Re: [PATCH v1 01/14] mm: introduce bpf struct ops for OOM handling

[Roman Gushchin <roman.gushchin@linux.dev>]

Suren Baghdasaryan <surenb@google.com> writes:

> On Mon, Aug 18, 2025 at 10:01 AM Roman Gushchin
> <roman.gushchin@linux.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@linux.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.

>
>> +
>> +       /* 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@linux.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

>
>
>> +       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.