[PATCH bpf-next v4 0/4] bpf: arena: handle memory.max on fault-in with reclaim/OOM

[PATCH bpf-next v4 0/4] bpf: arena: handle memory.max on fault-in with reclaim/OOM

[Jiayuan Chen]

Since commit e66fe1bc6d25 ("bpf: arena: Reintroduce memcg accounting"),
arena pages are charged to the memcg of the process that created the arena.
That accounting exposes two problems in the arena user page fault path.

1. The fault-in allocation runs under arena->spinlock, so it can only use the
   non-blocking allocator, which never reclaims. Once memory.current is at
   memory.max the allocation simply fails. Reaching memory.max is completely
   normal for a healthy application - e.g. reading a large file fills
   memory.current with page cache - so the process ends up killed for no real
   reason.

2. That failure is turned into VM_FAULT_SIGSEGV, which is misleading: the
   faulting address is a perfectly valid arena address. When we know it is an
   out-of-memory condition we can return VM_FAULT_OOM and let the memcg OOM
   path handle it properly.

Preallocate the page outside the lock (patch 2), the way do_anonymous_page()
does, so the allocation can sleep and go through reclaim and the OOM path.
This needs a sleepable allocator (patch 1), because can_alloc_pages() is a
conservative guess for BPF program context and always forces the non-blocking
allocator under PREEMPT_RT. patch 3&4 adds a selftest that faults an arena in
under a memory.max limit: without the fix the child gets SIGSEGV on a valid
address, with it the child is killed by the memcg OOM killer.


v3 -> v4:
   - rebase bpf-next and fix conflict
   - add Reviewed-by tag from Emil Tsalapatis

v2 -> v3:
   - selftest: check the memcg OOM via memory.events "oom_kill" instead of
     the exit signal; it only aims to pass on the fixed kernel, since the
     unfixed SIGSEGV is racy.

v1 -> v2:
   - Rebase on the separate deadlock fix (found by the Sashiko AI review),
     now applied to bpf-next.
   - Honor the map's NUMA node on fault-in.
   - Return VM_FAULT_SIGBUS for the non-recoverable faults (lock, range-tree
     and page-table failures); a scratch-page hole stays VM_FAULT_SIGSEGV
     only under BPF_F_SEGV_ON_FAULT. (Kumar Kartikeya Dwivedi)
   - Add read_cgroup_file() to cgroup_helpers instead of open-coding the
     /mnt/... path in the test. (Emil Tsalapatis)
   - Dump the cgroup memory stats on test failure to ease debugging.

v2:
https://lore.kernel.org/bpf/20260805091720.139924-1-jiayuan.chen@linux.dev/
v1:
https://lore.kernel.org/bpf/20260727062521.376231-1-jiayuan.chen@linux.dev/

Jiayuan Chen (4):
  bpf: Add a sleepable page allocator for map memory
  bpf: arena: allocate the fault-in page outside the lock
  selftests/bpf: Add read_cgroup_file() to cgroup_helpers
  selftests/bpf: Add a test for arena fault-in under memory.max

 include/linux/bpf.h                           |   1 +
 kernel/bpf/arena.c                            |  92 ++++++++---
 kernel/bpf/syscall.c                          |  21 ++-
 tools/testing/selftests/bpf/cgroup_helpers.c  |  67 ++++++++
 tools/testing/selftests/bpf/cgroup_helpers.h  |   4 +
 .../selftests/bpf/prog_tests/arena_memcg.c    | 156 ++++++++++++++++++
 .../testing/selftests/bpf/progs/arena_memcg.c |  24 +++
 7 files changed, 340 insertions(+), 25 deletions(-)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/arena_memcg.c
 create mode 100644 tools/testing/selftests/bpf/progs/arena_memcg.c

-- 
2.43.0

[PATCH bpf-next v4 1/4] bpf: Add a sleepable page allocator for map memory

[Jiayuan Chen]

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. Like the other bpf map allocators it places the page on the
map's numa_node and does not follow the faulting task's NUMA mempolicy;
arena memory is shared, so the map's node is the right placement policy.
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>
---
To sashiko:
- The alloc_pages_nolock() fallback still zeroes: it forces __GFP_ZERO
  internally and only accepts __GFP_ACCOUNT.
- __GFP_ZERO is unchanged from the existing __bpf_alloc_page(), and no
  arena-capable arch has D-cache aliasing, so there is no dcache concern.
---
 include/linux/bpf.h  |  1 +
 kernel/bpf/syscall.c | 21 +++++++++++++++++----
 2 files changed, 18 insertions(+), 4 deletions(-)

