdasaryan <surenb@...gle.com> writes:
>
> > On Tue, Aug 19, 2025 at 1:06 PM Roman Gushchin <roman.gushchin@...ux.dev> wrote:
> >>
> >> Suren Baghdasaryan <surenb@...gle.com> writes:
> >>
> >> > On Mon, Aug 18, 2025 at 10:01 AM Roman Gushchin
> >> > <roman.gushchin@...ux.dev> wrote:
> >> >>
> >> >> Introduce a bpf struct ops for implementing custom OOM handling policies.
> >> >>
> >> >> The struct ops provides the bpf_handle_out_of_memory() callback,
> >> >> which expected to return 1 if it was able to free some memory and 0
> >> >> otherwise.
> >> >>
> >> >> In the latter case it's guaranteed that the in-kernel OOM killer will
> >> >> be invoked. Otherwise the kernel also checks the bpf_memory_freed
> >> >> field of the oom_control structure, which is expected to be set by
> >> >> kfuncs suitable for releasing memory. It's a safety mechanism which
> >> >> prevents a bpf program to claim forward progress without actually
> >> >> releasing memory. The callback program is sleepable to enable using
> >> >> iterators, e.g. cgroup iterators.
> >> >>
> >> >> The callback receives struct oom_control as an argument, so it can
> >> >> easily filter out OOM's it doesn't want to handle, e.g. global vs
> >> >> memcg OOM's.
> >> >>
> >> >> The callback is executed just before the kernel victim task selection
> >> >> algorithm, so all heuristics and sysctls like panic on oom,
> >> >> sysctl_oom_kill_allocating_task and sysctl_oom_kill_allocating_task
> >> >> are respected.
> >> >>
> >> >> The struct ops also has the name field, which allows to define a
> >> >> custom name for the implemented policy. It's printed in the OOM report
> >> >> in the oom_policy=<policy> format. "default" is printed if bpf is not
> >> >> used or policy name is not specified.
> >> >>
> >> >> [  112.696676] test_progs invoked oom-killer: gfp_mask=0xcc0(GFP_KERNEL), order=0, oom_score_adj=0
> >> >>                oom_policy=bpf_test_policy
> >> >> [  112.698160] CPU: 1 UID: 0 PID: 660 Comm: test_progs Not tainted 6.16.0-00015-gf09eb0d6badc #102 PREEMPT(full)
> >> >> [  112.698165] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.17.0-5.fc42 04/01/2014
> >> >> [  112.698167] Call Trace:
> >> >> [  112.698177]  <TASK>
> >> >> [  112.698182]  dump_stack_lvl+0x4d/0x70
> >> >> [  112.698192]  dump_header+0x59/0x1c6
> >> >> [  112.698199]  oom_kill_process.cold+0x8/0xef
> >> >> [  112.698206]  bpf_oom_kill_process+0x59/0xb0
> >> >> [  112.698216]  bpf_prog_7ecad0f36a167fd7_test_out_of_memory+0x2be/0x313
> >> >> [  112.698229]  bpf__bpf_oom_ops_handle_out_of_memory+0x47/0xaf
> >> >> [  112.698236]  ? srso_alias_return_thunk+0x5/0xfbef5
> >> >> [  112.698240]  bpf_handle_oom+0x11a/0x1e0
> >> >> [  112.698250]  out_of_memory+0xab/0x5c0
> >> >> [  112.698258]  mem_cgroup_out_of_memory+0xbc/0x110
> >> >> [  112.698274]  try_charge_memcg+0x4b5/0x7e0
> >> >> [  112.698288]  charge_memcg+0x2f/0xc0
> >> >> [  112.698293]  __mem_cgroup_charge+0x30/0xc0
> >> >> [  112.698299]  do_anonymous_page+0x40f/0xa50
> >> >> [  112.698311]  __handle_mm_fault+0xbba/0x1140
> >> >> [  112.698317]  ? srso_alias_return_thunk+0x5/0xfbef5
> >> >> [  112.698335]  handle_mm_fault+0xe6/0x370
> >> >> [  112.698343]  do_user_addr_fault+0x211/0x6a0
> >> >> [  112.698354]  exc_page_fault+0x75/0x1d0
> >> >> [  112.698363]  asm_exc_page_fault+0x26/0x30
> >> >> [  112.698366] RIP: 0033:0x7fa97236db00
> >> >>
> >> >> It's possible to load multiple bpf struct programs. In the case of
> >> >> oom, they will be executed one by one in the same order they been
> >> >> loaded until one of them returns 1 and bpf_memory_freed is set to 1
> >> >> - an indication that the memory was freed. This allows to have
> >> >> multiple bpf programs to focus on different types of OOM's - e.g.
> >> >> one program can only handle memcg OOM's in one memory cgroup.
> >> >> But the filtering is done in bpf - so it's fully flexible.
