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

[Andrew Morton <akpm@linux-foundation.org>]

On Thu, 31 Oct 2019 10:35:21 -0700 Shakeel Butt <shakeelb@google.com> wrote:

> +Michal Hocko
> 
> On Thu, Oct 24, 2019 at 1:50 PM Tejun Heo <tj@kernel.org> 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?