[PATCH bpf-next v3] bpf: Better build_id caching in stack_map_get_build_id_offset()

[Ihor Solodrai <ihor.solodrai@linux.dev>]

This patch is a follow up to recent implementation of
stack_map_get_build_id_offset_sleepable() [1].

stack_map_get_build_id_offset() and its sleepable variant each cached
only the last successfully resolved VMA, with separate bookkeeping in
each function. A run of IPs in a VMA with no usable build ID will
repeat the lookup for every frame: find_vma() in the non-sleepable
path, a VMA lock and a blocking build_id_parse_file() in the sleepable.

Factor the per-call cache into a shared struct stack_map_build_id_cache
with two independent slots [2][3], used by both functions:

  * resolved   - last VMA that produced a build ID (file, build_id and
                 range), reused to skip the lookup and the parse;
  * unresolved - last VMA with no usable build ID (range only), reused to
                 emit a raw IP without another lookup or parse.

Keeping the slots independent means a build-ID-less VMA no longer evicts
the last resolved build ID, so a trace alternating between a binary and a
region without one stops re-resolving the binary on every return.

The shared lookup tests [vm_start, vm_end), matching the sleepable path;
the non-sleepable path previously reused a build ID for ip == vm_end
(range_in_vma() is inclusive) and now re-resolves it correctly.

[1] https://lore.kernel.org/bpf/20260525223948.1920986-1-ihor.solodrai@linux.dev/
[2] https://lore.kernel.org/bpf/CAEf4Bza2fRDGhLQoPE-EzM7F34xaEJfi5Exmxb-iWVUN3F06=g@mail.gmail.com/
[3] https://lore.kernel.org/bpf/CAEf4BzZXJFr=1iiVx937ht=4PYQkQHg=eFk810zhMDzXQG3ihw@mail.gmail.com/

Suggested-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Ihor Solodrai <ihor.solodrai@linux.dev>

---

v2->v3:
  * more refactoring, no functional changes
    * define and use struct stack_map_cached_vma as a cache slot type
    * use compound literals where appropriate
    * add `res = &cache.resolved` local in stack_map_get_build_id_offset_sleepable()
    * minor comment fixups
v2: https://lore.kernel.org/bpf/20260612220651.2507843-1-ihor.solodrai@linux.dev/

v1->v2:
  * nits in attempt to de-claudify the diff (Alexei)
    * factor out local variables in stack_map_build_id_set_from_cache()
      to make the conditions more readable
    * make comments more concise
  * I am ignoring the issue flagged by sashiko, since it's unrelated
    to the patch
v1: https://lore.kernel.org/bpf/20260609210749.3448062-1-ihor.solodrai@linux.dev/

---
 kernel/bpf/stackmap.c | 183 ++++++++++++++++++++++++++++++------------
 1 file changed, 130 insertions(+), 53 deletions(-)

diff --git a/kernel/bpf/stackmap.c b/kernel/bpf/stackmap.c
index 77ba03216c09..41fe87d7302f 100644
--- a/kernel/bpf/stackmap.c
+++ b/kernel/bpf/stackmap.c
@@ -175,6 +175,95 @@ static inline void stack_map_build_id_set_valid(struct bpf_stack_build_id *id,
 		memcpy(id->build_id, build_id, BUILD_ID_SIZE_MAX);
 }
 
