On Thu, Jul 27, 2023 at 2:59 PM Yonghong Song <yonghong.song@xxxxxxxxx> wrote:
>
>
>
> On 7/27/23 1:18 PM, YiFei Zhu wrote:
> > In internal testing of test_maps, we sometimes observed failures like:
> > test_maps: test_maps.c:173: void test_hashmap_percpu(unsigned int, void *):
> > Assertion `bpf_map_update_elem(fd, &key, value, BPF_ANY) == 0' failed.
> > where the errno is ENOMEM. After some troubleshooting and enabling
> > the warnings, we saw:
> > [ 91.304708] percpu: allocation failed, size=8 align=8 atomic=1, atomic alloc failed, no space left
> > [ 91.304716] CPU: 51 PID: 24145 Comm: test_maps Kdump: loaded Tainted: G N 6.1.38-smp-DEV #7
> > [ 91.304719] Hardware name: Google Astoria/astoria, BIOS 0.20230627.0-0 06/27/2023
> > [ 91.304721] Call Trace:
> > [ 91.304724] <TASK>
> > [ 91.304730] [<ffffffffa7ef83b9>] dump_stack_lvl+0x59/0x88
> > [ 91.304737] [<ffffffffa7ef83f8>] dump_stack+0x10/0x18
> > [ 91.304738] [<ffffffffa75caa0c>] pcpu_alloc+0x6fc/0x870
> > [ 91.304741] [<ffffffffa75ca302>] __alloc_percpu_gfp+0x12/0x20
> > [ 91.304743] [<ffffffffa756785e>] alloc_bulk+0xde/0x1e0
> > [ 91.304746] [<ffffffffa7566c02>] bpf_mem_alloc_init+0xd2/0x2f0
> > [ 91.304747] [<ffffffffa7547c69>] htab_map_alloc+0x479/0x650
> > [ 91.304750] [<ffffffffa751d6e0>] map_create+0x140/0x2e0
> > [ 91.304752] [<ffffffffa751d413>] __sys_bpf+0x5a3/0x6c0
> > [ 91.304753] [<ffffffffa751c3ec>] __x64_sys_bpf+0x1c/0x30
> > [ 91.304754] [<ffffffffa7ef847a>] do_syscall_64+0x5a/0x80
> > [ 91.304756] [<ffffffffa800009b>] entry_SYSCALL_64_after_hwframe+0x63/0xcd
> >
> > This makes sense, because in atomic context, percpu allocation would
> > not create new chunks; it would only create in non-atomic contexts.
> > And if during prefill all precpu chunks are full, -ENOMEM would
> > happen immediately upon next unit_alloc.
> >
> > Prefill phase does not actually run in atomic context, so we can
> > use this fact to allocate non-atomically with GFP_KERNEL instead
> > of GFP_NOWAIT. This avoids the immediate -ENOMEM.
> >
> > Unfortunately unit_alloc runs in atomic context, even from map
> > item allocation in syscalls, due to rcu_read_lock, so we can't do
> > non-atomic workarounds in unit_alloc.
>
> The above description is not clear to me. Do you mean
> GFP_NOWAIT has to be used in unit_alloc when bpf program runs
> in atomic context. Even if bpf program runs in non-atomic context,
> in most cases, rcu read lock is enabled for the program so
> GFP_NOWAIT is still needed.
I actually meant that in syscall BPF_MAP_UPDATE_ELEM, at least in the
case of hashmap_percpu the code path is rcu read locked, so one cannot
do non-atomic allocations even from syscalls. Hmm, shall I I change it
to something like this?
GFP_NOWAIT has to be used in unit_alloc when bpf program runs
in atomic context. Even if bpf program runs in non-atomic context,
in most cases, rcu read lock is enabled for the program so
GFP_NOWAIT is still needed. This is often also the case for
BPF_MAP_UPDATE_ELEM syscalls.
> The exception is sleepable bpf program in non-atomic context,
> e.g., sleepable bpf_iter program, sleepable fentry program
> in non-atomic context, and the unit_alloc is not inside
> bpf_rcu_read_lock kfunc. But this is too complicated and
> probably not worthwhile to special-case it.
Ack.
>
> >
> > Signed-off-by: YiFei Zhu <zhuyifei@xxxxxxxxxx>
> Acked-by: Yonghong Song <yhs@xxxxxx>
>
> > ---
> > v1->v2:
> > - Rebase from bpf to bpf-next
> > - Dropped second patch and edited commit message to include parts
> > of original cover letter, and dropped Fixes tag
> > ---
> > kernel/bpf/memalloc.c | 12 ++++++++----
> > 1 file changed, 8 insertions(+), 4 deletions(-)
> >
> > diff --git a/kernel/bpf/memalloc.c b/kernel/bpf/memalloc.c
> > index 14d9b1a9a4ca..9c49ae53deaf 100644
> > --- a/kernel/bpf/memalloc.c
> > +++ b/kernel/bpf/memalloc.c
> > @@ -201,12 +201,16 @@ static void add_obj_to_free_list(struct bpf_mem_cache *c, void *obj)
> > }
> >
> > /* Mostly runs from irq_work except __init phase. */
> > -static void alloc_bulk(struct bpf_mem_cache *c, int cnt, int node)
> > +static void alloc_bulk(struct bpf_mem_cache *c, int cnt, int node, bool atomic)
> > {
> > struct mem_cgroup *memcg = NULL, *old_memcg;
> > + gfp_t gfp;
> > void *obj;
> > int i;
> >
> > + gfp = __GFP_NOWARN | __GFP_ACCOUNT;
> > + gfp |= atomic ? GFP_NOWAIT : GFP_KERNEL;
> > +
> > for (i = 0; i < cnt; i++) {
> > /*
> > * For every 'c' llist_del_first(&c->free_by_rcu_ttrace); is
> > @@ -238,7 +242,7 @@ static void alloc_bulk(struct bpf_mem_cache *c, int cnt, int node)
> > * will allocate from the current numa node which is what we
> > * want here.
> > */
> > - obj = __alloc(c, node, GFP_NOWAIT | __GFP_NOWARN | __GFP_ACCOUNT);
> > + obj = __alloc(c, node, gfp);
> > if (!obj)
> > break;
> > add_obj_to_free_list(c, obj);
> > @@ -429,7 +433,7 @@ static void bpf_mem_refill(struct irq_work *work)
> > /* irq_work runs on this cpu and kmalloc will allocate
> > * from the current numa node which is what we want here.
> > */
> > - alloc_bulk(c, c->batch, NUMA_NO_NODE);
> > + alloc_bulk(c, c->batch, NUMA_NO_NODE, true);
> > else if (cnt > c->high_watermark)
> > free_bulk(c);
> >
> > @@ -477,7 +481,7 @@ static void prefill_mem_cache(struct bpf_mem_cache *c, int cpu)
> > * prog won't be doing more than 4 map_update_elem from
> > * irq disabled region
> > */
> > - alloc_bulk(c, c->unit_size <= 256 ? 4 : 1, cpu_to_node(cpu));
> > + alloc_bulk(c, c->unit_size <= 256 ? 4 : 1, cpu_to_node(cpu), false);
> > }
> >
> > /* When size != 0 bpf_mem_cache for each cpu.
