On Thu, 31 Oct 2019 10:35:21 -0700 Shakeel Butt <shakeelb@xxxxxxxxxx> wrote: > +Michal Hocko > > On Thu, Oct 24, 2019 at 1:50 PM Tejun Heo <tj@xxxxxxxxxx> wrote: > > > > sk_page_frag() optimizes skb_frag allocations by using per-task > > skb_frag cache when it knows it's the only user. The condition is > > determined by seeing whether the socket allocation mask allows > > blocking - if the allocation may block, it obviously owns the task's > > context and ergo exclusively owns current->task_frag. > > > > Unfortunately, this misses recursion through memory reclaim path. > > Please take a look at the following backtrace. > > > > [2] RIP: 0010:tcp_sendmsg_locked+0xccf/0xe10 > > ... > > tcp_sendmsg+0x27/0x40 > > sock_sendmsg+0x30/0x40 > > sock_xmit.isra.24+0xa1/0x170 [nbd] > > nbd_send_cmd+0x1d2/0x690 [nbd] > > nbd_queue_rq+0x1b5/0x3b0 [nbd] > > __blk_mq_try_issue_directly+0x108/0x1b0 > > blk_mq_request_issue_directly+0xbd/0xe0 > > blk_mq_try_issue_list_directly+0x41/0xb0 > > blk_mq_sched_insert_requests+0xa2/0xe0 > > blk_mq_flush_plug_list+0x205/0x2a0 > > blk_flush_plug_list+0xc3/0xf0 > > [1] blk_finish_plug+0x21/0x2e > > _xfs_buf_ioapply+0x313/0x460 > > __xfs_buf_submit+0x67/0x220 > > xfs_buf_read_map+0x113/0x1a0 > > xfs_trans_read_buf_map+0xbf/0x330 > > xfs_btree_read_buf_block.constprop.42+0x95/0xd0 > > xfs_btree_lookup_get_block+0x95/0x170 > > xfs_btree_lookup+0xcc/0x470 > > xfs_bmap_del_extent_real+0x254/0x9a0 > > __xfs_bunmapi+0x45c/0xab0 > > xfs_bunmapi+0x15/0x30 > > xfs_itruncate_extents_flags+0xca/0x250 > > xfs_free_eofblocks+0x181/0x1e0 > > xfs_fs_destroy_inode+0xa8/0x1b0 > > destroy_inode+0x38/0x70 > > dispose_list+0x35/0x50 > > prune_icache_sb+0x52/0x70 > > super_cache_scan+0x120/0x1a0 > > do_shrink_slab+0x120/0x290 > > shrink_slab+0x216/0x2b0 > > shrink_node+0x1b6/0x4a0 > > do_try_to_free_pages+0xc6/0x370 > > try_to_free_mem_cgroup_pages+0xe3/0x1e0 > > try_charge+0x29e/0x790 > > mem_cgroup_charge_skmem+0x6a/0x100 > > __sk_mem_raise_allocated+0x18e/0x390 > > __sk_mem_schedule+0x2a/0x40 > > [0] tcp_sendmsg_locked+0x8eb/0xe10 > > tcp_sendmsg+0x27/0x40 > > sock_sendmsg+0x30/0x40 > > ___sys_sendmsg+0x26d/0x2b0 > > __sys_sendmsg+0x57/0xa0 > > do_syscall_64+0x42/0x100 > > entry_SYSCALL_64_after_hwframe+0x44/0xa9 > > > > In [0], tcp_send_msg_locked() was using current->page_frag when it "tcp_sendmsg_locked" and "current->task_frag". Stuff like this makes review harder :( > > called sk_wmem_schedule(). It already calculated how many bytes can > > be fit into current->page_frag. Due to memory pressure, > > sk_wmem_schedule() called into memory reclaim path which called into > > xfs and then IO issue path. Because the filesystem in question is > > backed by nbd, the control goes back into the tcp layer - back into > > tcp_sendmsg_locked(). > > > > nbd sets sk_allocation to (GFP_NOIO | __GFP_MEMALLOC) which makes > > sense - it's in the process of freeing memory and wants to be able to, > > e.g., drop clean pages to make forward progress. However, this > > confused sk_page_frag() called from [2]. Because it only tests > > whether the allocation allows blocking which it does, it now thinks > > current->page_frag can be used again although it already was being > > used in [0]. > > > > After [2] used current->page_frag, the offset would be increased by > > the used amount. When the control returns to [0], > > current->page_frag's offset is increased and the previously calculated > > number of bytes now may overrun the end of allocated memory leading to > > silent memory corruptions. > > > > Fix it by adding gfpflags_normal_context() which tests sleepable && > > !reclaim and use it to determine whether to use current->task_frag. > > Dumb-but-obvious question. Rather than putzing with allocation modes, is it not feasible to change the net layer to copy the current value of current->task_frag into a local then restore its value when it has finished being used?
