lib/stackdepot: replace CONFIG_STACK_HASH_ORDER with automatic sizing

As Linus explained [1], setting the stackdepot hash table size as a config
option is suboptimal, especially as stackdepot becomes a dependency of
less "expert" subsystems than initially (e.g.  DRM, networking,
SLUB_DEBUG):

: (a) it introduces a new compile-time question that isn't sane to ask
: a regular user, but is now exposed to regular users.

: (b) this by default uses 1MB of memory for a feature that didn't in
: the past, so now if you have small machines you need to make sure you
: make a special kernel config for them.

Ideally we would employ rhashtable for fully automatic resizing, which
should be feasible for many of the new users, but problematic for the
original users with restricted context that call __stack_depot_save() with
can_alloc == false, i.e.  KASAN.

However we can easily remove the config option and scale the hash table
automatically with system memory.  The STACK_HASH_MASK constant becomes
stack_hash_mask variable and is used only in one mask operation, so the
overhead should be negligible to none.  For early allocation we can employ
the existing alloc_large_system_hash() function and perform similar
scaling for the late allocation.

The existing limits of the config option (between 4k and 1M buckets) are
preserved, and scaling factor is set to one bucket per 16kB memory so on
64bit the max 1M buckets (8MB memory) is achieved with 16GB system, while
a 1GB system will use 512kB.

Because KASAN is reported to need the maximum number of buckets even with
smaller amounts of memory [2], set it as such when kasan_enabled().

If needed, the automatic scaling could be complemented with a boot-time
kernel parameter, but it feels pointless to add it without a specific use
case.

[1] https://lore.kernel.org/all/CAHk-=wjC5nS+fnf6EzRD9yQRJApAhxx7gRB87ZV+pAWo9oVrTg@mail.gmail.com/
[2] https://lore.kernel.org/all/CACT4Y+Y4GZfXOru2z5tFPzFdaSUd+GFc6KVL=bsa0+1m197cQQ@mail.gmail.com/

Link: https://lkml.kernel.org/r/20220620150249.16814-1-vbabka@suse.cz
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Acked-by: Dmitry Vyukov <dvyukov@google.com>
Cc: Marco Elver <elver@google.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Andrey Konovalov <andreyknvl@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 0 insertions, 9 deletions

diff --git a/lib/Kconfig b/lib/Kconfig
index eaaad4d85bf2..986ea474836c 100644
--- a/lib/Kconfig
+++ b/lib/Kconfig
@@ -685,15 +685,6 @@ config STACKDEPOT_ALWAYS_INIT
 	bool
 	select STACKDEPOT
 
-config STACK_HASH_ORDER
-	int "stack depot hash size (12 => 4KB, 20 => 1024KB)"
-	range 12 20
-	default 20
-	depends on STACKDEPOT
-	help
-	 Select the hash size as a power of 2 for the stackdepot hash table.
-	 Choose a lower value to reduce the memory impact.
-
 config REF_TRACKER
 	bool
 	depends on STACKTRACE_SUPPORT