---------- Forwarded message ---------- From: "Joonsoo Kim" <iamjoonsoo.kim@xxxxxxx> Date: Jul 28, 2016 7:57 PM Subject: Re: [RFC] can we use vmalloc to alloc thread stack if compaction failed To: "Andy Lutomirski" <luto@xxxxxxxxxx> Cc: "Xishi Qiu" <qiuxishi@xxxxxxxxxx>, "Michal Hocko" <mhocko@xxxxxxxxxx>, "Tejun Heo" <tj@xxxxxxxxxx>, "Ingo Molnar" <mingo@xxxxxxxxxx>, "Peter Zijlstra" <peterz@xxxxxxxxxxxxx>, "LKML" <linux-kernel@xxxxxxxxxxxxxxx>, "Linux MM" <linux-mm@xxxxxxxxx>, "Yisheng Xie" <xieyisheng1@xxxxxxxxxx> > On Thu, Jul 28, 2016 at 08:07:51AM -0700, Andy Lutomirski wrote: > > On Thu, Jul 28, 2016 at 3:51 AM, Xishi Qiu <qiuxishi@xxxxxxxxxx> wrote: > > > On 2016/7/28 17:43, Michal Hocko wrote: > > > > > >> On Thu 28-07-16 16:45:06, Xishi Qiu wrote: > > >>> On 2016/7/28 15:58, Michal Hocko wrote: > > >>> > > >>>> On Thu 28-07-16 15:41:53, Xishi Qiu wrote: > > >>>>> On 2016/7/28 15:20, Michal Hocko wrote: > > >>>>> > > >>>>>> On Thu 28-07-16 15:08:26, Xishi Qiu wrote: > > >>>>>>> Usually THREAD_SIZE_ORDER is 2, it means we need to alloc 16kb continuous > > >>>>>>> physical memory during fork a new process. > > >>>>>>> > > >>>>>>> If the system's memory is very small, especially the smart phone, maybe there > > >>>>>>> is only 1G memory. So the free memory is very small and compaction is not > > >>>>>>> always success in slowpath(__alloc_pages_slowpath), then alloc thread stack > > >>>>>>> may be failed for memory fragment. > > >>>>>> > > >>>>>> Well, with the current implementation of the page allocator those > > >>>>>> requests will not fail in most cases. The oom killer would be invoked in > > >>>>>> order to free up some memory. > > >>>>>> > > >>>>> > > >>>>> Hi Michal, > > >>>>> > > >>>>> Yes, it success in most cases, but I did have seen this problem in some > > >>>>> stress-test. > > >>>>> > > >>>>> DMA free:470628kB, but alloc 2 order block failed during fork a new process. > > >>>>> There are so many memory fragments and the large block may be soon taken by > > >>>>> others after compact because of stress-test. > > >>>>> > > >>>>> --- dmesg messages --- > > >>>>> 07-13 08:41:51.341 <4>[309805.658142s][pid:1361,cpu5,sManagerService]sManagerService: page allocation failure: order:2, mode:0x2000d1 > > >>>> > > >>>> Yes but this is __GFP_DMA allocation. I guess you have already reported > > >>>> this failure and you've been told that this is quite unexpected for the > > >>>> kernel stack allocation. It is your out-of-tree patch which just makes > > >>>> things worse because DMA restricted allocations are considered "lowmem" > > >>>> and so they do not invoke OOM killer and do not retry like regular > > >>>> GFP_KERNEL allocations. > > >>> > > >>> Hi Michal, > > >>> > > >>> Yes, we add GFP_DMA, but I don't think this is the key for the problem. > > >> > > >> You are restricting the allocation request to a single zone which is > > >> definitely not good. Look at how many larger order pages are available > > >> in the Normal zone. > > >> > > >>> If we do oom-killer, maybe we will get a large block later, but there > > >>> is enough free memory before oom(although most of them are fragments). > > >> > > >> Killing a task is of course the last resort action. It would give you > > >> larger order blocks used for the victims thread. > > >> > > >>> I wonder if we can alloc success without kill any process in this situation. > > >> > > >> Sure it would be preferable to compact that memory but that might be > > >> hard with your restriction in place. Consider that DMA zone would tend > > >> to be less movable than normal zones as users would have to pin it for > > >> DMA. Your DMA is really large so this might turn out to just happen to > > >> work but note that the primary problem here is that you put a zone > > >> restriction for your allocations. > > >> > > >>> Maybe use vmalloc is a good way, but I don't know the influence. > > >> > > >> You can have a look at vmalloc patches posted by Andy. They are not that > > >> trivial. > > >> > > > > > > Hi Michal, > > > > > > Thank you for your comment, could you give me the link? > > > > > > > I've been keeping it mostly up to date in this branch: > > > > https://git.kernel.org/cgit/linux/kernel/git/luto/linux.git/log/?h=x86/vmap_stack > > > > It's currently out of sync due to a bunch of the patches being queued > > elsewhere for the merge window. > > Hello, Andy. > > I have some questions about it. > > IIUC, to turn on HAVE_ARCH_VMAP_STACK on different architecture, there > is nothing to be done in architecture side if the architecture doesn't > support lazily faults in top-level paging entries for the vmalloc > area. Is my understanding is correct? > There should be nothing fundamental that needs to be done. On the other hand, it might be good to make sure the arch code can print a clean stack trace on stack overflow. If it's helpful, I just pushed out anew > And, I'd like to know how you search problematic places using kernel > stack for DMA. > I did some searching for problematic sg_init_buf calls using Coccinelle. I'm not very good at Coccinelle, so I may have missed something. For the most part, DMA API debugging should have found the problems already. The ones I found were in drivers that didn't do real DMA: crypto users and virtio. > One note is that, stack overflow happens at the previous page of the > stack end position if stack grows down, but, guard page is placed at > the next page of the stack begin position. So, this stack overflow > detection depends on the fact that previous vmalloc-ed area is allocated > without VM_NO_GUARD. There isn't many users for this flag so there > would be no problem but just note. Yes, and that's a known weakness. It would be nice to improve it. --Andy -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. 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