netdev - [PATCH bpf-next 1/4] bpf: Make non-preallocated allocation low priority

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Message-Id: <20220629154832.56986-2-laoar.shao@gmail.com>
Date:   Wed, 29 Jun 2022 15:48:29 +0000
From:   Yafang Shao <laoar.shao@...il.com>
To:     ast@...nel.org, daniel@...earbox.net, andrii@...nel.org,
        kafai@...com, songliubraving@...com, yhs@...com,
        john.fastabend@...il.com, kpsingh@...nel.org, quentin@...valent.com
Cc:     netdev@...r.kernel.org, bpf@...r.kernel.org,
        Yafang Shao <laoar.shao@...il.com>,
        Roman Gushchin <roman.gushchin@...ux.dev>
Subject: [PATCH bpf-next 1/4] bpf: Make non-preallocated allocation low priority

GFP_ATOMIC doesn't cooperate well with memcg pressure so far, especially
if we allocate too much GFP_ATOMIC memory. For example, when we set the
memcg limit to limit a non-preallocated bpf memory, the GFP_ATOMIC can
easily break the memcg limit by force charge. So it is very dangerous to
use GFP_ATOMIC in non-preallocated case. One way to make it safe is to
remove __GFP_HIGH from GFP_ATOMIC, IOW, use (__GFP_ATOMIC |
__GFP_KSWAPD_RECLAIM) instead, then it will be limited if we allocate
too much memory.

We introduced BPF_F_NO_PREALLOC is because full map pre-allocation is
too memory expensive for some cases. That means removing __GFP_HIGH
doesn't break the rule of BPF_F_NO_PREALLOC, but has the same goal with
it-avoiding issues caused by too much memory. So let's remove it.

__GFP_KSWAPD_RECLAIM doesn't cooperate well with memcg pressure neither
currently. But the memcg code can be improved to make
__GFP_KSWAPD_RECLAIM work well under memcg pressure.

It also fixes a typo in the comment.

Signed-off-by: Yafang Shao <laoar.shao@...il.com>
Cc: Roman Gushchin <roman.gushchin@...ux.dev>
---
 kernel/bpf/hashtab.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/kernel/bpf/hashtab.c b/kernel/bpf/hashtab.c
index 17fb69c0e0dc..9d4559a1c032 100644
--- a/kernel/bpf/hashtab.c
+++ b/kernel/bpf/hashtab.c
@@ -61,7 +61,7 @@
  *
  * As regular device interrupt handlers and soft interrupts are forced into
  * thread context, the existing code which does
- *   spin_lock*(); alloc(GPF_ATOMIC); spin_unlock*();
+ *   spin_lock*(); alloc(GFP_ATOMIC); spin_unlock*();
  * just works.
  *
  * In theory the BPF locks could be converted to regular spinlocks as well,
@@ -978,7 +978,8 @@ static struct htab_elem *alloc_htab_elem(struct bpf_htab *htab, void *key,
 				goto dec_count;
 			}
 		l_new = bpf_map_kmalloc_node(&htab->map, htab->elem_size,
-					     GFP_ATOMIC | __GFP_NOWARN,
+					     __GFP_ATOMIC | __GFP_NOWARN |
+					     __GFP_KSWAPD_RECLAIM,
 					     htab->map.numa_node);
 		if (!l_new) {
 			l_new = ERR_PTR(-ENOMEM);
@@ -996,7 +997,8 @@ static struct htab_elem *alloc_htab_elem(struct bpf_htab *htab, void *key,
 		} else {
 			/* alloc_percpu zero-fills */
 			pptr = bpf_map_alloc_percpu(&htab->map, size, 8,
-						    GFP_ATOMIC | __GFP_NOWARN);
+						    __GFP_ATOMIC | __GFP_NOWARN |
+						    __GFP_KSWAPD_RECLAIM);
 			if (!pptr) {
 				kfree(l_new);
 				l_new = ERR_PTR(-ENOMEM);
-- 
2.17.1