On Tue 08-03-16 12:07:24, Hugh Dickins wrote: > On Tue, 8 Mar 2016, Michal Hocko wrote: > > On Mon 29-02-16 14:41:39, Michal Hocko wrote: > > > On Sun 28-02-16 19:19:11, Hugh Dickins wrote: > > > > On Tue, 23 Feb 2016, Michal Hocko wrote: > > > > > On Mon 22-02-16 17:36:07, David Rientjes wrote: > > > > > > > > > > > > Are we concerned about munlock_vma_pages_all() taking lock_page() and > > > > > > perhaps stalling forever, the same way it would stall in exit_mmap() for > > > > > > VM_LOCKED vmas, if another thread has locked the same page and is doing an > > > > > > allocation? > > > > > > > > > > This is a good question. I have checked for that particular case > > > > > previously and managed to convinced myself that this is OK(ish). > > > > > munlock_vma_pages_range locks only THP pages to prevent from the > > > > > parallel split-up AFAICS. > > > > > > > > I think you're mistaken on that: there is also the lock_page() > > > > on every page in Phase 2 of __munlock_pagevec(). > > > > > > Ohh, I have missed that one. Thanks for pointing it out! > > > > > > [...] > > > > > Just for the reference this is what I came up with (just compile tested). > > > > > > > > I tried something similar internally (on an earlier kernel). Like > > > > you I've set that work aside for now, there were quicker ways to fix > > > > the issue at hand. But it does continue to offend me that munlock > > > > demands all those page locks: so if you don't get back to it before me, > > > > I shall eventually. > > > > > > > > I didn't understand why you complicated yours with the "enforce" > > > > arg to munlock_vma_pages_range(): why not just trylock in all cases? > > > > > > Well, I have to confess that I am not really sure I understand all the > > > consequences of the locking here. It has always been subtle and weird > > > issues popping up from time to time. So I only wanted to have that > > > change limitted to the oom_reaper. So I would really appreciate if > > > somebody more knowledgeable had a look. We can drop the mlock patch for > > > now. > > > > According to the rc7 announcement it seems we are approaching the merge > > window. Should we drop the patch for now or the risk of the lockup is > > too low to care about and keep it in for now as it might be already > > useful and change the munlock path to not depend on page locks later on? > > > > I am OK with both ways. > > You're asking about the Subject patch, "oom reaper: handle mlocked pages", > I presume. Your Work-In-Progress mods to munlock_vma_pages_range() should > certainly be dropped for now, and revisited by one of us another time. I believe it hasn't landed in the mmotm yet. > I vote for dropping "oom reaper: handle mlocked pages" for now too. OK, Andrew, could you drop oom-reaper-handle-mlocked-pages.patch for now. We will revisit it later on after we make the munlock path page lock free. > If I understand correctly, the purpose of the oom reaper is to free up > as much memory from the targeted task as possible, while avoiding getting > stuck on locks; in advance of the task actually exiting and doing the > freeing itself, but perhaps getting stuck on locks as it does so. > > If that's a fair description, then it's inappropriate for the oom reaper > to call munlock_vma_pages_all(), with the risk of getting stuck on many > page locks; best leave that risk to the task when it exits as at present. > Of course we should come back to this later, fix munlock_vma_pages_range() > with trylocks (on the pages only? rmap mutexes also?), and then integrate > "oom reaper: handle mlocked pages". Fair enough. > (Or if we had the old mechanism for scanning unevictable lrus on demand, > perhaps simply not avoid the VM_LOCKED vmas in __oom_reap_vmas(), let > the clear_page_mlock() in page_remove_*rmap() handle all the singly > mapped and mlocked pages, and un-mlock the rest by scanning unevictables.) I will have a look at this possibility as well. -- Michal Hocko 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/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>