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lib/stackdepot: various comments clean-ups
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Clean up comments in include/linux/stackdepot.h and lib/stackdepot.c:

1. Rework the initialization comment in stackdepot.h.
2. Rework the header comment in stackdepot.c.
3. Various clean-ups for other comments.

Also adjust whitespaces for find_stack and depot_alloc_stack call sites.

No functional changes.

Link: https://lkml.kernel.org/r/e6d509e769bacc1842de868084e157787e6a3817.1675111415.git.andreyknvl@google.com
Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Evgenii Stepanov <eugenis@google.com>
Cc: Marco Elver <elver@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
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Andrey Konovalov authored and Andrew Morton committed Feb 1, 2023
1 parent dc196d5 commit bbb2f17
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36 changes: 19 additions & 17 deletions include/linux/stackdepot.h
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Original file line number Diff line number Diff line change
@@ -1,11 +1,11 @@
/* SPDX-License-Identifier: GPL-2.0-or-later */
/*
* A generic stack depot implementation
* Stack depot - a stack trace storage that avoids duplication.
*
* Author: Alexander Potapenko <glider@google.com>
* Copyright (C) 2016 Google, Inc.
*
* Based on code by Dmitry Chernenkov.
* Based on the code by Dmitry Chernenkov.
*/

#ifndef _LINUX_STACKDEPOT_H
Expand All @@ -17,35 +17,37 @@ typedef u32 depot_stack_handle_t;

/*
* Number of bits in the handle that stack depot doesn't use. Users may store
* information in them.
* information in them via stack_depot_set/get_extra_bits.
*/
#define STACK_DEPOT_EXTRA_BITS 5

/*
* Every user of stack depot has to call stack_depot_init() during its own init
* when it's decided that it will be calling stack_depot_save() later. This is
* recommended for e.g. modules initialized later in the boot process, when
* slab_is_available() is true.
* Using stack depot requires its initialization, which can be done in 3 ways:
*
* The alternative is to select STACKDEPOT_ALWAYS_INIT to have stack depot
* enabled as part of mm_init(), for subsystems where it's known at compile time
* that stack depot will be used.
* 1. Selecting CONFIG_STACKDEPOT_ALWAYS_INIT. This option is suitable in
* scenarios where it's known at compile time that stack depot will be used.
* Enabling this config makes the kernel initialize stack depot in mm_init().
*
* Another alternative is to call stack_depot_request_early_init(), when the
* decision to use stack depot is taken e.g. when evaluating kernel boot
* parameters, which precedes the enablement point in mm_init().
* 2. Calling stack_depot_request_early_init() during early boot, before
* stack_depot_early_init() in mm_init() completes. For example, this can
* be done when evaluating kernel boot parameters.
*
* 3. Calling stack_depot_init(). Possible after boot is complete. This option
* is recommended for modules initialized later in the boot process, after
* mm_init() completes.
*
* stack_depot_init() and stack_depot_request_early_init() can be called
* regardless of CONFIG_STACKDEPOT and are no-op when disabled. The actual
* save/fetch/print functions should only be called from code that makes sure
* CONFIG_STACKDEPOT is enabled.
* regardless of whether CONFIG_STACKDEPOT is enabled and are no-op when this
* config is disabled. The save/fetch/print stack depot functions can only be
* called from the code that makes sure CONFIG_STACKDEPOT is enabled _and_
* initializes stack depot via one of the ways listed above.
*/
#ifdef CONFIG_STACKDEPOT
int stack_depot_init(void);

void __init stack_depot_request_early_init(void);

/* This is supposed to be called only from mm_init() */
/* Must be only called from mm_init(). */
int __init stack_depot_early_init(void);
#else
static inline int stack_depot_init(void) { return 0; }
Expand Down
120 changes: 59 additions & 61 deletions lib/stackdepot.c
View file
Original file line number Diff line number Diff line change
@@ -1,22 +1,26 @@
// SPDX-License-Identifier: GPL-2.0-only
/*
* Generic stack depot for storing stack traces.
* Stack depot - a stack trace storage that avoids duplication.
*
* Some debugging tools need to save stack traces of certain events which can
* be later presented to the user. For example, KASAN needs to safe alloc and
* free stacks for each object, but storing two stack traces per object
* requires too much memory (e.g. SLUB_DEBUG needs 256 bytes per object for
* that).
* Stack depot is intended to be used by subsystems that need to store and
* later retrieve many potentially duplicated stack traces without wasting
* memory.
*
* Instead, stack depot maintains a hashtable of unique stacktraces. Since alloc
* and free stacks repeat a lot, we save about 100x space.
* Stacks are never removed from depot, so we store them contiguously one after
* another in a contiguous memory allocation.
* For example, KASAN needs to save allocation and free stack traces for each
* object. Storing two stack traces per object requires a lot of memory (e.g.
* SLUB_DEBUG needs 256 bytes per object for that). Since allocation and free
* stack traces often repeat, using stack depot allows to save about 100x space.
*
* Internally, stack depot maintains a hash table of unique stacktraces. The
* stack traces themselves are stored contiguously one after another in a set
* of separate page allocations.
*
* Stack traces are never removed from stack depot.
*
* Author: Alexander Potapenko <glider@google.com>
* Copyright (C) 2016 Google, Inc.
*
* Based on code by Dmitry Chernenkov.
* Based on the code by Dmitry Chernenkov.
*/

