[PATCH v2] mm/alloc_tag: clear codetag for pages allocated before page_ext initialization

[PATCH v2] mm/alloc_tag: clear codetag for pages allocated before page_ext initialization

[Hao Ge]

Due to initialization ordering, page_ext is allocated and initialized
relatively late during boot. Some pages have already been allocated
and freed before page_ext becomes available, leaving their codetag
uninitialized.

A clear example is in init_section_page_ext(): alloc_page_ext() calls
kmemleak_alloc(). If the slab cache has no free objects, it falls back
to the buddy allocator to allocate memory. However, at this point page_ext
is not yet fully initialized, so these newly allocated pages have no
codetag set. These pages may later be reclaimed by KASAN, which causes
the warning to trigger when they are freed because their codetag ref is
still empty.

Use a global array to track pages allocated before page_ext is fully
initialized. The array size is fixed at 8192 entries, and will emit
a warning if this limit is exceeded. When page_ext initialization
completes, set their codetag to empty to avoid warnings when they
are freed later.

The following warning is observed when this issue occurs:
[    9.582133] ------------[ cut here ]------------
[    9.582137] alloc_tag was not set
[    9.582139] WARNING: ./include/linux/alloc_tag.h:164 at __pgalloc_tag_sub+0x40f/0x550, CPU#5: systemd/1
[    9.582190] CPU: 5 UID: 0 PID: 1 Comm: systemd Not tainted 7.0.0-rc4 #1 PREEMPT(lazy)
[    9.582192] Hardware name: Red Hat KVM, BIOS rel-1.16.3-0-ga6ed6b701f0a-prebuilt.qemu.org 04/01/2014
[    9.582194] RIP: 0010:__pgalloc_tag_sub+0x40f/0x550
[    9.582196] Code: 00 00 4c 29 e5 48 8b 05 1f 88 56 05 48 8d 4c ad 00 48 8d 2c c8 e9 87 fd ff ff 0f 0b 0f 0b e9 f3 fe ff ff 48 8d 3d 61 2f ed 03 <67> 48 0f b9 3a e9 b3 fd ff ff 0f 0b eb e4 e8 5e cd 14 02 4c 89 c7
[    9.582197] RSP: 0018:ffffc9000001f940 EFLAGS: 00010246
[    9.582200] RAX: dffffc0000000000 RBX: 1ffff92000003f2b RCX: 1ffff110200d806c
[    9.582201] RDX: ffff8881006c0360 RSI: 0000000000000004 RDI: ffffffff9bc7b460
[    9.582202] RBP: 0000000000000000 R08: 0000000000000000 R09: fffffbfff3a62324
[    9.582203] R10: ffffffff9d311923 R11: 0000000000000000 R12: ffffea0004001b00
[    9.582204] R13: 0000000000002000 R14: ffffea0000000000 R15: ffff8881006c0360
[    9.582206] FS:  00007ffbbcf2d940(0000) GS:ffff888450479000(0000) knlGS:0000000000000000
[    9.582208] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[    9.582210] CR2: 000055ee3aa260d0 CR3: 0000000148b67005 CR4: 0000000000770ef0
[    9.582211] PKRU: 55555554
[    9.582212] Call Trace:
[    9.582213]  <TASK>
[    9.582214]  ? __pfx___pgalloc_tag_sub+0x10/0x10
[    9.582216]  ? check_bytes_and_report+0x68/0x140
[    9.582219]  __free_frozen_pages+0x2e4/0x1150
[    9.582221]  ? __free_slab+0xc2/0x2b0
[    9.582224]  qlist_free_all+0x4c/0xf0
[    9.582227]  kasan_quarantine_reduce+0x15d/0x180
[    9.582229]  __kasan_slab_alloc+0x69/0x90
[    9.582232]  kmem_cache_alloc_noprof+0x14a/0x500
[    9.582234]  do_getname+0x96/0x310
[    9.582237]  do_readlinkat+0x91/0x2f0
[    9.582239]  ? __pfx_do_readlinkat+0x10/0x10
[    9.582240]  ? get_random_bytes_user+0x1df/0x2c0
[    9.582244]  __x64_sys_readlinkat+0x96/0x100
[    9.582246]  do_syscall_64+0xce/0x650
[    9.582250]  ? __x64_sys_getrandom+0x13a/0x1e0
[    9.582252]  ? __pfx___x64_sys_getrandom+0x10/0x10
[    9.582254]  ? do_syscall_64+0x114/0x650
[    9.582255]  ? ksys_read+0xfc/0x1d0
[    9.582258]  ? __pfx_ksys_read+0x10/0x10
[    9.582260]  ? do_syscall_64+0x114/0x650
[    9.582262]  ? do_syscall_64+0x114/0x650
[    9.582264]  ? __pfx_fput_close_sync+0x10/0x10
[    9.582266]  ? file_close_fd_locked+0x178/0x2a0
[    9.582268]  ? __x64_sys_faccessat2+0x96/0x100
[    9.582269]  ? __x64_sys_close+0x7d/0xd0
[    9.582271]  ? do_syscall_64+0x114/0x650
[    9.582273]  ? do_syscall_64+0x114/0x650
[    9.582275]  ? clear_bhb_loop+0x50/0xa0
[    9.582277]  ? clear_bhb_loop+0x50/0xa0
[    9.582279]  entry_SYSCALL_64_after_hwframe+0x76/0x7e
[    9.582280] RIP: 0033:0x7ffbbda345ee
[    9.582282] Code: 0f 1f 40 00 48 8b 15 29 38 0d 00 f7 d8 64 89 02 48 c7 c0 ff ff ff ff c3 0f 1f 40 00 f3 0f 1e fa 49 89 ca b8 0b 01 00 00 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 8b 0d fa 37 0d 00 f7 d8 64 89 01 48
[    9.582284] RSP: 002b:00007ffe2ad8de58 EFLAGS: 00000202 ORIG_RAX: 000000000000010b
[    9.582286] RAX: ffffffffffffffda RBX: 000055ee3aa25570 RCX: 00007ffbbda345ee
[    9.582287] RDX: 000055ee3aa25570 RSI: 00007ffe2ad8dee0 RDI: 00000000ffffff9c
[    9.582288] RBP: 0000000000001000 R08: 0000000000000003 R09: 0000000000001001
[    9.582289] R10: 0000000000001000 R11: 0000000000000202 R12: 0000000000000033
[    9.582290] R13: 00007ffe2ad8dee0 R14: 00000000ffffff9c R15: 00007ffe2ad8deb0
[    9.582292]  </TASK>
[    9.582293] ---[ end trace 0000000000000000 ]---