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index ffa5626411ac..18e84ec4692d 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -2784,6 +2784,7 @@ struct bpf_prog *bpf_prog_get_curr_or_next(u32 *id);
 
 int bpf_map_alloc_pages(const struct bpf_map *map, int nid,
 			unsigned long nr_pages, struct page **page_array);
+struct page *bpf_map_alloc_page_sleepable(const struct bpf_map *map);
 #ifdef CONFIG_MEMCG
 void bpf_map_memcg_enter(const struct bpf_map *map, struct mem_cgroup **old_memcg,
 			 struct mem_cgroup **new_memcg);
diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index 6874ba1424af..f9b81638e537 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -602,15 +602,14 @@ static bool can_alloc_pages(void)
 		!IS_ENABLED(CONFIG_PREEMPT_RT);
 }
 
+#define BPF_PAGE_GFP (GFP_KERNEL | __GFP_ZERO | __GFP_ACCOUNT | __GFP_NOWARN)
+
 static struct page *__bpf_alloc_page(int nid)
 {
 	if (!can_alloc_pages())
 		return alloc_pages_nolock(__GFP_ACCOUNT, nid, 0);
 
-	return alloc_pages_node(nid,
-				GFP_KERNEL | __GFP_ZERO | __GFP_ACCOUNT
-				| __GFP_NOWARN,
-				0);
+	return alloc_pages_node(nid, BPF_PAGE_GFP, 0);
 }
 
 int bpf_map_alloc_pages(const struct bpf_map *map, int nid,
@@ -636,6 +635,20 @@ int bpf_map_alloc_pages(const struct bpf_map *map, int nid,
 	return ret;
 }
 
+/*
+ * For callers that know they run in a sleepable context, e.g. a user page
+ * fault handler. can_alloc_pages() is a conservative guess made for BPF
+ * program context - notably it is always false on PREEMPT_RT - so going
+ * through bpf_map_alloc_pages() there would needlessly pick the
+ * non-blocking allocator, which never reclaims and never engages the OOM
+ * machinery.
+ */
+struct page *bpf_map_alloc_page_sleepable(const struct bpf_map *map)
+{
+	might_sleep();
+	return alloc_pages_node(map->numa_node, BPF_PAGE_GFP, 0);
+}
+
 static int btf_field_cmp(const void *a, const void *b)
 {
 	const struct btf_field *f1 = a, *f2 = b;
-- 
2.43.0

[PATCH bpf-next v4 2/4] bpf: arena: allocate the fault-in page outside the lock

[Jiayuan Chen]

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.

Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>
Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
---
 kernel/bpf/arena.c | 92 +++++++++++++++++++++++++++++++++++-----------
 1 file changed, 71 insertions(+), 21 deletions(-)

diff --git a/kernel/bpf/arena.c b/kernel/bpf/arena.c
index 7b6847200b43..50dac9ba0f5e 100644
--- a/kernel/bpf/arena.c
+++ b/kernel/bpf/arena.c
@@ -481,7 +481,8 @@ static vm_fault_t arena_vm_fault(struct vm_fault *vmf)
 	struct bpf_map *map = vmf->vma->vm_file->private_data;
 	struct bpf_arena *arena = container_of(map, struct bpf_arena, map);
 	struct mem_cgroup *new_memcg, *old_memcg;
-	struct page *page;
+	struct page *page, *new_page = NULL;
+	vm_fault_t fault_ret;
 	long kbase, kaddr;
 	unsigned long flags;
 	int ret;
@@ -489,59 +490,108 @@ static vm_fault_t arena_vm_fault(struct vm_fault *vmf)
 	kbase = bpf_arena_get_kern_vm_start(arena);
 	kaddr = kbase + (u32)(vmf->address);
 
-	if (raw_res_spin_lock_irqsave(&arena->spinlock, flags))
+	page = vmalloc_to_page((void *)kaddr);
+	if (!page && !(arena->map.map_flags & BPF_F_SEGV_ON_FAULT)) {
+		/*
+		 * We run in process context here, so preallocate the page
+		 * outside the lock with an explicitly sleepable allocator. It
+		 * can then go through reclaim (both memcg and global) and the
+		 * OOM path, the way do_anonymous_page() does; under
+		 * arena->spinlock only the non-blocking allocator is available,
+		 * which never reclaims. That also decides the return value:
+		 * VM_FAULT_OOM below is only meaningful if the OOM machinery was
+		 * actually engaged, which the non-blocking allocator never does.
+		 */
+		bpf_map_memcg_enter(&arena->map, &old_memcg, &new_memcg);
+		new_page = bpf_map_alloc_page_sleepable(map);
+		bpf_map_memcg_exit(old_memcg, new_memcg);
+		if (!new_page)
+			return VM_FAULT_OOM;
+	}
+
+	if (raw_res_spin_lock_irqsave(&arena->spinlock, flags)) {
 		/*
 		 * A failed lock means a possible deadlock was detected. Don't
 		 * return VM_FAULT_RETRY: this handler never took mmap_lock, but
 		 * the fault path would re-take it on retry and deadlock. Fail.
 		 */
+		if (new_page)
+			free_pages_nolock(new_page, 0);
 		return VM_FAULT_SIGBUS;
+	}
 
