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Date:   Fri, 15 Oct 2021 10:57:13 +0200
From:   Vlastimil Babka <>
To:     Andrew Morton <>
Cc:,,,,, Dmitry Vyukov <>,
        Marco Elver <>,
        Vijayanand Jitta <>,
        Maarten Lankhorst <>,
        Maxime Ripard <>,
        Thomas Zimmermann <>,
        David Airlie <>,
        Daniel Vetter <>,
        Andrey Ryabinin <>,
        Alexander Potapenko <>,
        Andrey Konovalov <>,
        Geert Uytterhoeven <>,
        Oliver Glitta <>,
        Imran Khan <>,
        Mike Rapoport <>,
        kernel test robot <>
Subject: Re: [PATCH v3] lib/stackdepot: allow optional init and stack_table
 allocation by kvmalloc()

On 10/13/21 09:30, Vlastimil Babka wrote:
> Currently, enabling CONFIG_STACKDEPOT means its stack_table will be allocated
> from memblock, even if stack depot ends up not actually used. The default size
> of stack_table is 4MB on 32-bit, 8MB on 64-bit.
> This is fine for use-cases such as KASAN which is also a config option and
> has overhead on its own. But it's an issue for functionality that has to be
> actually enabled on boot (page_owner) or depends on hardware (GPU drivers)
> and thus the memory might be wasted. This was raised as an issue [1] when
> attempting to add stackdepot support for SLUB's debug object tracking
> functionality. It's common to build kernels with CONFIG_SLUB_DEBUG and enable
> slub_debug on boot only when needed, or create only specific kmem caches with
> debugging for testing purposes.
> It would thus be more efficient if stackdepot's table was allocated only when
> actually going to be used. This patch thus makes the allocation (and whole
> stack_depot_init() call) optional:
> - Add a CONFIG_STACKDEPOT_ALWAYS_INIT flag to keep using the current
>   well-defined point of allocation as part of mem_init(). Make CONFIG_KASAN
>   select this flag.
> - Other users have to call stack_depot_init() as part of their own init when
>   it's determined that stack depot will actually be used. This may depend on
>   both config and runtime conditions. Convert current users which are
>   page_owner and several in the DRM subsystem. Same will be done for SLUB
>   later.
> - Because the init might now be called after the boot-time memblock allocation
>   has given all memory to the buddy allocator, change stack_depot_init() to
>   allocate stack_table with kvmalloc() when memblock is no longer available.
>   Also handle allocation failure by disabling stackdepot (could have
>   theoretically happened even with memblock allocation previously), and don't
>   unnecessarily align the memblock allocation to its own size anymore.
> [1]
> ---
> Changes in v3:
> - stack_depot_init_mutex made static and moved inside stack_depot_init()
>   Reported-by: kernel test robot <>
> - use !stack_table condition instead of stack_table == NULL
>   reported by checkpatch on patchwork

The last change above was missing because I forgot git commit --amend before
git format-patch. More importantly there was a bot report for FLATMEM. Please
add this fixup. Thanks.

>From a971a1670491f8fbbaab579eef3c756a5263af95 Mon Sep 17 00:00:00 2001
From: Vlastimil Babka <>
Date: Thu, 7 Oct 2021 10:49:09 +0200
Subject: [PATCH] lib/stackdepot: allow optional init and stack_table
 allocation by kvmalloc() - fixup

On FLATMEM, we call page_ext_init_flatmem_late() just before kmem_cache_init()
which means stack_depot_init() (called by page owner init) will not recognize
properly it should use kvmalloc() and not memblock_alloc(). memblock_alloc()
will also not issue a warning and return a block memory that can be invalid and
cause kernel page fault when saving stacks, as reported by the kernel test
robot [1].

Fix this by moving page_ext_init_flatmem_late() below kmem_cache_init() so that
slab_is_available() is true during stack_depot_init(). SPARSEMEM doesn't have
this issue, as it doesn't do page_ext_init_flatmem_late(), but a different
page_ext_init() even later in the boot process.

Thanks to Mike Rapoport for pointing out the FLATMEM init ordering issue.

While at it, also actually resolve a checkpatch warning in stack_depot_init()
from DRM CI, which was supposed to be in the original patch already.


Signed-off-by: Vlastimil Babka <>
Reported-by: kernel test robot <>
 init/main.c      | 7 +++++--
 lib/stackdepot.c | 2 +-
 2 files changed, 6 insertions(+), 3 deletions(-)

diff --git a/init/main.c b/init/main.c
index ca2765c8e45c..0ab632f681c5 100644
--- a/init/main.c
+++ b/init/main.c
@@ -845,9 +845,12 @@ static void __init mm_init(void)
-	/* page_owner must be initialized after buddy is ready */
-	page_ext_init_flatmem_late();
+	/*
+	 * page_owner must be initialized after buddy is ready, and also after
+	 * slab is ready so that stack_depot_init() works properly
+	 */
+	page_ext_init_flatmem_late();
diff --git a/lib/stackdepot.c b/lib/stackdepot.c
index 049d7d025d78..1f8ea6d0899b 100644
--- a/lib/stackdepot.c
+++ b/lib/stackdepot.c
@@ -172,7 +172,7 @@ __ref int stack_depot_init(void)
 	static DEFINE_MUTEX(stack_depot_init_mutex);
-	if (!stack_depot_disable && stack_table == NULL) {
+	if (!stack_depot_disable && !stack_table) {
 		size_t size = (STACK_HASH_SIZE * sizeof(struct stack_record *));
 		int i;

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