Subject: [PATCH] mm: Disable zsmalloc on PREEMPT_RT
From: Sebastian Andrzej Siewior <[email-masked]>
Date: Thu, 23 Sep 2021 15:51:48 +0200

For efficiency reasons, zsmalloc is using a slim `handle'. The value is
the address of a memory allocation of 4 or 8 bytes depending on the size
of the long data type. The lowest bit in that allocated memory is used
as a bit spin lock.
The usage of the bit spin lock is problematic because with the bit spin
lock held zsmalloc acquires a rwlock_t and spinlock_t which are both
sleeping locks on PREEMPT_RT and therefore must not be acquired with
disabled preemption.

There is a patch which extends the handle on PREEMPT_RT so that a full
spinlock_t fits (even with lockdep enabled) and then eliminates the bit
spin lock. I'm not sure how sensible zsmalloc on PREEMPT_RT is given
that it is used to store compressed user memory.

Disable ZSMALLOC on PREEMPT_RT. If there is need for it, we can try to
get it to work.

Signed-off-by: Sebastian Andrzej Siewior <[email-masked]>
Link: https://lkml.kernel.org/r/[email-masked]
---
 mm/Kconfig |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -640,6 +640,7 @@ config ZSWAP_ZPOOL_DEFAULT_Z3FOLD
 
 config ZSWAP_ZPOOL_DEFAULT_ZSMALLOC
 	bool "zsmalloc"
+	depends on !PREEMPT_RT
 	select ZSMALLOC
 	help
 	  Use the zsmalloc allocator as the default allocator.
@@ -690,7 +691,7 @@ config Z3FOLD
 
 config ZSMALLOC
 	tristate "Memory allocator for compressed pages"
-	depends on MMU
+	depends on MMU && !PREEMPT_RT
 	help
 	  zsmalloc is a slab-based memory allocator designed to store
 	  compressed RAM pages.  zsmalloc uses virtual memory mapping