#define pr_fmt(fmt) "stackdepot: " fmt
Expand Down Expand Up @@ -50,7 +54,7 @@
(((1LL << (DEPOT_SLAB_INDEX_BITS)) < DEPOT_SLABS_CAP) ? \
(1LL << (DEPOT_SLAB_INDEX_BITS)) : DEPOT_SLABS_CAP)

/* The compact structure to store the reference to stacks. */
/* Compact structure that stores a reference to a stack. */
union handle_parts {
depot_stack_handle_t handle;
struct {
Expand All @@ -62,11 +66,11 @@ union handle_parts {
};

struct stack_record {
struct stack_record *next; /* Link in the hashtable */
u32 hash; /* Hash in the hastable */
u32 size; /* Number of frames in the stack */
struct stack_record *next; /* Link in the hash table */
u32 hash; /* Hash in the hash table */
u32 size; /* Number of stored frames */
union handle_parts handle;
unsigned long entries[]; /* Variable-sized array of entries. */
unsigned long entries[]; /* Variable-sized array of frames */
};

static bool stack_depot_disabled;
Expand Down Expand Up @@ -306,17 +310,17 @@ depot_alloc_stack(unsigned long *entries, int size, u32 hash, void **prealloc)
return stack;
}

/* Calculate hash for a stack */
/* Calculates the hash for a stack. */
static inline u32 hash_stack(unsigned long *entries, unsigned int size)
{
return jhash2((u32 *)entries,
array_size(size, sizeof(*entries)) / sizeof(u32),
STACK_HASH_SEED);
}

/* Use our own, non-instrumented version of memcmp().
*
* We actually don't care about the order, just the equality.
/*
* Non-instrumented version of memcmp().
* Does not check the lexicographical order, only the equality.
*/
static inline
int stackdepot_memcmp(const unsigned long *u1, const unsigned long *u2,
Expand All @@ -329,7 +333,7 @@ int stackdepot_memcmp(const unsigned long *u1, const unsigned long *u2,
return 0;
}

/* Find a stack that is equal to the one stored in entries in the hash */
/* Finds a stack in a bucket of the hash table. */
static inline struct stack_record *find_stack(struct stack_record *bucket,
unsigned long *entries, int size,
u32 hash)
Expand All @@ -346,27 +350,27 @@ static inline struct stack_record *find_stack(struct stack_record *bucket,
}

/**
* __stack_depot_save - Save a stack trace from an array
* __stack_depot_save - Save a stack trace to stack depot
*
* @entries: Pointer to storage array
* @nr_entries: Size of the storage array
* @alloc_flags: Allocation gfp flags
* @entries: Pointer to the stack trace
* @nr_entries: Number of frames in the stack
* @alloc_flags: Allocation GFP flags
* @can_alloc: Allocate stack slabs (increased chance of failure if false)
*
* Saves a stack trace from @entries array of size @nr_entries. If @can_alloc is
* %true, is allowed to replenish the stack slab pool in case no space is left
* %true, stack depot can replenish the stack slab pool in case no space is left
* (allocates using GFP flags of @alloc_flags). If @can_alloc is %false, avoids
* any allocations and will fail if no space is left to store the stack trace.
* any allocations and fails if no space is left to store the stack trace.
*
* If the stack trace in @entries is from an interrupt, only the portion up to
* interrupt entry is saved.
* If the provided stack trace comes from the interrupt context, only the part
* up to the interrupt entry is saved.
*
* Context: Any context, but 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 neither %GFP_ATOMIC nor
* this is the case for contexts where neither %GFP_ATOMIC nor
* %GFP_NOWAIT can be used (NMI, raw_spin_lock).
*
* Return: The handle of the stack struct stored in depot, 0 on failure.
* Return: 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,
Expand All @@ -381,11 +385,11 @@ depot_stack_handle_t __stack_depot_save(unsigned long *entries,

/*
* If this stack trace is from an interrupt, including anything before
* interrupt entry usually leads to unbounded stackdepot growth.
* interrupt entry usually leads to unbounded stack depot growth.
*
* Because use of filter_irq_stacks() is a requirement to ensure
* stackdepot can efficiently deduplicate interrupt stacks, always
* filter_irq_stacks() to simplify all callers' use of stackdepot.
* Since use of filter_irq_stacks() is a requirement to ensure stack
* depot can efficiently deduplicate interrupt stacks, always
* filter_irq_stacks() to simplify all callers' use of stack depot.
*/
nr_entries = filter_irq_stacks(entries, nr_entries);

