+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
> 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@xxxxxxxxxx>
> Cc: Josef Bacik <josef@xxxxxxxxxxxxxx>
> Cc: Eric Dumazet <eric.dumazet@xxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> ---
> include/linux/gfp.h | 23 +++++++++++++++++++++++
> include/net/sock.h | 11 ++++++++---
> 2 files changed, 31 insertions(+), 3 deletions(-)
>
> diff --git a/include/linux/gfp.h b/include/linux/gfp.h
> index fb07b503dc45..61f2f6ff9467 100644
> --- a/include/linux/gfp.h
> +++ b/include/linux/gfp.h
> @@ -325,6 +325,29 @@ static inline bool gfpflags_allow_blocking(const gfp_t gfp_flags)
> return !!(gfp_flags & __GFP_DIRECT_RECLAIM);
> }
>
> +/**
> + * 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_DIRECT_RECLAIM | __GFP_MEMALLOC)) ==
> + __GFP_DIRECT_RECLAIM;
I think we should be checking PF_MEMALLOC here instead. Something like:
return gfpflags_allow_blocking(gfp_flags) && !(current->flags & PF_MEMALLOC);
In my limited understanding, __GFP_MEMALLOC gives access to reserve
but we have overloaded PF_MEMALLOC to also define the reclaim context.
There are PF_MEMALLOC users which does not use __GFP_MEMALLOC like
iscsi_sw_tcp_pdu_xmit() which can call sock_sendmsg().
> +}
> +
> #ifdef CONFIG_HIGHMEM
> #define OPT_ZONE_HIGHMEM ZONE_HIGHMEM
> #else
> diff --git a/include/net/sock.h b/include/net/sock.h
> index f69b58bff7e5..c31a9ed86d5a 100644
> --- a/include/net/sock.h
> +++ b/include/net/sock.h
> @@ -2242,12 +2242,17 @@ struct sk_buff *sk_stream_alloc_skb(struct sock *sk, int size, gfp_t gfp,
> * 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.
> + *
> + * gfpflags_allow_blocking() 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 (gfpflags_allow_blocking(sk->sk_allocation))
> + if (gfpflags_normal_context(sk->sk_allocation))
> return ¤t->task_frag;
>
> return &sk->sk_frag;
