On Wed 12-12-18 12:42:49, Kirill A. Shutemov wrote: > On Tue, Dec 11, 2018 at 02:26:45PM +0100, Michal Hocko wrote: > > 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. > > Side node: > > Do we have PG_writeback vs. PG_locked ordering documentated somewhere? > > IIUC, the trace from task2 suggests that we must not wait for writeback > on the locked page. Well, waiting on writeback of page A when A is locked has always been fine. After all that's the only easy way to make sure you really have a page for which no IO is running as page lock protects you from new writeback attempt starting. Waiting on writeback of page B while having page A locked *is* problematic and prone to deadlocks due to code paths like in task2. > But that not what I see for many wait_on_page_writeback() users: it usally > called with the page locked. I see it for truncate, shmem, swapfile, > splice... > > Maybe the problem is within task2 codepath after all? So ->writepages() methods in filesystems have the property that to complete writeback on page with index X, they may need page lock from page X+1. I agree that this is a bit hairy but from fs point of view it makes a lot of sense and AFAIK nothing besides that memcg IO throttling can create deadlocks with such a locking scheme. Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR
