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authorOscar Salvador <osalvador@suse.de>2024-02-16 00:59:01 +0300
committerAndrew Morton <akpm@linux-foundation.org>2024-02-24 04:48:16 +0300
commit3ee34eabac2abb6b1b6fcdebffe18870719ad000 (patch)
tree5971c8b0cee3a36737818e61ee5d209f7bfb37e6 /lib/stackdepot.c
parent1f1183c4c0bc609c98b71ab9281ff72533d89bb0 (diff)
downloadlinux-3ee34eabac2abb6b1b6fcdebffe18870719ad000.tar.xz
lib/stackdepot: fix first entry having a 0-handle
Patch series "page_owner: print stacks and their outstanding allocations", v10. page_owner is a great debug functionality tool that lets us know about all pages that have been allocated/freed and their specific stacktrace. This comes very handy when debugging memory leaks, since with some scripting we can see the outstanding allocations, which might point to a memory leak. In my experience, that is one of the most useful cases, but it can get really tedious to screen through all pages and try to reconstruct the stack <-> allocated/freed relationship, becoming most of the time a daunting and slow process when we have tons of allocation/free operations. This patchset aims to ease that by adding a new functionality into page_owner. This functionality creates a new directory called 'page_owner_stacks' under 'sys/kernel//debug' with a read-only file called 'show_stacks', which prints out all the stacks followed by their outstanding number of allocations (being that the times the stacktrace has allocated but not freed yet). This gives us a clear and a quick overview of stacks <-> allocated/free. We take advantage of the new refcount_f field that stack_record struct gained, and increment/decrement the stack refcount on every __set_page_owner() (alloc operation) and __reset_page_owner (free operation) call. Unfortunately, we cannot use the new stackdepot api STACK_DEPOT_FLAG_GET because it does not fulfill page_owner needs, meaning we would have to special case things, at which point makes more sense for page_owner to do its own {dec,inc}rementing of the stacks. E.g: Using STACK_DEPOT_FLAG_PUT, once the refcount reaches 0, such stack gets evicted, so page_owner would lose information. This patchset also creates a new file called 'set_threshold' within 'page_owner_stacks' directory, and by writing a value to it, the stacks which refcount is below such value will be filtered out. A PoC can be found below: # cat /sys/kernel/debug/page_owner_stacks/show_stacks > page_owner_full_stacks.txt # head -40 page_owner_full_stacks.txt prep_new_page+0xa9/0x120 get_page_from_freelist+0x801/0x2210 __alloc_pages+0x18b/0x350 alloc_pages_mpol+0x91/0x1f0 folio_alloc+0x14/0x50 filemap_alloc_folio+0xb2/0x100 page_cache_ra_unbounded+0x96/0x180 filemap_get_pages+0xfd/0x590 filemap_read+0xcc/0x330 blkdev_read_iter+0xb8/0x150 vfs_read+0x285/0x320 ksys_read+0xa5/0xe0 do_syscall_64+0x80/0x160 entry_SYSCALL_64_after_hwframe+0x6e/0x76 stack_count: 521 prep_new_page+0xa9/0x120 get_page_from_freelist+0x801/0x2210 __alloc_pages+0x18b/0x350 alloc_pages_mpol+0x91/0x1f0 folio_alloc+0x14/0x50 filemap_alloc_folio+0xb2/0x100 __filemap_get_folio+0x14a/0x490 ext4_write_begin+0xbd/0x4b0 [ext4] generic_perform_write+0xc1/0x1e0 ext4_buffered_write_iter+0x68/0xe0 [ext4] ext4_file_write_iter+0x70/0x740 [ext4] vfs_write+0x33d/0x420 ksys_write+0xa5/0xe0 do_syscall_64+0x80/0x160 entry_SYSCALL_64_after_hwframe+0x6e/0x76 stack_count: 4609 ... ... # echo 5000 > /sys/kernel/debug/page_owner_stacks/set_threshold # cat /sys/kernel/debug/page_owner_stacks/show_stacks > page_owner_full_stacks_5000.txt # head -40 page_owner_full_stacks_5000.txt prep_new_page+0xa9/0x120 get_page_from_freelist+0x801/0x2210 __alloc_pages+0x18b/0x350 