Re: [PATCH] mm: Disable zsmalloc on PREEMPT_RT

[Minchan Kim <minchan@kernel.org>]

On Thu, Sep 23, 2021 at 07:01:21PM +0200, Sebastian Andrzej Siewior wrote:
> 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.

I am not sure how long the preemption disabled section takes since it
just disable for a page copy mostly.

> 
> 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.

I don't see what's relation between PREEMPT_RT and compressed user
memory so you can reach such conclustion. Disable zsmalloc also makes
disable zram. I think in-compress memory swap rather than storage
swap/block sometimes would be useful for even RT.

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

Please send the patch which extends handle with spin_lock rather than
simply disabing.

> 
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> ---
>  mm/Kconfig | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/Kconfig b/mm/Kconfig
> index 9f1e0098522c2..541371e64c477 100644
> --- 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
> -- 
> 2.33.0
> 
>