> >> >>
> >> >> Signed-off-by: Roman Gushchin <roman.gushchin@...ux.dev>
> >> >> ---
> >> >>  include/linux/bpf_oom.h |  49 +++++++++++++
> >> >>  include/linux/oom.h     |   8 ++
> >> >>  mm/Makefile             |   3 +
> >> >>  mm/bpf_oom.c            | 157 ++++++++++++++++++++++++++++++++++++++++
> >> >>  mm/oom_kill.c           |  22 +++++-
> >> >>  5 files changed, 237 insertions(+), 2 deletions(-)
> >> >>  create mode 100644 include/linux/bpf_oom.h
> >> >>  create mode 100644 mm/bpf_oom.c
> >> >>
> >> >> diff --git a/include/linux/bpf_oom.h b/include/linux/bpf_oom.h
> >> >> new file mode 100644
> >> >> index 000000000000..29cb5ea41d97
> >> >> --- /dev/null
> >> >> +++ b/include/linux/bpf_oom.h
> >> >> @@ -0,0 +1,49 @@
> >> >> +/* SPDX-License-Identifier: GPL-2.0+ */
> >> >> +
> >> >> +#ifndef __BPF_OOM_H
> >> >> +#define __BPF_OOM_H
> >> >> +
> >> >> +struct bpf_oom;
> >> >> +struct oom_control;
> >> >> +
> >> >> +#define BPF_OOM_NAME_MAX_LEN 64
> >> >> +
> >> >> +struct bpf_oom_ops {
> >> >> +       /**
> >> >> +        * @handle_out_of_memory: Out of memory bpf handler, called before
> >> >> +        * the in-kernel OOM killer.
> >> >> +        * @oc: OOM control structure
> >> >> +        *
> >> >> +        * Should return 1 if some memory was freed up, otherwise
> >> >> +        * the in-kernel OOM killer is invoked.
> >> >> +        */
> >> >> +       int (*handle_out_of_memory)(struct oom_control *oc);
> >> >> +
> >> >> +       /**
> >> >> +        * @name: BPF OOM policy name
> >> >> +        */
> >> >> +       char name[BPF_OOM_NAME_MAX_LEN];
> >> >
> >> > Why should the name be a part of ops structure? IMO it's not an
> >> > attribute of the operations but rather of the oom handler which is
> >> > represented by bpf_oom here.
> >>
> >> The ops structure describes a user-defined oom policy. Currently
> >> it's just one handler and the policy name. Later additional handlers
> >> can be added, e.g. a handler to control the dmesg output.
> >>
> >> bpf_oom is an implementation detail: it's basically an extension
> >> to struct bpf_oom_ops which contains "private" fields required
> >> for the internal machinery.
> >
> > Ok. I hope we can come up with some more descriptive naming but I
> > can't think of something good ATM.
> >
> >>
> >> >
> >> >> +
> >> >> +       /* Private */
> >> >> +       struct bpf_oom *bpf_oom;
> >> >> +};
> >> >> +
> >> >> +#ifdef CONFIG_BPF_SYSCALL
> >> >> +/**
> >> >> + * @bpf_handle_oom: handle out of memory using bpf programs
> >> >> + * @oc: OOM control structure
> >> >> + *
> >> >> + * Returns true if a bpf oom program was executed, returned 1
> >> >> + * and some memory was actually freed.
> >> >
> >> > The above comment is unclear, please clarify.
> >>
> >> Fixed, thanks.
> >>
> >> /**
> >>  * @bpf_handle_oom: handle out of memory condition using bpf
> >>  * @oc: OOM control structure
> >>  *
> >>  * Returns true if some memory was freed.
> >>  */
> >> bool bpf_handle_oom(struct oom_control *oc);
> >>
> >>
> >> >
> >> >> + */
> >> >> +bool bpf_handle_oom(struct oom_control *oc);
> >> >> +
> >> >> +#else /* CONFIG_BPF_SYSCALL */
> >> >> +static inline bool bpf_handle_oom(struct oom_control *oc)
> >> >> +{
> >> >> +       return false;
> >> >> +}
> >> >> +
> >> >> +#endif /* CONFIG_BPF_SYSCALL */
> >> >> +
> >> >> +#endif /* __BPF_OOM_H */
> >> >> diff --git a/include/linux/oom.h b/include/linux/oom.h
> >> >> index 1e0fc6931ce9..ef453309b7ea 100644
> >> >> --- a/include/linux/oom.h
> >> >> +++ b/include/linux/oom.h
> >> >> @@ -51,6 +51,14 @@ struct oom_control {
> >> >>
> >> >>         /* Used to print the constraint info. */
> >> >>         enum oom_constraint constraint;
> >> >> +
> >> >> +#ifdef CONFIG_BPF_SYSCALL
> >> >> +       /* Used by the bpf oom implementation to mark the forward progress */
> >> >> +       bool bpf_memory_freed;
> >> >> +
> >> >> +       /* Policy name */
> >> >> +       const char *bpf_policy_name;
> >> >> +#endif
> >> >>  };
> >> >>
> >> >>  extern struct mutex oom_lock;
> >> >> diff --git a/mm/Makefile b/mm/Makefile