 	page = vmalloc_to_page((void *)kaddr);
 	if (page) {
-		if (page == arena->scratch_page)
-			/* BPF triggered scratch here; don't lazy-alloc over it */
-			goto out_sigsegv;
+		if (page == arena->scratch_page) {
+			/*
+			 * A scratch page marks a hole. Segfault only if the user
+			 * asked for it; otherwise we could lazy-allocate but
+			 * choose not to over a hole, so report a bus error.
+			 */
+			fault_ret = (arena->map.map_flags & BPF_F_SEGV_ON_FAULT) ?
+				    VM_FAULT_SIGSEGV : VM_FAULT_SIGBUS;
+			goto out_err_locked;
+		}
 		/* already have a page vmap-ed */
 		goto out;
 	}
 
+	if (arena->map.map_flags & BPF_F_SEGV_ON_FAULT) {
+		/* User space requested to segfault when page is not allocated by bpf prog */
+		fault_ret = VM_FAULT_SIGSEGV;
+		goto out_err_locked;
+	}
+
 	bpf_map_memcg_enter(&arena->map, &old_memcg, &new_memcg);
 
-	if (arena->map.map_flags & BPF_F_SEGV_ON_FAULT)
-		/* User space requested to segfault when page is not allocated by bpf prog */
-		goto out_sigsegv_memcg;
+	if (!new_page) {
+		/*
+		 * Very rare race: the bpf program had allocated a page here, so
+		 * the lockless probe saw it and we skipped preallocation, but it
+		 * freed the page before we took the lock. Now we do need one;
+		 * sleeping is not allowed here, so fall back to the non-blocking
+		 * allocator and give up if it fails.
+		 */
+		ret = bpf_map_alloc_pages(map, map->numa_node, 1, &new_page);
+		if (ret) {
+			fault_ret = VM_FAULT_SIGBUS;
+			goto out_err_locked_memcg;
+		}
+	}
 
 	ret = range_tree_clear(&arena->rt, vmf->pgoff, 1);
-	if (ret)
-		goto out_sigsegv_memcg;
-
-	struct apply_range_data data = { .arena = arena, .pages = &page, .i = 0 };
-	/* Account into memcg of the process that created bpf_arena */
-	ret = bpf_map_alloc_pages(map, NUMA_NO_NODE, 1, &page);
 	if (ret) {
-		range_tree_set(&arena->rt, vmf->pgoff, 1);
-		goto out_sigsegv_memcg;
+		fault_ret = VM_FAULT_SIGBUS;
+		goto out_err_locked_memcg;
 	}
+	struct apply_range_data data = { .arena = arena, .pages = &new_page, .i = 0 };
 
 	ret = apply_to_page_range(&init_mm, kaddr, PAGE_SIZE, apply_range_set_cb, &data);
 	if (ret) {
 		range_tree_set(&arena->rt, vmf->pgoff, 1);
-		free_pages_nolock(page, 0);
-		goto out_sigsegv_memcg;
+		fault_ret = VM_FAULT_SIGBUS;
+		goto out_err_locked_memcg;
 	}
 	flush_vmap_cache(kaddr, PAGE_SIZE);
 	bpf_map_memcg_exit(old_memcg, new_memcg);
+	/* new_page was consumed */
+	page = new_page;
+	new_page = NULL;
 out:
 	page_ref_add(page, 1);
 	raw_res_spin_unlock_irqrestore(&arena->spinlock, flags);
+	if (new_page)
+		free_pages_nolock(new_page, 0);
 	vmf->page = page;
 	return 0;
-out_sigsegv_memcg:
+
+out_err_locked_memcg:
 	bpf_map_memcg_exit(old_memcg, new_memcg);
-out_sigsegv:
+out_err_locked:
 	raw_res_spin_unlock_irqrestore(&arena->spinlock, flags);
-	return VM_FAULT_SIGSEGV;
+	if (new_page)
+		free_pages_nolock(new_page, 0);
+	return fault_ret;
 }
 
 static const struct vm_operations_struct arena_vm_ops = {
-- 
2.43.0

Re: [PATCH bpf-next v4 2/4] bpf: arena: allocate the fault-in page outside the lock

[bot+bpf-ci]

[-- Attachment #1: Type: text/plain, Size: 3960 bytes --]

> diff --git a/kernel/bpf/arena.c b/kernel/bpf/arena.c
> index 7b6847200b43..50dac9ba0f5e 100644
> --- a/kernel/bpf/arena.c
> +++ b/kernel/bpf/arena.c

[ ... ]

