Re: [PATCH v2] net: fix sk_page_frag() recursion from memory reclaim

[Shakeel Butt]

+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 &current->task_frag;
>
>         return &sk->sk_frag;