bpf: arena: handle memory.max on fault-in with reclaim/OOM - #8625
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bpf_map_alloc_pages() picks the allocator via can_alloc_pages(), a conservative guess for BPF program context that is always false under PREEMPT_RT. So even a caller that really is sleepable gets the non-blocking allocator, which never reclaims and never engages the OOM machinery. Add bpf_map_alloc_page_sleepable() for callers that know they are sleepable. It allocates from the map's numa_node, like the other map allocators. The next patch uses it from the arena page fault handler. Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev> Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>
arena_vm_fault() allocated the page while holding arena->spinlock, so it could only use the non-blocking allocator. Once the memcg is at memory.max that allocation just fails, the fault turns into VM_FAULT_SIGSEGV, and the process gets a SIGSEGV on a perfectly valid arena address. Hitting memory.max is routine (e.g. page cache from reading a big file), so this kills innocent processes. Rework the fault handler: - Preallocate the page before taking the lock, like do_anonymous_page() does, so it can sleep, reclaim and go through the OOM path, and return VM_FAULT_OOM on failure so the memcg OOM handler runs instead of a fake segfault. - A lockless probe skips that preallocation when a page is already mapped (e.g. allocated by the bpf program), so the common case wastes no allocation. The rare race where such a page is freed before we take the lock falls back to the non-blocking allocator under the lock. - Return VM_FAULT_SIGBUS for the non-recoverable errors (lock failure, range-tree and page-table failures) instead of VM_FAULT_SIGSEGV; only BPF_F_SEGV_ON_FAULT, and a scratch-page hole under that flag, is a real user addressing error and keeps VM_FAULT_SIGSEGV. - Tidy up the error labels. Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
cgroup_helpers has write_cgroup_file()/write_cgroup_file_parent() but no read counterpart. Add read_cgroup_file() and read_cgroup_file_parent() so a forked child can read a cgroup file (e.g. memory.current) from the work dir owned by the parent that set the environment up, without hand-building the /mnt/... path. Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
A child joins a memcg capped at 64M and faults an arena in until it runs out of the budget. Without the kernel fix the child dies with SIGSEGV on a valid arena address; with it, the child is killed by the memcg OOM killer. With the fix: serial_test_arena_memcg:PASS:child killed by signal serial_test_arena_memcg:PASS:not killed by SIGSEGV #5 arena_memcg:OK # dmesg arena_vm_fault+0x655/0xa90 Memory cgroup out of memory: Killed process 512, file-rss:67920kB Without the fix: serial_test_arena_memcg:PASS:child killed by signal serial_test_arena_memcg:FAIL:not killed by SIGSEGV: actual 11 #5 arena_memcg:FAIL # dmesg test_progs[508]: segfault at 100004025000 ... Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
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Pull request for series with
subject: bpf: arena: handle memory.max on fault-in with reclaim/OOM
version: 2
url: https://patchwork.kernel.org/project/netdevbpf/list/?series=1140666