3.16.80-rc1 review patch. If anyone has any objections, please let me know.
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From: Tejun Heo tj@kernel.org
commit 20eb4f29b60286e0d6dc01d9c260b4bd383c58fb upstream.
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 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.
v2: Eric didn't like gfp flags being tested twice. Introduce a new helper gfpflags_normal_context() and combine the two tests.
Signed-off-by: Tejun Heo tj@kernel.org Cc: Josef Bacik josef@toxicpanda.com Cc: Eric Dumazet eric.dumazet@gmail.com Signed-off-by: David S. Miller davem@davemloft.net [bwh: Backported to 3.16: Keep testing __GFP_WAIT flag instead of __GFP_DIRECT_RECLAIM.] Signed-off-by: Ben Hutchings ben@decadent.org.uk --- --- a/include/linux/gfp.h +++ b/include/linux/gfp.h @@ -168,6 +168,28 @@ static inline int allocflags_to_migratet ((gfp_flags & __GFP_RECLAIMABLE) != 0); }
+/** + * gfpflags_normal_context - is gfp_flags a normal sleepable context? + * @gfp_flags: gfp_flags to test + * + * Test whether @gfp_flags indicates that the allocation is from the + * %current context and allowed to sleep. + * + * An allocation being allowed to block doesn't mean it owns the %current + * context. When direct reclaim path tries to allocate memory, the + * allocation context is nested inside whatever %current was doing at the + * time of the original allocation. The nested allocation may be allowed + * to block but modifying anything %current owns can corrupt the outer + * context's expectations. + * + * %true result from this function indicates that the allocation context + * can sleep and use anything that's associated with %current. + */ +static inline bool gfpflags_normal_context(const gfp_t gfp_flags) +{ + return (gfp_flags & (__GFP_WAIT | __GFP_MEMALLOC)) == __GFP_WAIT; +} + #ifdef CONFIG_HIGHMEM #define OPT_ZONE_HIGHMEM ZONE_HIGHMEM #else --- a/include/net/sock.h +++ b/include/net/sock.h @@ -2103,12 +2103,17 @@ struct sk_buff *sk_stream_alloc_skb(stru * sk_page_frag - return an appropriate page_frag * @sk: socket * - * If socket allocation mode allows current thread to sleep, it means its - * safe to use the per task page_frag instead of the per socket one. + * Use the per task page_frag instead of the per socket one for + * optimization when we know that we're in the normal context and owns + * everything that's associated with %current. + * + * Testing __GFP_WAIT isn't enough here as direct reclaim may nest + * inside other socket operations and end up recursing into sk_page_frag() + * while it's already in use. */ static inline struct page_frag *sk_page_frag(struct sock *sk) { - if (sk->sk_allocation & __GFP_WAIT) + if (gfpflags_normal_context(sk->sk_allocation)) return ¤t->task_frag;
return &sk->sk_frag;