linux-kernel - [PATCH bpf-next 0/3] bpf: Fix unintended eviction when updating lru hash maps

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Message-ID: <20251202153032.10118-1-leon.hwang@linux.dev>
Date: Tue,  2 Dec 2025 23:30:29 +0800
From: Leon Hwang <leon.hwang@...ux.dev>
To: bpf@...r.kernel.org
Cc: Alexei Starovoitov <ast@...nel.org>,
	Daniel Borkmann <daniel@...earbox.net>,
	Andrii Nakryiko <andrii@...nel.org>,
	Martin KaFai Lau <martin.lau@...ux.dev>,
	Eduard Zingerman <eddyz87@...il.com>,
	Song Liu <song@...nel.org>,
	Yonghong Song <yonghong.song@...ux.dev>,
	John Fastabend <john.fastabend@...il.com>,
	KP Singh <kpsingh@...nel.org>,
	Stanislav Fomichev <sdf@...ichev.me>,
	Hao Luo <haoluo@...gle.com>,
	Jiri Olsa <jolsa@...nel.org>,
	Shuah Khan <shuah@...nel.org>,
	Leon Hwang <leon.hwang@...ux.dev>,
	Saket Kumar Bhaskar <skb99@...ux.ibm.com>,
	"David S . Miller" <davem@...emloft.net>,
	linux-kernel@...r.kernel.org,
	linux-kselftest@...r.kernel.org,
	kernel-patches-bot@...com
Subject: [PATCH bpf-next 0/3] bpf: Fix unintended eviction when updating lru hash maps

This unintended LRU eviction issue was observed while developing the
selftest for
"[PATCH bpf-next v10 0/8] bpf: Introduce BPF_F_CPU and BPF_F_ALL_CPUS flags for percpu maps" [1].

When updating an existing element in lru_hash or lru_percpu_hash maps,
the current implementation calls prealloc_lru_pop() to get a new node
before checking if the key already exists. If the map is full, this
triggers LRU eviction and removes an existing element, even though the
update operation only needs to modify the value in-place.

In the selftest, this was to be worked around by reserving an extra entry to
avoid triggering eviction in __htab_lru_percpu_map_update_elem().
However, the underlying issue remains problematic because:

1. Users may unexpectedly lose entries when updating existing keys in a
   full map.
2. The eviction overhead is unnecessary for existing key updates.

This patchset fixes the issue by first checking if the key exists before
allocating a new node. If the key is found, update the value in-place,
refresh the LRU reference, and return immediately without triggering any
eviction. Only proceed with node allocation if the key does not exist.

Links:
[1] https://lore.kernel.org/bpf/20251117162033.6296-1-leon.hwang@linux.dev/

Leon Hwang (3):
  bpf: Avoid unintended eviction when updating lru_hash maps
  bpf: Avoid unintended eviction when updating lru_percpu_hash maps
  selftests/bpf: Add tests to verify no unintended eviction when
    updating lru hash maps

 kernel/bpf/hashtab.c                          | 43 +++++++++++
 .../selftests/bpf/prog_tests/htab_update.c    | 73 +++++++++++++++++++
 2 files changed, 116 insertions(+)

--
2.52.0