> >> >> index 1a7a11d4933d..a714aba03759 100644
> >> >> --- a/mm/Makefile
> >> >> +++ b/mm/Makefile
> >> >> @@ -105,6 +105,9 @@ obj-$(CONFIG_MEMCG) += memcontrol.o vmpressure.o
> >> >>  ifdef CONFIG_SWAP
> >> >>  obj-$(CONFIG_MEMCG) += swap_cgroup.o
> >> >>  endif
> >> >> +ifdef CONFIG_BPF_SYSCALL
> >> >> +obj-y += bpf_oom.o
> >> >> +endif
> >> >>  obj-$(CONFIG_CGROUP_HUGETLB) += hugetlb_cgroup.o
> >> >>  obj-$(CONFIG_GUP_TEST) += gup_test.o
> >> >>  obj-$(CONFIG_DMAPOOL_TEST) += dmapool_test.o
> >> >> diff --git a/mm/bpf_oom.c b/mm/bpf_oom.c
> >> >> new file mode 100644
> >> >> index 000000000000..47633046819c
> >> >> --- /dev/null
> >> >> +++ b/mm/bpf_oom.c
> >> >> @@ -0,0 +1,157 @@
> >> >> +// SPDX-License-Identifier: GPL-2.0-or-later
> >> >> +/*
> >> >> + * BPF-driven OOM killer customization
> >> >> + *
> >> >> + * Author: Roman Gushchin <roman.gushchin@...ux.dev>
> >> >> + */
> >> >> +
> >> >> +#include <linux/bpf.h>
> >> >> +#include <linux/oom.h>
> >> >> +#include <linux/bpf_oom.h>
> >> >> +#include <linux/srcu.h>
> >> >> +
> >> >> +DEFINE_STATIC_SRCU(bpf_oom_srcu);
> >> >> +static DEFINE_SPINLOCK(bpf_oom_lock);
> >> >> +static LIST_HEAD(bpf_oom_handlers);
> >> >> +
> >> >> +struct bpf_oom {
> >> >
> >> > Perhaps bpf_oom_handler ? Then bpf_oom_ops->bpf_oom could be called
> >> > bpf_oom_ops->handler.
> >>
> >> I don't think it's a handler, it's more like a private part
> >> of bpf_oom_ops. Maybe bpf_oom_impl? Idk
> >
> > Yeah, we need to come up with some nomenclature and name these structs
> > accordingly. In my mind ops means a structure that contains only
> > operations, so current naming does not sit well but maybe that's just
> > me...
> >
> >>
> >> >
> >> >
> >> >> +       struct bpf_oom_ops *ops;
> >> >> +       struct list_head node;
> >> >> +       struct srcu_struct srcu;
> >> >> +};
> >> >> +
> >> >> +bool bpf_handle_oom(struct oom_control *oc)
> >> >> +{
> >> >> +       struct bpf_oom_ops *ops;
> >> >> +       struct bpf_oom *bpf_oom;
> >> >> +       int list_idx, idx, ret = 0;
> >> >> +
> >> >> +       oc->bpf_memory_freed = false;
> >> >> +
> >> >> +       list_idx = srcu_read_lock(&bpf_oom_srcu);
> >> >> +       list_for_each_entry_srcu(bpf_oom, &bpf_oom_handlers, node, false) {
> >> >> +               ops = READ_ONCE(bpf_oom->ops);
> >> >> +               if (!ops || !ops->handle_out_of_memory)
> >> >> +                       continue;
> >> >> +               idx = srcu_read_lock(&bpf_oom->srcu);
> >> >> +               oc->bpf_policy_name = ops->name[0] ? &ops->name[0] :
> >> >> +                       "bpf_defined_policy";
> >> >> +               ret = ops->handle_out_of_memory(oc);
> >> >> +               oc->bpf_policy_name = NULL;
> >> >> +               srcu_read_unlock(&bpf_oom->srcu, idx);
> >> >> +
> >> >> +               if (ret && oc->bpf_memory_freed)
> >> >
> >> > IIUC ret and oc->bpf_memory_freed seem to reflect the same state:
> >> > handler successfully freed some memory. Could you please clarify when
> >> > they differ?
> >>
> >> The idea here is to provide an additional safety measure:
> >> if the bpf program simple returns 1 without doing anything,
> >> the system won't deadlock.
> >>
> >> oc->bpf_memory_freed is set by the bpf_oom_kill_process() helper
> >> (and potentially some other helpers in the future, e.g.
> >> bpf_oom_rm_tmpfs_file()) and can't be modified by the bpf
> >> program directly.
> >
> > I see. Then maybe we use only oc->bpf_memory_freed and
> > handle_out_of_memory() does not return anything?
>
> Idk, I think it's neat to have an ability to pass to the in-kernel
> OOM killer even after killing a task.
> Also, I believe, bpf programs have to return an int anyway,
> so we can ignore it, but I don't necessary see the point.

Ok, so you want this parameter for the bpf oom-handler to say "even if
something got killed, proceed with the next oom-handler anyway?". I
can't think of a case when someone would do that... In any case, the
use for this return value should be clearly documented.

>

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