alloc_pages_mpol+0x91/0x1f0 folio_alloc+0x14/0x50 filemap_alloc_folio+0xb2/0x100 __filemap_get_folio+0x14a/0x490 ext4_write_begin+0xbd/0x4b0 [ext4] generic_perform_write+0xc1/0x1e0 ext4_buffered_write_iter+0x68/0xe0 [ext4] ext4_file_write_iter+0x70/0x740 [ext4] vfs_write+0x33d/0x420 ksys_pwrite64+0x75/0x90 do_syscall_64+0x80/0x160 entry_SYSCALL_64_after_hwframe+0x6e/0x76 stack_count: 6781 prep_new_page+0xa9/0x120 get_page_from_freelist+0x801/0x2210 __alloc_pages+0x18b/0x350 pcpu_populate_chunk+0xec/0x350 pcpu_balance_workfn+0x2d1/0x4a0 process_scheduled_works+0x84/0x380 worker_thread+0x12a/0x2a0 kthread+0xe3/0x110 ret_from_fork+0x30/0x50 ret_from_fork_asm+0x1b/0x30 stack_count: 8641 This patch (of 7): The very first entry of stack_record gets a handle of 0, but this is wrong because stackdepot treats a 0-handle as a non-valid one. E.g: See the check in stack_depot_fetch() Fix this by adding and offset of 1. This bug has been lurking since the very beginning of stackdepot, but no one really cared as it seems. Because of that I am not adding a Fixes tag. Link: https://lkml.kernel.org/r/20240215215907.20121-1-osalvador@suse.de Link: https://lkml.kernel.org/r/20240215215907.20121-2-osalvador@suse.de Co-developed-by: Marco Elver <elver@google.com> Signed-off-by: Marco Elver <elver@google.com> Signed-off-by: Oscar Salvador <osalvador@suse.de> Acked-by: Vlastimil Babka <vbabka@suse.cz> Acked-by: Andrey Konovalov <andreyknvl@gmail.com> Cc: Alexander Potapenko <glider@google.com> Cc: Michal Hocko <mhocko@suse.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Diffstat (limited to 'lib/stackdepot.c')
-rw-r--r--lib/stackdepot.c16
1 files changed, 9 insertions, 7 deletions
diff --git a/lib/stackdepot.c b/lib/stackdepot.c
index 4a7055a63d9f..c043a4186bc5 100644
--- a/lib/stackdepot.c
+++ b/lib/stackdepot.c
@@ -45,15 +45,16 @@
#define DEPOT_POOL_INDEX_BITS (DEPOT_HANDLE_BITS - DEPOT_OFFSET_BITS - \
STACK_DEPOT_EXTRA_BITS)
#define DEPOT_POOLS_CAP 8192
+/* The pool_index is offset by 1 so the first record does not have a 0 handle. */
#define DEPOT_MAX_POOLS \
- (((1LL << (DEPOT_POOL_INDEX_BITS)) < DEPOT_POOLS_CAP) ? \
- (1LL << (DEPOT_POOL_INDEX_BITS)) : DEPOT_POOLS_CAP)
+ (((1LL << (DEPOT_POOL_INDEX_BITS)) - 1 < DEPOT_POOLS_CAP) ? \
+ (1LL << (DEPOT_POOL_INDEX_BITS)) - 1 : DEPOT_POOLS_CAP)
/* Compact structure that stores a reference to a stack. */
union handle_parts {
depot_stack_handle_t handle;
struct {
- u32 pool_index : DEPOT_POOL_INDEX_BITS;
+ u32 pool_index : DEPOT_POOL_INDEX_BITS; /* pool_index is offset by 1 */
u32 offset : DEPOT_OFFSET_BITS;
u32 extra : STACK_DEPOT_EXTRA_BITS;
};
@@ -372,7 +373,7 @@ static struct stack_record *depot_pop_free_pool(void **prealloc, size_t size)
stack = current_pool + pool_offset;
/* Pre-initialize handle once. */
- stack->handle.pool_index = pool_index;
+ stack->handle.pool_index = pool_index + 1;
stack->handle.offset = pool_offset >> DEPOT_STACK_ALIGN;
stack->handle.extra = 0;
INIT_LIST_HEAD(&stack->hash_list);
@@ -483,18 +484,19 @@ static struct stack_record *depot_fetch_stack(depot_stack_handle_t handle)
const int pools_num_cached = READ_ONCE(pools_num);
union handle_parts parts = { .handle = handle };
void *pool;
+ u32 pool_index = parts.pool_index - 1;
size_t offset = parts.offset << DEPOT_STACK_ALIGN;
struct stack_record *stack;
lockdep_assert_not_held(&pool_lock);
- if (parts.pool_index > pools_num_cached) {
+ if (pool_index > pools_num_cached) {
WARN(1, "pool index %d out of bounds (%d) for stack id %08x\n",
- parts.pool_index, pools_num_cached, handle);
+ pool_index, pools_num_cached, handle);
return NULL;
}
- pool = stack_pools[parts.pool_index];
+ pool = stack_pools[pool_index];
if (WARN_ON(!pool))
return NULL;