+/*
+ * A cached VMA lookup result. The range [vm_start, vm_end) is always set.
+ * vm_pgoff, file, build_id are set only when the build ID was resolved.
+ * Zero vm_end marks the slot empty. build_id aliases the id_offs[] entry.
+ */
+struct stack_map_cached_vma {
+	unsigned long vm_start;
+	unsigned long vm_end;
+	unsigned long vm_pgoff;
+	struct file *file; /* pinned in the sleepable path; NULL otherwise */
+	const unsigned char *build_id;
+};
+
+/*
+ * Per stack_map_get_build_id_offset() call cache of the last VMA with a build ID
+ * resolved and the last VMA with no usable build ID. Adjacent stack frames tend
+ * to land in the same VMA or the same backing file, so caching the last result
+ * of each kind lets us skip unnecessary VMA lookups and build ID parse calls.
+ * Keeping the two slots independent means a build-ID-less VMA doesn't evict the
+ * last resolved build ID.
+ */
+struct stack_map_build_id_cache {
+	struct stack_map_cached_vma resolved;
+	struct stack_map_cached_vma unresolved;
+};
+
+/*
+ * Fill @id from a cached range covering @ip. On a hit this writes @id (resolved
+ * range -> build ID + offset, unresolved range -> raw ip) and returns 0; on a
+ * miss it leaves @id untouched and returns -ENOENT.
+ */
+static int stack_map_build_id_set_from_cache(struct stack_map_build_id_cache *cache,
+					     struct bpf_stack_build_id *id, u64 ip)
+{
+	unsigned long vm_start, vm_end, vm_pgoff;
+	u64 offset;
+
+	vm_start = cache->resolved.vm_start;
+	vm_end = cache->resolved.vm_end;
+	if (vm_end && ip >= vm_start && ip < vm_end) {
+		vm_pgoff = cache->resolved.vm_pgoff;
+		offset = stack_map_build_id_offset(vm_pgoff, vm_start, ip);
+		stack_map_build_id_set_valid(id, offset, cache->resolved.build_id);
+		return 0;
+	}
+
+	vm_start = cache->unresolved.vm_start;
+	vm_end = cache->unresolved.vm_end;
+	if (vm_end && ip >= vm_start && ip < vm_end) {
+		stack_map_build_id_set_ip(id);
+		return 0;
+	}
+
+	return -ENOENT;
+}
+
+/*
+ * Record @vma's build ID as the last resolved one. @file is the pinned backing
+ * file in the sleepable path (released when evicted), or NULL otherwise.
+ */
+static void stack_map_build_id_cache_set_resolved(struct stack_map_build_id_cache *cache,
+						  struct file *file,
+						  const unsigned char *build_id,
+						  unsigned long vm_start,
+						  unsigned long vm_end,
+						  unsigned long vm_pgoff)
+{
+	if (cache->resolved.file)
+		fput(cache->resolved.file);
+	cache->resolved = (struct stack_map_cached_vma){
+		.vm_start = vm_start,
+		.vm_end = vm_end,
+		.vm_pgoff = vm_pgoff,
+		.file = file,
+		.build_id = build_id,
+	};
+}
+
+/* Record [vm_start, vm_end) as a range with no usable build ID. */
+static void stack_map_build_id_cache_set_unresolved(struct stack_map_build_id_cache *cache,
+						    unsigned long vm_start,
+						    unsigned long vm_end)
+{
+	cache->unresolved = (struct stack_map_cached_vma){
+		.vm_start = vm_start,
+		.vm_end = vm_end,
+	};
+}
+
 struct stack_map_vma_lock {
 	struct vm_area_struct *vma;
 	struct mm_struct *mm;
@@ -244,15 +333,9 @@ static void stack_map_unlock_vma(struct stack_map_vma_lock *lock)
 static void stack_map_get_build_id_offset_sleepable(struct bpf_stack_build_id *id_offs,
 						    u32 trace_nr)
 {
-	struct mm_struct *mm = current->mm;
-	struct stack_map_vma_lock lock = { .mm = mm };
-	struct {
-		struct file *file;
-		const unsigned char *build_id;
-		unsigned long vm_start;
-		unsigned long vm_end;
-		unsigned long vm_pgoff;
-	} cache = {};
+	struct stack_map_vma_lock lock = { .mm = current->mm };
+	struct stack_map_build_id_cache cache = {};
+	struct stack_map_cached_vma *res = &cache.resolved;
 	unsigned long vm_pgoff, vm_start, vm_end;
 	struct vm_area_struct *vma;
 	struct file *file;
@@ -262,44 +345,39 @@ static void stack_map_get_build_id_offset_sleepable(struct bpf_stack_build_id *i
 	for (u32 i = 0; i < trace_nr; i++) {
 		ip = READ_ONCE(id_offs[i].ip);
 
-		/*
-		 * Range cache fast path: if ip falls within the previously
-		 * resolved VMA range, reuse the cache build_id without
-		 * re-acquiring the VMA lock.
-		 */
-		if (cache.build_id && ip >= cache.vm_start && ip < cache.vm_end) {
-			offset = stack_map_build_id_offset(cache.vm_pgoff, cache.vm_start, ip);
-			stack_map_build_id_set_valid(&id_offs[i], offset, cache.build_id);
+		if (!stack_map_build_id_set_from_cache(&cache, &id_offs[i], ip))
 			continue;
-		}
 
 		vma = stack_map_lock_vma(&lock, ip);
 		if (!vma) {
 			stack_map_build_id_set_ip(&id_offs[i]);
 			continue;
 		}
+
+		vm_pgoff = vma->vm_pgoff;
+		vm_start = vma->vm_start;
+		vm_end = vma->vm_end;
+
 		if (vma_is_anonymous(vma) || !vma->vm_file) {
-			stack_map_build_id_set_ip(&id_offs[i]);
 			stack_map_unlock_vma(&lock);
+			stack_map_build_id_set_ip(&id_offs[i]);
+			stack_map_build_id_cache_set_unresolved(&cache, vm_start, vm_end);
 			continue;
 		}
 
 		file = vma->vm_file;
-		vm_pgoff = vma->vm_pgoff;
-		vm_start = vma->vm_start;
-		vm_end = vma->vm_end;
 		offset = stack_map_build_id_offset(vm_pgoff, vm_start, ip);
 
