The patch titled
Subject: mm: disable zsmalloc on PREEMPT_RT
has been added to the -mm tree. Its filename is
mm-disable-zsmalloc-on-preempt_rt.patch
This patch should soon appear at
https://ozlabs.org/~akpm/mmots/broken-out/mm-disable-zsmalloc-on-preempt_rt.patch
and later at
https://ozlabs.org/~akpm/mmotm/broken-out/mm-disable-zsmalloc-on-preempt_rt.patch
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------------------------------------------------------
From: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
Subject: mm: disable zsmalloc on PREEMPT_RT
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.
Link: https://lkml.kernel.org/r/20210923170121.1860133-1-bigeasy@xxxxxxxxxxxxx
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Minchan Kim <minchan@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/Kconfig | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)
--- a/mm/Kconfig~mm-disable-zsmalloc-on-preempt_rt
+++ a/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
_
Patches currently in -mm which might be from bigeasy@xxxxxxxxxxxxx are
mm-disable-zsmalloc-on-preempt_rt.patch
