Subject: Re: [PATCH RFC net-next v1 3/5] net: add get_netmem/put_netmem support

From: Stanislav Fomichev <[email-masked]>
Date: Thu, 26 Dec 2024 11:07:01 -0800

On 12/21, Mina Almasry wrote:
> Currently net_iovs support only pp ref counts, and do not support a
> page ref equivalent.
> 
> This is fine for the RX path as net_iovs are used exclusively with the
> pp and only pp refcounting is needed there. The TX path however does not
> use pp ref counts, thus, support for get_page/put_page equivalent is
> needed for netmem.
> 
> Support get_netmem/put_netmem. Check the type of the netmem before
> passing it to page or net_iov specific code to obtain a page ref
> equivalent.
> 
> For dmabuf net_iovs, we obtain a ref on the underlying binding. This
> ensures the entire binding doesn't disappear until all the net_iovs have
> been put_netmem'ed. We do not need to track the refcount of individual
> dmabuf net_iovs as we don't allocate/free them from a pool similar to
> what the buddy allocator does for pages.
> 
> This code is written to be extensible by other net_iov implementers.
> get_netmem/put_netmem will check the type of the netmem and route it to
> the correct helper:
> 
> pages -> [get|put]_page()
> dmabuf net_iovs -> net_devmem_[get|put]_net_iov()
> new net_iovs ->	new helpers
> 
> Signed-off-by: Mina Almasry <[email-masked]>
> 
> ---
>  include/linux/skbuff_ref.h |  4 ++--
>  include/net/netmem.h       |  3 +++
>  net/core/devmem.c          | 10 ++++++++++
>  net/core/devmem.h          | 11 +++++++++++
>  net/core/skbuff.c          | 30 ++++++++++++++++++++++++++++++
>  5 files changed, 56 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/skbuff_ref.h b/include/linux/skbuff_ref.h
> index 0f3c58007488..9e49372ef1a0 100644
> --- a/include/linux/skbuff_ref.h
> +++ b/include/linux/skbuff_ref.h
> @@ -17,7 +17,7 @@
>   */
>  static inline void __skb_frag_ref(skb_frag_t *frag)
>  {
> -	get_page(skb_frag_page(frag));
> +	get_netmem(skb_frag_netmem(frag));
>  }
>  
>  /**
> @@ -40,7 +40,7 @@ static inline void skb_page_unref(netmem_ref netmem, bool recycle)
>  	if (recycle && napi_pp_put_page(netmem))
>  		return;
>  #endif

[..]

> -	put_page(netmem_to_page(netmem));
> +	put_netmem(netmem);

I moved the release operation onto a workqueue in my series [1] to avoid
calling dmabuf detach (which can sleep) from the socket close path
(which is called with bh disabled). You should probably do something similar,
see the trace attached below.

1: https://github.com/fomichev/linux/commit/3b3ad4f36771a376c204727e5a167c4993d4c65a#diff-3c58b866674b2f9beb5ac7349f81566e4df595c25c647710203549589d450f2dR436

(the condition to trigger that is to have an skb in the write queue
and call close from the userspace)

[    1.548495] BUG: sleeping function called from invalid context at drivers/dma-buf/dma-buf.c:1255
[    1.548741] in_atomic(): 1, irqs_disabled(): 0, non_block: 0, pid: 149, name: ncdevmem
[    1.548926] preempt_count: 201, expected: 0
[    1.549026] RCU nest depth: 0, expected: 0
[    1.549197]
[    1.549237] =============================
[    1.549331] [ BUG: Invalid wait context ]
[    1.549425] 6.13.0-rc3-00770-gbc9ef9606dc9-dirty #15 Tainted: G        W
[    1.549609] -----------------------------
[    1.549704] ncdevmem/149 is trying to lock:
[    1.549801] ffff8880066701c0 (reservation_ww_class_mutex){+.+.}-{4:4}, at: dma_buf_unmap_attachment_unlocked+0x4b/0x90
[    1.550051] other info that might help us debug this:
[    1.550167] context-{5:5}
[    1.550229] 3 locks held by ncdevmem/149:
[    1.550322]  #0: ffff888005730208 (&sb->s_type->i_mutex_key#11){+.+.}-{4:4}, at: sock_close+0x40/0xf0
[    1.550530]  #1: ffff88800b148f98 (sk_lock-AF_INET6){+.+.}-{0:0}, at: tcp_close+0x19/0x80
[    1.550731]  #2: ffff88800b148f18 (slock-AF_INET6){+.-.}-{3:3}, at: __tcp_close+0x185/0x4b0
[    1.550921] stack backtrace:
[    1.550990] CPU: 0 UID: 0 PID: 149 Comm: ncdevmem Tainted: G        W          6.13.0-rc3-00770-gbc9ef9606dc9-dirty #15
[    1.551233] Tainted: [W]=WARN
[    1.551304] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Arch Linux 1.16.3-1-1 04/01/2014
[    1.551518] Call Trace:
[    1.551584]  <TASK>
[    1.551636]  dump_stack_lvl+0x86/0xc0
[    1.551723]  __lock_acquire+0xb0f/0xc30
[    1.551814]  ? dma_buf_unmap_attachment_unlocked+0x4b/0x90
[    1.551941]  lock_acquire+0xf1/0x2a0
[    1.552026]  ? dma_buf_unmap_attachment_unlocked+0x4b/0x90
[    1.552152]  ? dma_buf_unmap_attachment_unlocked+0x4b/0x90
[    1.552281]  ? dma_buf_unmap_attachment_unlocked+0x4b/0x90
[    1.552408]  __ww_mutex_lock+0x121/0x1060
[    1.552503]  ? dma_buf_unmap_attachment_unlocked+0x4b/0x90
[    1.552648]  ww_mutex_lock+0x3d/0xa0
[    1.552733]  dma_buf_unmap_attachment_unlocked+0x4b/0x90
[    1.552857]  __net_devmem_dmabuf_binding_free+0x56/0xb0
[    1.552979]  skb_release_data+0x120/0x1f0
[    1.553074]  __kfree_skb+0x29/0xa0
[    1.553156]  tcp_write_queue_purge+0x41/0x310
[    1.553259]  tcp_v4_destroy_sock+0x127/0x320
[    1.553363]  ? __tcp_close+0x169/0x4b0
[    1.553452]  inet_csk_destroy_sock+0x53/0x130
[    1.553560]  __tcp_close+0x421/0x4b0
[    1.553646]  tcp_close+0x24/0x80
[    1.553724]  inet_release+0x5d/0x90
[    1.553806]  sock_close+0x4a/0xf0
[    1.553886]  __fput+0x9c/0x2b0
[    1.553960]  task_work_run+0x89/0xc0
[    1.554046]  do_exit+0x27f/0x980
[    1.554125]  do_group_exit+0xa4/0xb0
[    1.554211]  __x64_sys_exit_group+0x17/0x20
[    1.554309]  x64_sys_call+0x21a0/0x21a0
[    1.554400]  do_syscall_64+0xec/0x1d0
[    1.554487]  ? exc_page_fault+0x8a/0xf0
[    1.554585]  entry_SYSCALL_64_after_hwframe+0x77/0x7f
[    1.554703] RIP: 0033:0x7f2f8a27abcd