Hi Motohiro, On Mon, Jan 06, 2014 at 08:47:23AM -0800, Motohiro Kosaki wrote: > > >> -----Original Message----- >> From: linus971@xxxxxxxxx [mailto:linus971@xxxxxxxxx] 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 <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(). >> >> 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. Could you point out when page is locked during 1)? Regards, Wanpeng Li >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. > -- 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>