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

[Hyeonggon Yoo <42.hyeyoo@gmail.com>]

On Thu, Feb 24, 2022 at 07:16:11PM +0100, Vlastimil Babka wrote:
> 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?
>

My thought was similar. But after testing I noticed that &n->list_lock prevents
race between __slab_free() and deactivate_slab().

> 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?
>

It's so tricky.

I tried to simplify more as you said. Seeing frozen slab on list was not
problem. But the problem was that something might interfere between
cmpxchg_double() and taking spinlock.

This is what I faced:

	CPU A				CPU B
deactivate_slab() {			__slab_free() {
	/* slab is full */
	slab.frozen = 0;
	cmpxchg_double();
						/* Hmm... 
						slab->frozen == 0 &&
						slab->freelist != NULL?
						Oh This must be on the list.. */
						
						spin_lock_irqsave();
						cmpxchg_double();
						/* Corruption: slab
						 * was not yet inserted to
						 * list but try removing */
						remove_full();
						spin_unlock_irqrestore();
					}
	spin_lock_irqsave();
	add_full();
	spin_unlock_irqrestore();
}

I think it's quite confusing because it's protecting code, not data.

Would you have an idea to solve it, or should we add a comment for this?

> > ---
> >  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);
> 

-- 
Thank you, You are awesome!
Hyeonggon :-)