From: Michal Hocko <mhocko@xxxxxxxx> Liu Bo has experienced a deadlock between memcg (legacy) reclaim and the ext4 writeback task1: [<ffffffff811aaa52>] wait_on_page_bit+0x82/0xa0 [<ffffffff811c5777>] shrink_page_list+0x907/0x960 [<ffffffff811c6027>] shrink_inactive_list+0x2c7/0x680 [<ffffffff811c6ba4>] shrink_node_memcg+0x404/0x830 [<ffffffff811c70a8>] shrink_node+0xd8/0x300 [<ffffffff811c73dd>] do_try_to_free_pages+0x10d/0x330 [<ffffffff811c7865>] try_to_free_mem_cgroup_pages+0xd5/0x1b0 [<ffffffff8122df2d>] try_charge+0x14d/0x720 [<ffffffff812320cc>] memcg_kmem_charge_memcg+0x3c/0xa0 [<ffffffff812321ae>] memcg_kmem_charge+0x7e/0xd0 [<ffffffff811b68a8>] __alloc_pages_nodemask+0x178/0x260 [<ffffffff8120bff5>] alloc_pages_current+0x95/0x140 [<ffffffff81074247>] pte_alloc_one+0x17/0x40 [<ffffffff811e34de>] __pte_alloc+0x1e/0x110 [<ffffffffa06739de>] alloc_set_pte+0x5fe/0xc20 [<ffffffff811e5d93>] do_fault+0x103/0x970 [<ffffffff811e6e5e>] handle_mm_fault+0x61e/0xd10 [<ffffffff8106ea02>] __do_page_fault+0x252/0x4d0 [<ffffffff8106ecb0>] do_page_fault+0x30/0x80 [<ffffffff8171bce8>] page_fault+0x28/0x30 [<ffffffffffffffff>] 0xffffffffffffffff task2: [<ffffffff811aadc6>] __lock_page+0x86/0xa0 [<ffffffffa02f1e47>] mpage_prepare_extent_to_map+0x2e7/0x310 [ext4] [<ffffffffa08a2689>] ext4_writepages+0x479/0xd60 [<ffffffff811bbede>] do_writepages+0x1e/0x30 [<ffffffff812725e5>] __writeback_single_inode+0x45/0x320 [<ffffffff81272de2>] writeback_sb_inodes+0x272/0x600 [<ffffffff81273202>] __writeback_inodes_wb+0x92/0xc0 [<ffffffff81273568>] wb_writeback+0x268/0x300 [<ffffffff81273d24>] wb_workfn+0xb4/0x390 [<ffffffff810a2f19>] process_one_work+0x189/0x420 [<ffffffff810a31fe>] worker_thread+0x4e/0x4b0 [<ffffffff810a9786>] kthread+0xe6/0x100 [<ffffffff8171a9a1>] ret_from_fork+0x41/0x50 [<ffffffffffffffff>] 0xffffffffffffffff He adds : task1 is waiting for the PageWriteback bit of the page that task2 has : collected in mpd->io_submit->io_bio, and tasks2 is waiting for the LOCKED : bit the page which tasks1 has locked. More precisely task1 is handling a page fault and it has a page locked while it charges a new page table to a memcg. That in turn hits a memory limit reclaim and the memcg reclaim for legacy controller is waiting on the writeback but that is never going to finish because the writeback itself is waiting for the page locked in the #PF path. So this is essentially ABBA deadlock. Waiting for the writeback in legacy memcg controller is a workaround for pre-mature OOM killer invocations because there is no dirty IO throttling available for the controller. There is no easy way around that unfortunately. Therefore fix this specific issue by pre-allocating the page table outside of the page lock. We have that handy infrastructure for that already so simply reuse the fault-around pattern which already does this. Reported-and-Debugged-by: Liu Bo <bo.liu@xxxxxxxxxxxxxxxxx> Signed-off-by: Michal Hocko <mhocko@xxxxxxxx> --- Hi, this has been originally reported here [1]. While it could get worked around in the fs, catching the allocation early sounds like a preferable approach. Liu Bo has noted that he is not able to reproduce this anymore because kmem accounting has been disabled in their workload but this should be quite straightforward to review. There are probably other hidden __GFP_ACCOUNT | GFP_KERNEL allocations from under a fs page locked but they should be really rare. I am not aware of a better solution unfortunately. I would appreciate if Kirril could have a look and double check I am not doing something stupid here. Debugging lock_page deadlocks is an absolute PITA considering a lack of lockdep support so I would mark it for stable. [1] http://lkml.kernel.org/r/1540858969-75803-1-git-send-email-bo.liu@xxxxxxxxxxxxxxxxx mm/memory.c | 11 +++++++++++ 1 file changed, 11 insertions(+) diff --git a/mm/memory.c b/mm/memory.c index 4ad2d293ddc2..1a73d2d4659e 100644 --- a/mm/memory.c +++ b/mm/memory.c @@ -2993,6 +2993,17 @@ static vm_fault_t __do_fault(struct vm_fault *vmf) struct vm_area_struct *vma = vmf->vma; vm_fault_t ret; + /* + * Preallocate pte before we take page_lock because this might lead to + * deadlocks for memcg reclaim which waits for pages under writeback. + */ + if (pmd_none(*vmf->pmd) && !vmf->prealloc_pte) { + vmf->prealloc_pte = pte_alloc_one(vmf->vma->vm>mm, vmf->address); + if (!vmf->prealloc_pte) + return VM_FAULT_OOM; + smp_wmb(); /* See comment in __pte_alloc() */ + } + ret = vma->vm_ops->fault(vmf); if (unlikely(ret & (VM_FAULT_ERROR | VM_FAULT_NOPAGE | VM_FAULT_RETRY | VM_FAULT_DONE_COW))) -- 2.19.2
