Re: [PATCH] zsmalloc: do not use bit_spin_lock

[Minchan Kim]

On Sun, Dec 20, 2020 at 02:22:28AM +0200, Vitaly Wool wrote:
> zsmalloc takes bit spinlock in its _map() callback and releases it
> only in unmap() which is unsafe and leads to zswap complaining
> about scheduling in atomic context.
> 
> To fix that and to improve RT properties of zsmalloc, remove that
> bit spinlock completely and use a bit flag instead.

I don't want to use such open code for the lock.

I see from Mike's patch, recent zswap change introduced the lockdep
splat bug and you want to improve zsmalloc to fix the zswap bug and
introduce this patch with allowing preemption enabling.

https://lore.kernel.org/linux-mm/fae85e4440a8ef6f13192476bd33a4826416fc58.camel@xxxxxx/

zs_[un/map]_object is designed to be used in fast path(i.e.,
zs_map_object/4K page copy/zs_unmap_object) so the spinlock is
perfectly fine for API point of view. However, zswap introduced
using the API with mutex_lock/crypto_wait_req where allowing
preemption, which was wrong.

Furthermore, the zs_map_object already has a few more places where
disablepreemptions(migrate_read_lock, get_cpu_var and kmap_atomic).

Without making those locks preemptible all at once, zswap will still
see the lockdep warning.

> 
> Signed-off-by: Vitaly Wool <vitaly.wool@xxxxxxxxxxxx>
> ---
>  mm/zsmalloc.c | 13 ++++++++-----
>  1 file changed, 8 insertions(+), 5 deletions(-)
> 
> diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
> index 7289f502ffac..ff26546a7fed 100644
> --- a/mm/zsmalloc.c
> +++ b/mm/zsmalloc.c
> @@ -876,22 +876,25 @@ static unsigned long obj_to_head(struct page *page, void *obj)
>  
>  static inline int testpin_tag(unsigned long handle)
>  {
> -	return bit_spin_is_locked(HANDLE_PIN_BIT, (unsigned long *)handle);
> +	return test_bit(HANDLE_PIN_BIT, (unsigned long *)handle);
>  }
>  
>  static inline int trypin_tag(unsigned long handle)
>  {
> -	return bit_spin_trylock(HANDLE_PIN_BIT, (unsigned long *)handle);
> +	return !test_and_set_bit(HANDLE_PIN_BIT, (unsigned long *)handle);
>  }
>  
> -static void pin_tag(unsigned long handle) __acquires(bitlock)
> +static void pin_tag(unsigned long handle)
>  {
> -	bit_spin_lock(HANDLE_PIN_BIT, (unsigned long *)handle);
> +	preempt_disable();
> +	while(test_and_set_bit(HANDLE_PIN_BIT, (unsigned long *)handle))
> +		cpu_relax();
> +	preempt_enable();
>  }
>  
>  static void unpin_tag(unsigned long handle) __releases(bitlock)
>  {
> -	bit_spin_unlock(HANDLE_PIN_BIT, (unsigned long *)handle);
> +	clear_bit(HANDLE_PIN_BIT, (unsigned long *)handle);
>  }
>  
>  static void reset_page(struct page *page)
> -- 
> 2.20.1
>