On Fri, Aug 12, 2011 at 05:36:16PM +0200, Andrea Arcangeli wrote: > On Tue, Aug 09, 2011 at 04:04:21AM -0700, Michel Lespinasse wrote: > > - Use my proposed page count lock in order to avoid the race. One > > would have to convert all get_page_unless_zero() sites to use it. I > > expect the cost would be low but still measurable. > > I didn't yet focus at your problem after we talked about it at MM > summit, but I seem to recall I suggested there to just get to the head > page and always take the lock on it. split_huge_page only works at 2M > aligned pages, the rest you don't care about. Getting to the head page > compound_lock should be always safe. And that will still scale > incredibly better than taking the lru_lock for the whole zone (which > would also work). And it seems the best way to stop split_huge_page > without having to alter the put_page fast path when it works on head > pages (the only thing that gets into put_page complex slow path is the > release of tail pages after get_user_pages* so it'd be nice if > put_page fast path still didn't need to take locks). > > > - It'd be sweet if one could somehow record the time a THP page was > > created, and wait for at least one RCU grace period *starting from the > > recorded THP creation time* before splitting huge pages. In practice, > > we would be very unlikely to have to wait since the grace period would > > be already expired. However, I don't think RCU currently provides such > > a mechanism - Paul, is this something that would seem easy to > > implement or not ? It should not be hard. I already have an API for rcutorture testing use, but it is not appropriate for your use because it is unsynchronized. We need to be careful with what I give you and how you interpret it. The most effective approach would be for me to give you an API that filled in a cookie given a pointer to one, then another API that took pointers to a pair of cookies and returned saying whether or not a grace period had elapsed. You would do something like the following: rcu_get_gp_cookie(&pagep->rcucookie); . . . rcu_get_gp_cookie(&autovarcookie); if (!rcu_cookie_gp_elapsed(&pagep->rcucookie, &autovarcookie)) synchronize_rcu(); So, how much space do I get for ->rcucookie? By default, it is a pair of unsigned longs, but I could live with as small as a single byte if you didn't mind a high probability of false negatives (me telling you to do a grace period despite 16 of them having happened in the meantime due to overflow of a 4-bit field in the byte). That covers TREE_RCU and TREE_PREEMPT_RCU, on to TINY_RCU and TINY_PREEMPT_RCU. TINY_RCU will require more thought, as it doesn't bother counting grace periods. Ah, but in TINY_RCU, synchronize_rcu() is free, so I simply make rcu_cookie_gp_elapsed() always return false. OK, TINY_PREEMPT_RCU... It doesn't count grace periods, either. But it is able to reliably detect if there are any RCU readers in flight, and there normally won't be, so synchronize_rcu() is again free in the common case. And no, I don't want to count grace periods as this would increase the memory footprint. And the whole point of TINY_PREEMPT_RCU is to be tiny, after all. ;-) If you need SRCU, you are out of luck until I get my act together and merge it in with the other RCU implementations, which might be awhile still. For TREE_*RCU, the calls to rcu_get_gp_cookie() will cost you a lock round trip. I am hoping to be able to use the counters stored in the rcu_data structure, which means that I would need to disable preemption and re-enable it. Or maybe disable and re-enable irqs instead, not yet sure which. This might require me to be conservative and make rcu_cookie_gp_elapsed() unless two grace periods have elapsed. Things get a bit tricky -- yes, I could just use the global counters, but that would mean that rcu_get_gp_cookie() would need to acquire a global lock, and I suspect that you intend to invoke it too often for that to be a winning strategy. Thoughts? And how many bits do I get for the cookie? Thanx, Paul > This looks sweet. We could store a quiescent points generation counter > in the page[1].something, if the page has the same generation of the > last RCU quiescent point (vs rcu_read_lock) we synchronize_rcu before > starting split_huge_page. split_huge_page is serialized through the > anon_vma lock however, so we'd need to release the anon_vma lock, > synchronize_rcu and retry and this time the page[1].something sequence > counter would be older than the rcu generation counter and it'll > proceed (maybe another thread or process will get there first but > that's ok). > > I didn't have better ideas than yours above, but I'll keep thinking. > > > > When I make deactivate_page, I didn't consider that honestly. > > > IMHO, It shouldn't be a problem as deactive_page hold a reference > > > of page by pagevec_lookup so the page shouldn't be gone under us. > > > > Agree - it seems like you are guaranteed to already hold a reference > > (but then a straight get_page should be sufficient, right ?) > > I hope this is not an issue because of the fact the page is guaranteed > not to be THP when get_page_unless_zero runs on it. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. 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