 		/*
-		 * Same backing file as previous (e.g. different VMAs
-		 * of the same ELF binary). Reuse the cache build_id.
+		 * Same backing file as the last resolved VMA (another mapping
+		 * of the same ELF binary): reuse its build_id without re-parsing.
 		 */
-		if (file == cache.file) {
+		if (file == res->file) {
 			stack_map_unlock_vma(&lock);
-			stack_map_build_id_set_valid(&id_offs[i], offset, cache.build_id);
-			cache.vm_start = vm_start;
-			cache.vm_end = vm_end;
-			cache.vm_pgoff = vm_pgoff;
+			stack_map_build_id_set_valid(&id_offs[i], offset, res->build_id);
+			res->vm_start = vm_start;
+			res->vm_end = vm_end;
+			res->vm_pgoff = vm_pgoff;
 			continue;
 		}
 
@@ -310,21 +388,17 @@ static void stack_map_get_build_id_offset_sleepable(struct bpf_stack_build_id *i
 		if (build_id_parse_file(file, id_offs[i].build_id, NULL)) {
 			stack_map_build_id_set_ip(&id_offs[i]);
 			fput(file);
+			stack_map_build_id_cache_set_unresolved(&cache, vm_start, vm_end);
 			continue;
 		}
 
 		stack_map_build_id_set_valid(&id_offs[i], offset, id_offs[i].build_id);
-		if (cache.file)
-			fput(cache.file);
-		cache.file = file;
-		cache.build_id = id_offs[i].build_id;
-		cache.vm_start = vm_start;
-		cache.vm_end = vm_end;
-		cache.vm_pgoff = vm_pgoff;
+		stack_map_build_id_cache_set_resolved(&cache, file, id_offs[i].build_id,
+						      vm_start, vm_end, vm_pgoff);
 	}
 
-	if (cache.file)
-		fput(cache.file);
+	if (res->file)
+		fput(res->file);
 }
 
 /*
@@ -343,8 +417,8 @@ static void stack_map_get_build_id_offset(struct bpf_stack_build_id *id_offs,
 	struct mmap_unlock_irq_work *work = NULL;
 	bool irq_work_busy = bpf_mmap_unlock_get_irq_work(&work);
 	bool has_user_ctx = user && current && current->mm;
-	struct vm_area_struct *vma, *prev_vma = NULL;
-	const unsigned char *prev_build_id = NULL;
+	struct stack_map_build_id_cache cache = {};
+	struct vm_area_struct *vma;
 	int i;
 
 	if (may_fault && has_user_ctx) {
@@ -365,27 +439,30 @@ static void stack_map_get_build_id_offset(struct bpf_stack_build_id *id_offs,
 
 	for (i = 0; i < trace_nr; i++) {
 		u64 ip = READ_ONCE(id_offs[i].ip);
-		u64 offset;
 
-		if (prev_build_id && range_in_vma(prev_vma, ip, ip)) {
-			vma = prev_vma;
-			offset = stack_map_build_id_offset(vma->vm_pgoff, vma->vm_start, ip);
-			stack_map_build_id_set_valid(&id_offs[i], offset, prev_build_id);
+		if (!stack_map_build_id_set_from_cache(&cache, &id_offs[i], ip))
 			continue;
-		}
+
 		vma = find_vma(current->mm, ip);
 		if (!vma || vma_is_anonymous(vma) ||
 		    fetch_build_id(vma, id_offs[i].build_id, may_fault)) {
-			/* per entry fall back to ips */
+			/* per entry fall back to ips; cache build-ID-less range */
 			stack_map_build_id_set_ip(&id_offs[i]);
-			prev_vma = vma;
-			prev_build_id = NULL;
+			if (vma)
+				stack_map_build_id_cache_set_unresolved(&cache,
+						vma->vm_start, vma->vm_end);
 			continue;
 		}
-		offset = stack_map_build_id_offset(vma->vm_pgoff, vma->vm_start, ip);
-		stack_map_build_id_set_valid(&id_offs[i], offset, id_offs[i].build_id);
-		prev_vma = vma;
-		prev_build_id = id_offs[i].build_id;
+		/*
+		 * mmap_lock is held for the whole loop, so the cached VMA
+		 * fields stay valid; no file pinning is needed here.
+		 */
+		stack_map_build_id_set_valid(&id_offs[i],
+			stack_map_build_id_offset(vma->vm_pgoff, vma->vm_start, ip),
+			id_offs[i].build_id);
+		stack_map_build_id_cache_set_resolved(&cache, NULL, id_offs[i].build_id,
+						      vma->vm_start, vma->vm_end,
+						      vma->vm_pgoff);
 	}
 	bpf_mmap_unlock_mm(work, current->mm);
 }
-- 
2.54.0