> @@ -489,59 +490,108 @@ static vm_fault_t arena_vm_fault(struct vm_fault *vmf)
>  	kbase = bpf_arena_get_kern_vm_start(arena);
>  	kaddr = kbase + (u32)(vmf->address);
>
> +	page = vmalloc_to_page((void *)kaddr);
> +	if (!page && !(arena->map.map_flags & BPF_F_SEGV_ON_FAULT)) {
> +		/*
> +		 * We run in process context here, so preallocate the page
> +		 * outside the lock with an explicitly sleepable allocator. It
> +		 * can then go through reclaim (both memcg and global) and the
> +		 * OOM path, the way do_anonymous_page() does; under
> +		 * arena->spinlock only the non-blocking allocator is available,
> +		 * which never reclaims. That also decides the return value:
> +		 * VM_FAULT_OOM below is only meaningful if the OOM machinery was
> +		 * actually engaged, which the non-blocking allocator never does.
> +		 */
> +		bpf_map_memcg_enter(&arena->map, &old_memcg, &new_memcg);
> +		new_page = bpf_map_alloc_page_sleepable(map);
> +		bpf_map_memcg_exit(old_memcg, new_memcg);
> +		if (!new_page)
> +			return VM_FAULT_OOM;
> +	}

Can this return VM_FAULT_OOM when the allocation is charged to a foreign
memcg?

The preallocation charges the arena creator's memcg, not current's:

    bpf_map_memcg_enter(&arena->map, &old_memcg, &new_memcg);
    /* set_active_memcg(bpf_map_get_memcg(map)) */

    new_page = bpf_map_alloc_page_sleepable(map);
    /* GFP_KERNEL|__GFP_ZERO|__GFP_ACCOUNT|__GFP_NOWARN */

For an order-0 GFP_KERNEL allocation the only realistic NULL is a memcg
charge rejection. mem_cgroup_oom() calls out_of_memory() with oom_control
pointing at the arena owner's memcg, so the OOM killer can only select
victims inside that cgroup.

VM_FAULT_OOM reaches pagefault_out_of_memory() (mm/oom_kill.c:1180-1193),
which only resolves current->memcg_in_oom; otherwise it prints the
rate-limited "Huh VM_FAULT_OOM leaked out to the #PF handler. Retrying PF"
message and returns. Because the over-limit memcg is foreign, killing
tasks there never makes fatal_signal_pending(current) true, so
try_charge_memcg()'s force-charge escape hatch never fires.

If the owner memcg's charge is dominated by memory its tasks cannot free
(for example the arena's own pages, which are only freed by the bpf program
or by map destruction and are unreclaimable), or if that cgroup has no
OOM-eligible tasks left, mem_cgroup_oom() returns false, try_charge_memcg()
returns -ENOMEM, and the instruction is retried in a loop.

This creates an unbounded fault/retry livelock burning CPU in the faulting
task plus dmesg spam, where the pre-image returned a bounded
VM_FAULT_SIGSEGV. Before this patch the in-lock allocator never engaged
reclaim or the OOM killer, so a user touching an arena page could not kill
anything. Now any task that can mmap the arena (the fd can be passed via
SCM_RIGHTS or a bpffs pin, so the faulting task need not be in the owner's
cgroup at all) can drive the memcg OOM killer in the owner's cgroup one
task at a time simply by faulting in pages.

VM_FAULT_OOM promises the caller that the OOM machinery was engaged on a
domain where retrying can succeed. The in-code comment acknowledges the
general point ("VM_FAULT_OOM below is only meaningful if the OOM machinery
was actually engaged") but not the foreign-memcg case. Would it make sense
to use a bounded status for the charge-failure case (VM_FAULT_SIGBUS, or
VM_FAULT_OOM only when the charged memcg == current's memcg)? That would
keep the fix's benefit (reclaim now runs, which is the routine memory.max
case the changelog cares about) without the unbounded retry.


---
AI reviewed your patch. Please fix the bug or email reply why it's not a bug.
See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md

CI run summary: https://github.com/kernel-patches/bpf/actions/runs/32450158690

[PATCH bpf-next v4 3/4] selftests/bpf: Add read_cgroup_file() to cgroup_helpers

[Jiayuan Chen]

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.

Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>
Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
---
 tools/testing/selftests/bpf/cgroup_helpers.c | 67 ++++++++++++++++++++
 tools/testing/selftests/bpf/cgroup_helpers.h |  4 ++
 2 files changed, 71 insertions(+)

diff --git a/tools/testing/selftests/bpf/cgroup_helpers.c b/tools/testing/selftests/bpf/cgroup_helpers.c
index 45cd0b479fe3..4183ff6150c2 100644
--- a/tools/testing/selftests/bpf/cgroup_helpers.c
+++ b/tools/testing/selftests/bpf/cgroup_helpers.c
@@ -188,6 +188,73 @@ int write_cgroup_file_parent(const char *relative_path, const char *file,
 	return __write_cgroup_file(cgroup_path, file, buf);
 }
 
+static int __read_cgroup_file(const char *cgroup_path, const char *file,
+			      char *buf, size_t len)
+{
+	char file_path[PATH_MAX + 1];
+	ssize_t got;
+	int fd;
+
+	snprintf(file_path, sizeof(file_path), "%s/%s", cgroup_path, file);
+	fd = open(file_path, O_RDONLY);
+	if (fd < 0) {
+		log_err("Opening %s", file_path);
+		return 1;
+	}
+
+	got = read(fd, buf, len - 1);
+	if (got < 0) {
+		log_err("Reading %s", file_path);
+		close(fd);
+		return 1;
+	}
+	buf[got] = '\0';
+	close(fd);
+	return 0;
+}
+
+/**
+ * read_cgroup_file() - Read from a cgroup file
+ * @relative_path: The cgroup path, relative to the workdir
+ * @file: The name of the file in cgroupfs to read from
+ * @buf: Buffer to read into, NUL-terminated on success
+ * @len: Size of @buf
+ *
+ * Read from a file in the given cgroup's directory.
+ *
+ * If successful, 0 is returned.
+ */
+int read_cgroup_file(const char *relative_path, const char *file,
+		     char *buf, size_t len)
+{
+	char cgroup_path[PATH_MAX - 24];
+
+	format_cgroup_path(cgroup_path, relative_path);
+	return __read_cgroup_file(cgroup_path, file, buf, len);
+}
+
+/**
+ * read_cgroup_file_parent() - Read from a cgroup file in the parent process
+ *                             workdir
+ * @relative_path: The cgroup path, relative to the parent process workdir
+ * @file: The name of the file in cgroupfs to read from
+ * @buf: Buffer to read into, NUL-terminated on success
+ * @len: Size of @buf
+ *
+ * Read from a file in the given cgroup's directory under the parent process
+ * workdir.
+ *
+ * If successful, 0 is returned.
+ */
+int read_cgroup_file_parent(const char *relative_path, const char *file,
+			    char *buf, size_t len)
+{
+	char cgroup_path[PATH_MAX - 24];
+
+	format_parent_cgroup_path(cgroup_path, relative_path);
+	return __read_cgroup_file(cgroup_path, file, buf, len);
+}
+
 /**
  * setup_cgroup_environment() - Setup the cgroup environment
  *
diff --git a/tools/testing/selftests/bpf/cgroup_helpers.h b/tools/testing/selftests/bpf/cgroup_helpers.h
index 3857304be874..d42d2e13044e 100644
--- a/tools/testing/selftests/bpf/cgroup_helpers.h
+++ b/tools/testing/selftests/bpf/cgroup_helpers.h
@@ -15,6 +15,10 @@ int write_cgroup_file(const char *relative_path, const char *file,
 		      const char *buf);
 int write_cgroup_file_parent(const char *relative_path, const char *file,
 			     const char *buf);
+int read_cgroup_file(const char *relative_path, const char *file,
+		     char *buf, size_t len);
+int read_cgroup_file_parent(const char *relative_path, const char *file,
+			    char *buf, size_t len);
 int cgroup_setup_and_join(const char *relative_path);
 int get_root_cgroup(void);
 int create_and_get_cgroup(const char *relative_path);
-- 
2.43.0

[PATCH bpf-next v4 4/4] selftests/bpf: Add a test for arena fault-in under memory.max

[Jiayuan Chen]

A child joins a memcg capped 64M above its post-load usage and faults an
arena in until it runs out of that budget.

With the fix the arena page comes from the sleepable allocator, so
hitting memory.max goes through the memcg OOM path and the child is
OOM-killed, which the test checks via memory.events "oom_kill".

Without the fix the test may still pass, because a concurrent blocking
allocation in the child (e.g. a COW fault on an inherited page) can hit
memory.max and OOM-kill it first. The goal is only that the fixed kernel
passes reliably.

  # test_progs -v -t arena_memcg
  serial_test_arena_memcg:PASS:child killed by signal
  serial_test_arena_memcg:PASS:memcg oom_kill
  #5       arena_memcg:OK

  # dmesg (the OOM comes from the arena sleepable allocation)
  test_progs invoked oom-killer: gfp_mask=GFP_KERNEL_ACCOUNT|__GFP_ZERO
   arena_vm_fault+0x4bc/0xad0
  Memory cgroup out of memory: Killed process 473 (test_progs)

Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>
Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
---
 .../selftests/bpf/prog_tests/arena_memcg.c    | 156 ++++++++++++++++++
 .../testing/selftests/bpf/progs/arena_memcg.c |  24 +++
 2 files changed, 180 insertions(+)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/arena_memcg.c
 create mode 100644 tools/testing/selftests/bpf/progs/arena_memcg.c

diff --git a/tools/testing/selftests/bpf/prog_tests/arena_memcg.c b/tools/testing/selftests/bpf/prog_tests/arena_memcg.c
new file mode 100644
index 000000000000..af49d8b1b995
--- /dev/null
+++ b/tools/testing/selftests/bpf/prog_tests/arena_memcg.c
@@ -0,0 +1,156 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <test_progs.h>
+#include <fcntl.h>
+#include <signal.h>
+#include <sys/mman.h>
+#include <sys/wait.h>
+#include <unistd.h>
+#include <sys/user.h>
+#ifndef PAGE_SIZE /* on some archs it comes in sys/user.h */
+#include <unistd.h>
+#define PAGE_SIZE getpagesize()
+#endif
+
+#include "cgroup_helpers.h"
+#include "arena_memcg.skel.h"
+
+#define CG_PATH		"/arena_memcg"
+
+/* Budget the arena gets on top of whatever is already charged after load. */
+#define ARENA_BUDGET	(64 * 1024 * 1024)
+
+static void dump_memcg(int (*rd)(const char *, const char *, char *, size_t))
+{
+	char buf[512];
+
+	/*
+	 * memory.current reads 0 once the child has left the cgroup, so it only
+	 * carries information when dumped from the live child; memory.peak and
+	 * memory.events survive the child and tell the story either way.
+	 */
+	if (!rd(CG_PATH, "memory.current", buf, sizeof(buf)))
+		fprintf(stderr, "memory.current: %s", buf);
+	if (!rd(CG_PATH, "memory.max", buf, sizeof(buf)))
+		fprintf(stderr, "memory.max: %s", buf);
+	if (!rd(CG_PATH, "memory.peak", buf, sizeof(buf)))
+		fprintf(stderr, "memory.peak: %s", buf);
+	if (!rd(CG_PATH, "memory.events", buf, sizeof(buf)))
+		fprintf(stderr, "memory.events:\n%s", buf);
+	fflush(NULL); /* _exit() in the child would not flush stdio otherwise */
+}
+
+/* Read one key from a flat keyed cgroup file, e.g. "oom_kill" in memory.events. */
+static long cg_read_key(const char *cg, const char *file, const char *key)
+{
+	char buf[512], *p;
+
+	if (read_cgroup_file(cg, file, buf, sizeof(buf)))
+		return -1;
+	p = strstr(buf, key);
+	if (!p)
+		return -1;
+	return strtol(p + strlen(key), NULL, 10);
+}
+
+void serial_test_arena_memcg(void)
+{
+	int cgroup_fd = -1, status;
+	const long ps = PAGE_SIZE;
+	char buf[64];
+	pid_t pid;
+
+	if (setup_cgroup_environment())
+		return;
+
+	cgroup_fd = create_and_get_cgroup(CG_PATH);
+	if (!ASSERT_OK_FD(cgroup_fd, "create_and_get_cgroup"))
+		goto out;
+
+	/* No memory controller -> nothing to test. */
+	if (read_cgroup_file(CG_PATH, "memory.current", buf, sizeof(buf))) {
+		test__skip();
+		goto out;
+	}
+
+	pid = fork();
+	if (!ASSERT_GE(pid, 0, "fork"))
+		goto out;
+	if (pid == 0) {
+		struct arena_memcg *cskel;
+		__u32 i, npages;
+		char *base;
+		size_t sz;
+		long cur;
+
+		/*
+		 * Do everything from the child: the arena vma is VM_DONTCOPY so
+		 * it would not survive fork(), only the child should be under the
+		 * limit so that a memcg OOM cannot pick test_progs, and a map is
+		 * charged to the memcg of the task that creates it - so join
+		 * before load. The cgroup work dir belongs to the parent that set
+		 * the environment up, so reach it with the _parent() helpers.
+		 * Errors are reported to the parent through the exit code, since
+		 * ASSERT_* in a forked child does not reach it.
+		 */
+		snprintf(buf, sizeof(buf), "%d", getpid());
+		if (write_cgroup_file_parent(CG_PATH, "cgroup.procs", buf))
+			_exit(2);
+
+		cskel = arena_memcg__open_and_load();
+		if (!cskel)
+			_exit(3);
+
+		base = bpf_map__initial_value(cskel->maps.arena, &sz);
+		if (!base)
+			_exit(4);
+		npages = bpf_map__max_entries(cskel->maps.arena);
+
+		/*
+		 * Cap only now, after load: everything but the fault-in is
+		 * charged, so the arena gets a fixed budget regardless of what
+		 * the load itself cost, and the load can never hit the limit.
+		 */
+		if (read_cgroup_file_parent(CG_PATH, "memory.current", buf, sizeof(buf)))
+			_exit(5);
+		cur = strtol(buf, NULL, 10);
+		snprintf(buf, sizeof(buf), "%ld", cur + ARENA_BUDGET);
+		if (write_cgroup_file_parent(CG_PATH, "memory.max", buf))
+			_exit(6);
+
+		for (i = 0; i < npages; i++)
+			base[(size_t)i * ps] = 1;
+		/* Faulted everything without dying: no pressure built, dump why. */
+		dump_memcg(read_cgroup_file_parent);
+		_exit(0);
+	}
+
+	if (!ASSERT_EQ(waitpid(pid, &status, 0), pid, "waitpid"))
+		goto out;
+
+	/* A non-zero exit means the child failed to set up; the code says where. */
+	if (WIFEXITED(status) && WEXITSTATUS(status)) {
+		ASSERT_OK(WEXITSTATUS(status), "child setup");
+		goto out;
+	}
+
+	/*
+	 * Faulting a valid arena address until memory.max is hit must not look
+	 * like an invalid access. Without the fix the fault path allocated with
+	 * the non-blocking allocator, turned its -ENOMEM into VM_FAULT_SIGSEGV,
+	 * and the child died with SIGSEGV on a valid address; now it is handled
+	 * by the memcg OOM path instead. A SIGKILL alone would not prove the
+	 * memcg OOM killer did it (a global OOM or an unrelated crash could also
+	 * kill the child), so check memory.events.oom_kill, which records the
+	 * memcg OOM and survives the child.
+	 */
+	if (!ASSERT_TRUE(WIFSIGNALED(status), "child killed by signal"))
+		goto out;
+	if (!ASSERT_GE(cg_read_key(CG_PATH, "memory.events", "oom_kill"), 1,
+		       "memcg oom_kill"))
+		dump_memcg(read_cgroup_file);
+out:
+	if (cgroup_fd >= 0)
+		close(cgroup_fd);
+	cleanup_cgroup_environment();
+}
diff --git a/tools/testing/selftests/bpf/progs/arena_memcg.c b/tools/testing/selftests/bpf/progs/arena_memcg.c
new file mode 100644
index 000000000000..88259cfea06b
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/arena_memcg.c
@@ -0,0 +1,24 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <vmlinux.h>
+#include <bpf/bpf_helpers.h>
+#include "bpf_arena_common.h"
+
+struct {
+	__uint(type, BPF_MAP_TYPE_ARENA);
+	__uint(map_flags, BPF_F_MMAPABLE);
+	__uint(max_entries, 50000); /* number of pages */
+#ifdef __TARGET_ARCH_arm64
+	__ulong(map_extra, 0x1ull << 32); /* start of mmap() region */
+#else
+	__ulong(map_extra, 0x1ull << 44); /* start of mmap() region */
+#endif
+} arena SEC(".maps");
+
+SEC("syscall")
+int noop(void *ctx)
+{
+	return 0;
+}
+
+char _license[] SEC("license") = "GPL";
-- 
2.43.0

