I was offline for several days for the holidays and I'm not back online properly until Jan 4th, hence the delay in responding. On Fri, Dec 23, 2011 at 11:08:19AM -0800, Hugh Dickins wrote: > Sorry, Mel, I've had to revert this patch (and its two little children) > from my 3.2.0-rc6-next-20111222 testing: you really do need a page flag > (or substitute) for your "immediate" lru. > Don't be sorry at all. I prefer that this was caught before merging to mainline and thanks for catching this. > How else can a del_page_from_lru[_list]() know whether to decrement > the count of the immediate or the inactive list? You are right, it cannot and because pages are removed from the LRU list in contexts such as invalidating a mapping, we cannot be sure whether a page is on the immediate LRU or inactive_file in all cases. It is further complicated by the fact that PageReclaim and PageReadhead use the same page flag. > page_lru() says to > decrement the count of the inactive list, so in due course that wraps > to a gigantic number, and then page reclaim livelocks trying to wring > pages out of an empty list. It's the memcg case I've been hitting, > but presumably the same happens with global counts. > I've verified that the accounting can break. I did not see it wrap negative because in my testing it was rare the problem occurred but it would happen eventually. I considered a few ways of fixing this. The obvious one is to add a new page flag but that is difficult to justify as the high-cpu-usage problem should only occur when there is a lot of writeback to slow storage which I believe is a rare case. It is not a suitable use for an extended page flag. The second was to keep these PageReclaim pages off the LRU but this leads to complications of its own. The third was to use a combination of flags to mark pages that are on the immediate LRU such as how PG_compound and PG_reclaim in combination mark tail pages. This would not be free of races and would eventually cause corruption. There is also the problem that we cannot atomically set multiple bits so setting the bits in contexts such as set_page_dirty() may be problematic. Andrew, as there is not an easy uncontroversial fix can you remove the following patches from mmotm please? mm-isolate-pages-for-immediate-reclaim-on-their-own-lru.patch mm-isolate-pages-for-immediate-reclaim-on-their-own-lru-fix.patch mm-isolate-pages-for-immediate-reclaim-on-their-own-lru-fix-2.patch The impact is that users writing to slow stage may see higher CPU usage as the pages under writeback have to be skipped by scanning once the dirty pages move to the end of the LRU list. I'm assuming once they are removed from mmotm that they also get removed from linux-next. > There is another such accounting bug in -next, been there longer and > not so easy to hit: I'm fairly sure it will turn out to be memcg > misaccounting a THPage somewhere, I'll have a look around shortly. > > p.s. Immediate? Isn't that an odd name for a list of pages which are > not immediately freeable? Maybe Rik's launder/laundry name would be > better: pages which are currently being cleaned. That is potentially very misleading as not all pages being laundered are on that list. reclaim_writeback might be a better name. Thanks. -- Mel Gorman SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>