On Tue 23-09-14 13:05:25, Johannes Weiner wrote: [...] > #include <trace/events/vmscan.h> > > -int page_counter_sub(struct page_counter *counter, unsigned long nr_pages) > +/** > + * page_counter_cancel - take pages out of the local counter > + * @counter: counter > + * @nr_pages: number of pages to cancel > + * > + * Returns whether there are remaining pages in the counter. > + */ > +int page_counter_cancel(struct page_counter *counter, unsigned long nr_pages) > { > long new; > > new = atomic_long_sub_return(nr_pages, &counter->count); > > - if (WARN_ON(unlikely(new < 0))) > - atomic_long_set(&counter->count, 0); > + if (WARN_ON_ONCE(unlikely(new < 0))) > + atomic_long_add(nr_pages, &counter->count); > > return new > 0; > }
I am not sure I understand this correctly. The original res_counter code has protection against < 0 because it used unsigned longs and wanted to protect from really disturbing effects of underflow I guess (this wasn't documented anywhere). But you are using long so even underflow shouldn't be a big problem so why do we need a fixup? The only way how we can end up < 0 would be a cancel without pairing charge AFAICS. A charge should always appear before uncharge because both of them are using atomics which imply memory barriers (atomic_*_return). So do I understand correctly that your motivation is to fix up those cancel-without-charge automatically? This would definitely ask for a fat comment. Or am I missing something? Besides that do we need to have any memory barrier there? Thanks! -- Michal Hocko SUSE Labs -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/