Re: [PATCH bpf-next v4 4/4] selftests/bpf: Add a test for arena fault-in under memory.max

[bot+bpf-ci]

[-- Attachment #1: Type: text/plain, Size: 5731 bytes --]

> diff --git a/tools/testing/selftests/bpf/prog_tests/arena_memcg.c b/tools/testing/selftests/bpf/prog_tests/arena_memcg.c
> new file mode 100644
> index 000000000000..af49d8b1b995
> --- /dev/null
> +++ b/tools/testing/selftests/bpf/prog_tests/arena_memcg.c
> @@ -0,0 +1,156 @@

[ ... ]

> +static void dump_memcg(int (*rd)(const char *, const char *, char *, size_t))
> +{
> +	char buf[512];
> +
> +	/*
> +	 * memory.current reads 0 once the child has left the cgroup, so it only
> +	 * carries information when dumped from the live child; memory.peak and
> +	 * memory.events survive the child and tell the story either way.
> +	 */
> +	if (!rd(CG_PATH, "memory.current", buf, sizeof(buf)))
> +		fprintf(stderr, "memory.current: %s", buf);
> +	if (!rd(CG_PATH, "memory.max", buf, sizeof(buf)))
> +		fprintf(stderr, "memory.max: %s", buf);
> +	if (!rd(CG_PATH, "memory.peak", buf, sizeof(buf)))
> +		fprintf(stderr, "memory.peak: %s", buf);
> +	if (!rd(CG_PATH, "memory.events", buf, sizeof(buf)))
> +		fprintf(stderr, "memory.events:\n%s", buf);
> +	fflush(NULL); /* _exit() in the child would not flush stdio otherwise */
> +}

Looking at fflush(NULL) - does this need to flush every open stream?
Under -v with output redirected, any test_progs output still buffered in
stdout at fork() time would be emitted a second time by the child.
Would fflush(stderr) be sufficient here?