Fixes: 93d5440ece3c ("alloc_tag: uninline code gated by mem_alloc_profiling_key in page allocator")
Suggested-by: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Hao Ge <hao.ge@linux.dev>
---
v2:
  - Replace spin_lock_irqsave() with atomic_try_cmpxchg() to avoid potential
     deadlock in NMI context
  - Change EARLY_ALLOC_PFN_MAX from 256 to 8192
  - Add pr_warn_once() when the limit is exceeded
  - Check ref.ct before clearing to avoid overwriting valid tags
  - Use function pointer (alloc_tag_add_early_pfn_ptr) instead of state
---
 include/linux/alloc_tag.h   |  2 +
 include/linux/pgalloc_tag.h |  2 +-
 lib/alloc_tag.c             | 92 +++++++++++++++++++++++++++++++++++++
 mm/page_alloc.c             |  7 +++
 4 files changed, 102 insertions(+), 1 deletion(-)

diff --git a/include/linux/alloc_tag.h b/include/linux/alloc_tag.h
index d40ac39bfbe8..bf226c2be2ad 100644
--- a/include/linux/alloc_tag.h
+++ b/include/linux/alloc_tag.h
@@ -74,6 +74,8 @@ static inline void set_codetag_empty(union codetag_ref *ref)
 
 #ifdef CONFIG_MEM_ALLOC_PROFILING
 
+void alloc_tag_add_early_pfn(unsigned long pfn);
+
 #define ALLOC_TAG_SECTION_NAME	"alloc_tags"
 
 struct codetag_bytes {
diff --git a/include/linux/pgalloc_tag.h b/include/linux/pgalloc_tag.h
index 38a82d65e58e..951d33362268 100644
--- a/include/linux/pgalloc_tag.h
+++ b/include/linux/pgalloc_tag.h
@@ -181,7 +181,7 @@ static inline struct alloc_tag *__pgalloc_tag_get(struct page *page)
 
 	if (get_page_tag_ref(page, &ref, &handle)) {
 		alloc_tag_sub_check(&ref);
-		if (ref.ct)
+		if (ref.ct && !is_codetag_empty(&ref))
 			tag = ct_to_alloc_tag(ref.ct);
 		put_page_tag_ref(handle);
 	}
diff --git a/lib/alloc_tag.c b/lib/alloc_tag.c
index 58991ab09d84..7b1812768af9 100644
--- a/lib/alloc_tag.c
+++ b/lib/alloc_tag.c
@@ -6,6 +6,7 @@
 #include <linux/kallsyms.h>
 #include <linux/module.h>
 #include <linux/page_ext.h>
+#include <linux/pgalloc_tag.h>
 #include <linux/proc_fs.h>
 #include <linux/seq_buf.h>
 #include <linux/seq_file.h>
@@ -26,6 +27,96 @@ static bool mem_profiling_support;
 
 static struct codetag_type *alloc_tag_cttype;
 
