Messages in this thread | | | Date | Fri, 15 Jul 2022 16:05:29 +0800 | Subject | Re: [PATCH 1/3] mm/slub: fix the race between validate_slab and slab_free | From | Rongwei Wang <> |
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On 6/17/22 5:40 PM, Vlastimil Babka wrote: > On 6/8/22 14:23, Christoph Lameter wrote: >> On Wed, 8 Jun 2022, Rongwei Wang wrote: >> >>> If available, I think document the issue and warn this incorrect behavior is >>> OK. But it still prints a large amount of confusing messages, and disturbs us? >> >> Correct it would be great if you could fix this in a way that does not >> impact performance. >> >>>> are current operations on the slab being validated. >>> And I am trying to fix it in following way. In a short, these changes only >>> works under the slub debug mode, and not affects the normal mode (I'm not >>> sure). It looks not elegant enough. And if all approve of this way, I can >>> submit the next version. >> >> >>> >>> Anyway, thanks for your time:). >>> -wrw >>> >>> @@ -3304,7 +3300,7 @@ static void __slab_free(struct kmem_cache *s, >> struct >>> slab *slab, >>> >>> { >>> void *prior; >>> - int was_frozen; >>> + int was_frozen, to_take_off = 0; >>> struct slab new; >> >> to_take_off has the role of !n ? Why is that needed? >> >>> - do { >>> - if (unlikely(n)) { >>> + spin_lock_irqsave(&n->list_lock, flags); >>> + ret = free_debug_processing(s, slab, head, tail, cnt, addr); >> >> Ok so the idea is to take the lock only if kmem_cache_debug. That looks >> ok. But it still adds a number of new branches etc to the free loop. > Hi, Vlastimil, sorry for missing your message long time. > It also further complicates the already tricky code. I wonder if we should > make more benefit from the fact that for kmem_cache_debug() caches we don't > leave any slabs on percpu or percpu partial lists, and also in > free_debug_processing() we aready take both list_lock and slab_lock. If we > just did the freeing immediately there under those locks, we would be > protected against other freeing cpus by that list_lock and don't need the > double cmpxchg tricks. enen, I'm not sure get your "don't need the double cmpxchg tricks" means completely. What you want to say is that replace cmpxchg_double_slab() here with following code when kmem_cache_debug(s)?
__slab_lock(slab); if (slab->freelist == freelist_old && slab->counters == counters_old){ slab->freelist = freelist_new; slab->counters = counters_new; __slab_unlock(slab); local_irq_restore(flags); return true; } __slab_unlock(slab);
If I make mistakes for your words, please let me know. Thanks! > > What about against allocating cpus? More tricky as those will currently end > up privatizing the freelist via get_partial(), only to deactivate it again, > so our list_lock+slab_lock in freeing path would not protect in the > meanwhile. But the allocation is currently very inefficient for debug > caches, as in get_partial() it will take the list_lock to take the slab from > partial list and then in most cases again in deactivate_slab() to return it. It seems that I need speed some time to eat these words. Anyway, thanks. > > If instead the allocation path for kmem_cache_debug() cache would take a > single object from the partial list (not whole freelist) under list_lock, it > would be ultimately more efficient, and protect against freeing using > list_lock. Sounds like an idea worth trying to me?
Hyeonggon had a similar advice that split freeing and allocating slab from debugging, likes below:
__slab_alloc() { if (kmem_cache_debug(s)) slab_alloc_debug() else ___slab_alloc() }
I guess that above code aims to solve your mentioned problem (idea)?
slab_free() { if (kmem_cache_debug(s)) slab_free_debug() else __do_slab_free() }
Currently, I only modify the code of freeing slab to fix the confusing messages of "slabinfo -v". If you agree, I can try to realize above mentioned slab_alloc_debug() code. Maybe it's also a challenge to me.
Thanks for your time.
> And of course we would stop creating the 'validate' sysfs files for > non-debug caches.
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