On 01/04/2014 01:18 AM, Linus Torvalds wrote: > On Fri, Jan 3, 2014 at 3:36 PM, Vlastimil Babka <vbabka@xxxxxxx> wrote: >> >> I'm for going with the removal of BUG_ON. The TestSetPageMlocked should provide enough >> race protection. > > Maybe. But dammit, that's subtle, and I don't think you're even right. > > It basically depends on mlock_vma_page() and munlock_vma_page() being > able to run CONCURRENTLY on the same page. In particular, you could > have a mlock_vma_page() set the bit on one CPU, and munlock_vma_page() > immediately clearing it on another, and then the rest of those > functions could run with a totally arbitrary interleaving when working > with the exact same page. > > They both do basically > > if (!isolate_lru_page(page)) > putback_lru_page(page); > > but one or the other would randomly win the race (it's internally > protected by the lru lock), and *if* the munlock_vma_page() wins it, > it would also do > > try_to_munlock(page); > > but if mlock_vma_page() wins it, that wouldn't happen. That looks > entirely broken - you end up with the PageMlocked bit clear, but > try_to_munlock() was never called on that page, because > mlock_vma_page() got to the page isolation before the "subsequent" > munlock_vma_page(). I got the impression (see e.g. munlock_vma_page() comments) that the whole thing is designed with this possibility in mind. isolate_lru_page() may fail (presumably also in other scenarios than this) and if try_to_munlock() was not called here, then yes the page might lose the PageMlocked bit and go to LRU instead of inevictable list, but try_to_unmap() should catch and fix this. That would also explain why mlock_vma_page() is called from try_to_unmap_cluster(). So if I understand correctly, PageMlocked bit is not something that has to be correctly set 100% of the time, but when it's set correctly most of the time, then most of these pages will go to inevictable list and spare vmscan's time. > And this is very much what the page lock serialization would prevent. > So no, the PageMlocked in *no* way gives serialization. It's an atomic > bit op, yes, but that only "serializes" in one direction, not when you > can have a mix of bit setting and clearing. > > So quite frankly, I think you're wrong. The BUG_ON() is correct, or at > least enforces some kind of ordering. And try_to_unmap_cluster() is > just broken in calling that without the page being locked. That's my > opinion. There may be some *other* reason why it all happens to work, > but no, "TestSetPageMlocked should provide enough race protection" is > simply not true, and even if it were, it's way too subtle and odd to > be a good rule. Right, it was stupid of me to write such strong statement without any details. I wanted to review that patch when back at work next week, but since it came up now, I just wanted to point out that it's in the pipeline for this bug. > So I really object to just removing the BUG_ON(). Not with a *lot* > more explanation as to why these kinds of issues wouldn't matter. > > Linus > -- 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/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>