Re: [PATCH 5/5] mm/slub: Refactor deactivate_slab()

[Vlastimil Babka <vbabka@suse.cz>]

On 2/21/22 11:53, Hyeonggon Yoo wrote:
> Simply deactivate_slab() by removing variable 'lock' and replacing
> 'l' and 'm' with 'mode'. Instead, remove slab from list and unlock
> n->list_lock when cmpxchg_double() fails, and then retry.
> 
> One slight functional change is releasing and taking n->list_lock again
> when cmpxchg_double() fails. This is not harmful because SLUB avoids
> deactivating slabs as much as possible.
> 
> Signed-off-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>

Hm I wonder if we could simplify even a bit more. Do we have to actually
place the slab on a partial (full) list before the cmpxchg, only to remove
it when cmpxchg fails? Seems it's to avoid anyone else seeing the slab
un-frozen, but not on the list, which would be unexpected. However if anyone
sees such slab, they have to take the list_lock first to start working with
the slab... so this should be safe, because we hold the list_lock here, and
will place the slab on the list before we release it. But it thus shouldn't
matter if the placement happens before or after a successful cmpxchg, no? So
we can only do it once after a successful cmpxchg and need no undo's?

Specifically AFAIK the only possible race should be with a __slab_free()
which might observe !was_frozen after we succeed an unfreezing cmpxchg and
go through the
"} else { /* Needs to be taken off a list */"
branch but then it takes the list_lock as the first thing, so will be able
to proceed only after the slab is actually on the list.

Do I miss anything or would you agree?

> ---
>  mm/slub.c | 74 +++++++++++++++++++++++++------------------------------
>  1 file changed, 33 insertions(+), 41 deletions(-)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index a4964deccb61..2d0663befb9e 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -2350,8 +2350,8 @@ static void deactivate_slab(struct kmem_cache *s, struct slab *slab,
>  {
>  	enum slab_modes { M_NONE, M_PARTIAL, M_FULL, M_FREE };
>  	struct kmem_cache_node *n = get_node(s, slab_nid(slab));
> -	int lock = 0, free_delta = 0;
> -	enum slab_modes l = M_NONE, m = M_NONE;
> +	int free_delta = 0;
> +	enum slab_modes mode = M_NONE;
>  	void *nextfree, *freelist_iter, *freelist_tail;
>  	int tail = DEACTIVATE_TO_HEAD;
>  	unsigned long flags = 0;
> @@ -2420,57 +2420,49 @@ static void deactivate_slab(struct kmem_cache *s, struct slab *slab,
>  	new.frozen = 0;
>  
>  	if (!new.inuse && n->nr_partial >= s->min_partial)
> -		m = M_FREE;
> +		mode = M_FREE;
>  	else if (new.freelist) {
> -		m = M_PARTIAL;
> -		if (!lock) {
> -			lock = 1;
> -			/*
> -			 * Taking the spinlock removes the possibility that
> -			 * acquire_slab() will see a slab that is frozen
> -			 */
> -			spin_lock_irqsave(&n->list_lock, flags);
> -		}
> -	} else {
> -		m = M_FULL;
> -		if (kmem_cache_debug_flags(s, SLAB_STORE_USER) && !lock) {
> -			lock = 1;
> -			/*
> -			 * This also ensures that the scanning of full
> -			 * slabs from diagnostic functions will not see
> -			 * any frozen slabs.
> -			 */
> -			spin_lock_irqsave(&n->list_lock, flags);
> -		}
> +		mode = M_PARTIAL;
> +		/*
> +		 * Taking the spinlock removes the possibility that
> +		 * acquire_slab() will see a slab that is frozen
> +		 */
> +		spin_lock_irqsave(&n->list_lock, flags);
> +		add_partial(n, slab, tail);
> +	} else if (kmem_cache_debug_flags(s, SLAB_STORE_USER)) {
> +		mode = M_FULL;
> +		/*
> +		 * This also ensures that the scanning of full
> +		 * slabs from diagnostic functions will not see
> +		 * any frozen slabs.
> +		 */
> +		spin_lock_irqsave(&n->list_lock, flags);
> +		add_full(s, n, slab);
>  	}
>  
> -	if (l != m) {
> -		if (l == M_PARTIAL)
> -			remove_partial(n, slab);
> -		else if (l == M_FULL)
> -			remove_full(s, n, slab);
>  
> -		if (m == M_PARTIAL)
> -			add_partial(n, slab, tail);
> -		else if (m == M_FULL)
> -			add_full(s, n, slab);
> -	}
> -
> -	l = m;
>  	if (!cmpxchg_double_slab(s, slab,
>  				old.freelist, old.counters,
>  				new.freelist, new.counters,
> -				"unfreezing slab"))
> +				"unfreezing slab")) {
> +		if (mode == M_PARTIAL) {
> +			remove_partial(n, slab);
> +			spin_unlock_irqrestore(&n->list_lock, flags);
> +		} else if (mode == M_FULL) {
> +			remove_full(s, n, slab);
> +			spin_unlock_irqrestore(&n->list_lock, flags);
> +		}
>  		goto redo;
> +	}
>  
> -	if (lock)
> -		spin_unlock_irqrestore(&n->list_lock, flags);
>  
> -	if (m == M_PARTIAL)
> +	if (mode == M_PARTIAL) {
> +		spin_unlock_irqrestore(&n->list_lock, flags);
>  		stat(s, tail);
> -	else if (m == M_FULL)
> +	} else if (mode == M_FULL) {
> +		spin_unlock_irqrestore(&n->list_lock, flags);
>  		stat(s, DEACTIVATE_FULL);
> -	else if (m == M_FREE) {
> +	} else if (mode == M_FREE) {
>  		stat(s, DEACTIVATE_EMPTY);
>  		discard_slab(s, slab);
>  		stat(s, FREE_SLAB);