> +
> +/* Read one key from a flat keyed cgroup file, e.g. "oom_kill" in memory.events. */
> +static long cg_read_key(const char *cg, const char *file, const char *key)
> +{
> +	char buf[512], *p;
> +
> +	if (read_cgroup_file(cg, file, buf, sizeof(buf)))
> +		return -1;
> +	p = strstr(buf, key);
> +	if (!p)
> +		return -1;
> +	return strtol(p + strlen(key), NULL, 10);
> +}
> +
> +void serial_test_arena_memcg(void)
> +{
> +	int cgroup_fd = -1, status;
> +	const long ps = PAGE_SIZE;
> +	char buf[64];
> +	pid_t pid;
> +
> +	if (setup_cgroup_environment())
> +		return;

Can setup_cgroup_environment() failing here result in a silent pass?
setup_cgroup_environment() can fail at several points - mkdir, unshare,
mount operations - meaning on a host without the right capabilities or
where cgroup2 cannot be mounted, the test returns before any ASSERT_*
runs and test_progs records arena_memcg:OK even though nothing was
tested. Since the arena/memcg interaction is the entire point of the
test, would wrapping this in ASSERT_OK() or adding an explicit skip be
clearer?

> +
> +	cgroup_fd = create_and_get_cgroup(CG_PATH);
> +	if (!ASSERT_OK_FD(cgroup_fd, "create_and_get_cgroup"))
> +		goto out;
> +
> +	/* No memory controller -> nothing to test. */
> +	if (read_cgroup_file(CG_PATH, "memory.current", buf, sizeof(buf))) {
> +		test__skip();
> +		goto out;
> +	}

A subsystem pattern flags this as potentially concerning: when the
memory controller is not available, test__skip() increments the skip
counter but prints no reason - a #N arena_memcg:SKIP line in a CI log
would give no indication of why. The comment records it in the source,
but nothing emits it at runtime. Every other arena test in this
directory prints an explicit reason with its skip:

  prog_tests/arena_htab.c:
    printf("%s:SKIP:compiler doesn't support arena_cast\n", __func__);

  prog_tests/arena_list.c, arena_spin_lock.c, arena_atomics.c:
    [similar patterns]

Would adding printf("%s:SKIP:no memory controller\n", __func__); before
test__skip() match the neighbours?

> +
> +	pid = fork();
> +	if (!ASSERT_GE(pid, 0, "fork"))
> +		goto out;
> +	if (pid == 0) {
> +		struct arena_memcg *cskel;
> +		__u32 i, npages;
> +		char *base;
> +		size_t sz;
> +		long cur;
> +
> +		/*
> +		 * Do everything from the child: the arena vma is VM_DONTCOPY so
> +		 * it would not survive fork(), only the child should be under the
> +		 * limit so that a memcg OOM cannot pick test_progs, and a map is
> +		 * charged to the memcg of the task that creates it - so join
> +		 * before load. The cgroup work dir belongs to the parent that set
> +		 * the environment up, so reach it with the _parent() helpers.
> +		 * Errors are reported to the parent through the exit code, since
> +		 * ASSERT_* in a forked child does not reach it.
> +		 */

[ ... ]

> +		for (i = 0; i < npages; i++)
> +			base[(size_t)i * ps] = 1;
> +		/* Faulted everything without dying: no pressure built, dump why. */
> +		dump_memcg(read_cgroup_file_parent);
> +		_exit(0);

Can the child's dump_memcg() output reach the parent's log?
stdio_hijack_init() redirects stderr to an in-memory stream in
non-verbose runs, and after fork() the child has a private copy-on-write
copy of that FILE. The fprintf() calls in dump_memcg() and fflush(NULL)
write only into the child's address space, then _exit(0) throws them
away. So in a default test_progs run - exactly where diagnostics are
needed - the parent then fails ASSERT_TRUE(WIFSIGNALED(status)) with no
memcg state attached. The sample output in the commit message was
captured with -v, the one mode where this path does work. The second
dump_memcg() call at the end of the function runs in the parent and is
unaffected. Would writing the numbers over a pipe, or having the parent
dump memory.peak/memory.events on the !WIFSIGNALED path, get them into
the log?

> +	}

[ ... ]

> +	if (!ASSERT_TRUE(WIFSIGNALED(status), "child killed by signal"))
> +		goto out;
> +	if (!ASSERT_GE(cg_read_key(CG_PATH, "memory.events", "oom_kill"), 1,
> +		       "memcg oom_kill"))
> +		dump_memcg(read_cgroup_file);

[ ... ]


---
AI reviewed your patch. Please fix the bug or email reply why it's not a bug.
See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md

CI run summary: https://github.com/kernel-patches/bpf/actions/runs/32450158690