Expand All @@ -400,8 +404,7 @@ depot_stack_handle_t __stack_depot_save(unsigned long *entries,
* The smp_load_acquire() here pairs with smp_store_release() to
* |bucket| below.
*/
found = find_stack(smp_load_acquire(bucket), entries,
nr_entries, hash);
found = find_stack(smp_load_acquire(bucket), entries, nr_entries, hash);
if (found)
goto exit;

Expand Down Expand Up @@ -431,7 +434,8 @@ depot_stack_handle_t __stack_depot_save(unsigned long *entries,

found = find_stack(*bucket, entries, nr_entries, hash);
if (!found) {
struct stack_record *new = depot_alloc_stack(entries, nr_entries, hash, &prealloc);
struct stack_record *new =
depot_alloc_stack(entries, nr_entries, hash, &prealloc);

if (new) {
new->next = *bucket;
Expand All @@ -444,16 +448,16 @@ depot_stack_handle_t __stack_depot_save(unsigned long *entries,
}
} else if (prealloc) {
/*
* We didn't need to store this stack trace, but let's keep
* the preallocated memory for the future.
* Stack depot already contains this stack trace, but let's
* keep the preallocated memory for the future.
*/
depot_init_slab(&prealloc);
}

raw_spin_unlock_irqrestore(&slab_lock, flags);
exit:
if (prealloc) {
/* Nobody used this memory, ok to free it. */
/* Stack depot didn't use this memory, free it. */
free_pages((unsigned long)prealloc, DEPOT_SLAB_ORDER);
}
if (found)
Expand All @@ -464,16 +468,16 @@ depot_stack_handle_t __stack_depot_save(unsigned long *entries,
EXPORT_SYMBOL_GPL(__stack_depot_save);

/**
* stack_depot_save - Save a stack trace from an array
* stack_depot_save - Save a stack trace to stack depot
*
* @entries: Pointer to storage array
* @nr_entries: Size of the storage array
* @alloc_flags: Allocation gfp flags
* @entries: Pointer to the stack trace
* @nr_entries: Number of frames in the stack
* @alloc_flags: Allocation GFP flags
*
* Context: Contexts where allocations via alloc_pages() are allowed.
* See __stack_depot_save() for more details.
*
* Return: The handle of the stack struct stored in depot, 0 on failure.
* Return: Handle of the stack trace stored in depot, 0 on failure
*/
depot_stack_handle_t stack_depot_save(unsigned long *entries,
unsigned int nr_entries,
Expand All @@ -484,13 +488,12 @@ depot_stack_handle_t stack_depot_save(unsigned long *entries,
EXPORT_SYMBOL_GPL(stack_depot_save);

/**
* stack_depot_fetch - Fetch stack entries from a depot
* stack_depot_fetch - Fetch a stack trace from stack depot
*
* @handle: Stack depot handle which was returned from
* stack_depot_save().
* @entries: Pointer to store the entries address
* @handle: Stack depot handle returned from stack_depot_save()
* @entries: Pointer to store the address of the stack trace
*
* Return: The number of trace entries for this depot.
* Return: Number of frames for the fetched stack
*/
unsigned int stack_depot_fetch(depot_stack_handle_t handle,
unsigned long **entries)
Expand Down Expand Up @@ -522,11 +525,9 @@ unsigned int stack_depot_fetch(depot_stack_handle_t handle,
EXPORT_SYMBOL_GPL(stack_depot_fetch);

/**
* stack_depot_print - print stack entries from a depot
*
* @stack: Stack depot handle which was returned from
* stack_depot_save().
* stack_depot_print - Print a stack trace from stack depot
*
* @stack: Stack depot handle returned from stack_depot_save()
*/
void stack_depot_print(depot_stack_handle_t stack)
{
Expand All @@ -540,17 +541,14 @@ void stack_depot_print(depot_stack_handle_t stack)
EXPORT_SYMBOL_GPL(stack_depot_print);

/**
* stack_depot_snprint - print stack entries from a depot into a buffer
* stack_depot_snprint - Print a stack trace from stack depot into a buffer
*
* @handle: Stack depot handle which was returned from
* stack_depot_save().
* @handle: Stack depot handle returned from stack_depot_save()
* @buf: Pointer to the print buffer
*
* @size: Size of the print buffer
*
* @spaces: Number of leading spaces to print
*
* Return: Number of bytes printed.
* Return: Number of bytes printed
*/
int stack_depot_snprint(depot_stack_handle_t handle, char *buf, size_t size,
int spaces)
Expand Down

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