> -----Original Message----- > From: linus...@gmail.com [mailto:linus...@gmail.com] On Behalf Of Linus > Torvalds > Sent: Friday, January 03, 2014 7:18 PM > To: Vlastimil Babka > Cc: Sasha Levin; Andrew Morton; Wanpeng Li; Michel Lespinasse; Bob Liu; > Nick Piggin; Motohiro Kosaki JP; Rik van Riel; David Rientjes; Mel Gorman; > Minchan Kim; Hugh Dickins; Johannes Weiner; linux-mm; Linux Kernel Mailing > List > Subject: Re: [PATCH] mm/mlock: fix BUG_ON unlocked page for nolinear > VMAs > > On Fri, Jan 3, 2014 at 3:36 PM, Vlastimil Babka <vba...@suse.cz> 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(). > > 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. > > 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.
I don't have a perfect answer. But I can explain a bit history. Let's me try. First off, 5 years ago, Lee's original putback_lru_page() implementation required page-lock, but I removed the restriction months later. That's why we can see strange BUG_ON here. 5 years ago, both mlock(2) and munlock(2) called do_mlock() and it was protected by mmap_sem (write mdoe). Then, mlock and munlock had no race. Now, __mm_populate() (called by mlock(2)) is only protected by mmap_sem read-mode. However it is enough to protect against munlock. Next, In case of mlock vs reclaim, the key is that mlock(2) has two step operation. 1) turn on VM_LOCKED under mmap_sem write-mode, 2) turn on Page_Mlocked under mmap_sem read-mode. If reclaim race against step (1), reclaim must lose because it uses trylock. On the other hand, if reclaim race against step (2), reclaim must detect VM_LOCKED because both VM_LOCKED modifier and observer take mmap-sem. By the way, page isolation is still necessary because we need to protect another page modification like page migration. My memory was alomostly flushed and I might lost some technical concern and past discussion. Please point me out, If I am overlooking something. Thanks.
