[PATCH 1/4] lib/stackdepot: introduce __stack_depot_save()

[Marco Elver]

Add __stack_depot_save(), which provides more fine-grained control over
stackdepot's memory allocation behaviour, in case stackdepot runs out of
"stack slabs".

Normally stackdepot uses alloc_pages() in case it runs out of space;
passing can_alloc==false to __stack_depot_save() prohibits this, at the
cost of more likely failure to record a stack trace.

Signed-off-by: Marco Elver <elver@xxxxxxxxxx>
---
 include/linux/stackdepot.h |  4 ++++
 lib/stackdepot.c           | 38 ++++++++++++++++++++++++++++++++------
 2 files changed, 36 insertions(+), 6 deletions(-)

diff --git a/include/linux/stackdepot.h b/include/linux/stackdepot.h
index 6bb4bc1a5f54..3735b97f62be 100644
--- a/include/linux/stackdepot.h
+++ b/include/linux/stackdepot.h
@@ -13,6 +13,10 @@
 
 typedef u32 depot_stack_handle_t;
 
+depot_stack_handle_t __stack_depot_save(unsigned long *entries,
+					unsigned int nr_entries,
+					gfp_t gfp_flags, bool can_alloc);
+
 depot_stack_handle_t stack_depot_save(unsigned long *entries,
 				      unsigned int nr_entries, gfp_t gfp_flags);
 
diff --git a/lib/stackdepot.c b/lib/stackdepot.c
index c80a9f734253..ec26845b4e29 100644
--- a/lib/stackdepot.c
+++ b/lib/stackdepot.c
@@ -248,17 +248,26 @@ unsigned int stack_depot_fetch(depot_stack_handle_t handle,
 EXPORT_SYMBOL_GPL(stack_depot_fetch);
 
 /**
- * stack_depot_save - Save a stack trace from an array
+ * __stack_depot_save - Save a stack trace from an array
+ *
+ * Save a stack trace from an array. If |can_alloc| is false, avoids allocating
+ * new stack slabs if no space is left in the current stack slab.
+ *
+ * Setting |can_alloc| to false is required if alloc_pages() cannot be used from
+ * the current context; currently this is the case from contexts where not even
+ * %GFP_ATOMIC or %GFP_NOWAIT can be used (NMI, raw_spin_lock).
  *
  * @entries:		Pointer to storage array
  * @nr_entries:		Size of the storage array
  * @alloc_flags:	Allocation gfp flags
+ * @can_alloc:		Allocate stack slabs (increased chance of failure if false)
  *
- * Return: The handle of the stack struct stored in depot
+ * Return: The handle of the stack struct stored in depot (0 on failure)
  */
-depot_stack_handle_t stack_depot_save(unsigned long *entries,
-				      unsigned int nr_entries,
-				      gfp_t alloc_flags)
+depot_stack_handle_t __stack_depot_save(unsigned long *entries,
+					unsigned int nr_entries,
+					gfp_t alloc_flags,
+					bool can_alloc)
 {
 	struct stack_record *found = NULL, **bucket;
 	depot_stack_handle_t retval = 0;
@@ -291,7 +300,7 @@ depot_stack_handle_t stack_depot_save(unsigned long *entries,
 	 * The smp_load_acquire() here pairs with smp_store_release() to
 	 * |next_slab_inited| in depot_alloc_stack() and init_stack_slab().
 	 */
-	if (unlikely(!smp_load_acquire(&next_slab_inited))) {
+	if (unlikely(can_alloc && !smp_load_acquire(&next_slab_inited))) {
 		/*
 		 * Zero out zone modifiers, as we don't have specific zone
 		 * requirements. Keep the flags related to allocation in atomic
@@ -339,6 +348,23 @@ depot_stack_handle_t stack_depot_save(unsigned long *entries,
 fast_exit:
 	return retval;
 }
+EXPORT_SYMBOL_GPL(__stack_depot_save);
+
+/**
+ * stack_depot_save - Save a stack trace from an array
+ *
+ * @entries:		Pointer to storage array
+ * @nr_entries:		Size of the storage array
+ * @alloc_flags:	Allocation gfp flags
+ *
+ * Return: The handle of the stack struct stored in depot (0 on failure)
+ */
+depot_stack_handle_t stack_depot_save(unsigned long *entries,
+				      unsigned int nr_entries,
+				      gfp_t alloc_flags)
+{
+	return __stack_depot_save(entries, nr_entries, alloc_flags, true);
+}
 EXPORT_SYMBOL_GPL(stack_depot_save);
 
 static inline int in_irqentry_text(unsigned long ptr)
-- 
2.33.0.153.gba50c8fa24-goog


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