+#ifdef CONFIG_MEM_ALLOC_PROFILING_DEBUG
+
+/*
+ * Track page allocations before page_ext is initialized.
+ * Some pages are allocated before page_ext becomes available, leaving
+ * their codetag uninitialized. Track these early PFNs so we can clear
+ * their codetag refs later to avoid warnings when they are freed.
+ *
+ * Early allocations include:
+ *   - Base allocations independent of CPU count
+ *   - Per-CPU allocations (e.g., CPU hotplug callbacks during smp_init,
+ *     such as trace ring buffers, scheduler per-cpu data)
+ *
+ * For simplicity, we fix the size to 8192.
+ * If insufficient, a warning will be triggered to alert the user.
+ */
+#define EARLY_ALLOC_PFN_MAX		8192
+
+static unsigned long early_pfns[EARLY_ALLOC_PFN_MAX] __initdata;
+static atomic_t early_pfn_count __initdata = ATOMIC_INIT(0);
+
+static void __init __alloc_tag_add_early_pfn(unsigned long pfn)
+{
+	int old_idx, new_idx;
+
+	do {
+		old_idx = atomic_read(&early_pfn_count);
+		if (old_idx >= EARLY_ALLOC_PFN_MAX) {
+			pr_warn_once("Early page allocations before page_ext init exceeded EARLY_ALLOC_PFN_MAX (%d)\n",
+				      EARLY_ALLOC_PFN_MAX);
+			return;
+		}
+		new_idx = old_idx + 1;
+	} while (!atomic_try_cmpxchg(&early_pfn_count, &old_idx, new_idx));
+
+	early_pfns[old_idx] = pfn;
+}
+
+static void (*alloc_tag_add_early_pfn_ptr)(unsigned long pfn) __refdata =
+		__alloc_tag_add_early_pfn;
+
+void alloc_tag_add_early_pfn(unsigned long pfn)
+{
+	if (static_key_enabled(&mem_profiling_compressed))
+		return;
+
+	if (alloc_tag_add_early_pfn_ptr)
+		alloc_tag_add_early_pfn_ptr(pfn);
+}
+
+static void __init clear_early_alloc_pfn_tag_refs(void)
+{
+	unsigned int i;
+
+	for (i = 0; i < atomic_read(&early_pfn_count); i++) {
+		unsigned long pfn = early_pfns[i];
+
+		if (pfn_valid(pfn)) {
+			struct page *page = pfn_to_page(pfn);
+			union pgtag_ref_handle handle;
+			union codetag_ref ref;
+
+			if (get_page_tag_ref(page, &ref, &handle)) {
+				/*
+				 * An early-allocated page could be freed and reallocated
+				 * after its page_ext is initialized but before we clear it.
+				 * In that case, it already has a valid tag set.
+				 * We should not overwrite that valid tag with CODETAG_EMPTY.
+				 */
+				if (ref.ct) {
+					put_page_tag_ref(handle);
+					continue;
+				}
+
+				set_codetag_empty(&ref);
+				update_page_tag_ref(handle, &ref);
+				put_page_tag_ref(handle);
+			}
+		}
+
+	}
+	atomic_set(&early_pfn_count, 0);
+
+	alloc_tag_add_early_pfn_ptr = NULL;
+}
+#else /* !CONFIG_MEM_ALLOC_PROFILING_DEBUG */
+inline void alloc_tag_add_early_pfn(unsigned long pfn) {}
+static inline void __init clear_early_alloc_pfn_tag_refs(void) {}
+#endif
+
 #ifdef CONFIG_ARCH_MODULE_NEEDS_WEAK_PER_CPU
 DEFINE_PER_CPU(struct alloc_tag_counters, _shared_alloc_tag);
 EXPORT_SYMBOL(_shared_alloc_tag);
@@ -760,6 +851,7 @@ static __init bool need_page_alloc_tagging(void)
 
 static __init void init_page_alloc_tagging(void)
 {
+	clear_early_alloc_pfn_tag_refs();
 }
 
 struct page_ext_operations page_alloc_tagging_ops = {
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 2d4b6f1a554e..8f9bda04403b 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1293,6 +1293,13 @@ void __pgalloc_tag_add(struct page *page, struct task_struct *task,
 		alloc_tag_add(&ref, task->alloc_tag, PAGE_SIZE * nr);
 		update_page_tag_ref(handle, &ref);
 		put_page_tag_ref(handle);
+	} else {
+		/*
+		 * page_ext is not available yet, record the pfn so we can
+		 * clear the tag ref later when page_ext is initialized.
+		 */
+		alloc_tag_add_early_pfn(page_to_pfn(page));
+		alloc_tag_set_inaccurate(current->alloc_tag);
 	}
 }
 
-- 
2.25.1