Subject: Re: [PATCH iwl-net] ice: recycle/free all of the fragments from multi-buffer packet

From: Simon Horman <[email-masked]>
Date: Fri, 12 May 2023 17:37:39 +0200

On Fri, May 12, 2023 at 03:23:30PM +0200, Maciej Fijalkowski wrote:
> The ice driver caches next_to_clean value at the beginning of
> ice_clean_rx_irq() in order to remember the first buffer that has to be
> freed/recycled after main Rx processing loop. The end is indicated by
> first descriptor of packet that the Rx processing loop has ended. Note
> that if mentioned loop ended in the middle of gathering a multi-buffer
> packet, next_to_clean would be pointing to the descriptor in the middle
> of the packet BUT freeing/recycling stage will stop at the first
> descriptor. This means that next iteration of ice_clean_rx_irq() will
> miss the (first_desc, next_to_clean - 1) entries.
> 
>  When running various 9K MTU workloads, such splats were observed,
> mostly related to rx_buf->page being NULL as behavior described in the
> paragraph above breaks ICE_RX_DESC_UNUSED() macro which is used when
> refilling Rx buffers:
> 
> [  540.780716] BUG: kernel NULL pointer dereference, address: 0000000000000000
> [  540.787787] #PF: supervisor read access in kernel mode
> [  540.793002] #PF: error_code(0x0000) - not-present page
> [  540.798218] PGD 0 P4D 0
> [  540.800801] Oops: 0000 [#1] PREEMPT SMP NOPTI
> [  540.805231] CPU: 18 PID: 3984 Comm: xskxceiver Tainted: G        W          6.3.0-rc7+ #96
> [  540.813619] Hardware name: Intel Corporation S2600WFT/S2600WFT, BIOS SE5C620.86B.02.01.0008.031920191559 03/19/2019
> [  540.824209] RIP: 0010:ice_clean_rx_irq+0x2b6/0xf00 [ice]
> [  540.829678] Code: 74 24 10 e9 aa 00 00 00 8b 55 78 41 31 57 10 41 09 c4 4d 85 ff 0f 84 83 00 00 00 49 8b 57 08 41 8b 4f 1c 65 8b 35 1a fa 4b 3f <48> 8b 02 48 c1 e8 3a 39 c6 0f 85 a2 00 00 00 f6 42 08 02 0f 85 98
> [  540.848717] RSP: 0018:ffffc9000f42fc50 EFLAGS: 00010282
> [  540.854029] RAX: 0000000000000004 RBX: 0000000000000002 RCX: 000000000000fffe
> [  540.861272] RDX: 0000000000000000 RSI: 0000000000000001 RDI: 00000000ffffffff
> [  540.868519] RBP: ffff88984a05ac00 R08: 0000000000000000 R09: dead000000000100
> [  540.875760] R10: ffff88983fffcd00 R11: 000000000010f2b8 R12: 0000000000000004
> [  540.883008] R13: 0000000000000003 R14: 0000000000000800 R15: ffff889847a10040
> [  540.890253] FS:  00007f6ddf7fe640(0000) GS:ffff88afdf800000(0000) knlGS:0000000000000000
> [  540.898465] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [  540.904299] CR2: 0000000000000000 CR3: 000000010d3da001 CR4: 00000000007706e0
> [  540.911542] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> [  540.918789] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> [  540.926032] PKRU: 55555554
> [  540.928790] Call Trace:
> [  540.931276]  <TASK>
> [  540.933418]  ice_napi_poll+0x4ca/0x6d0 [ice]
> [  540.937804]  ? __pfx_ice_napi_poll+0x10/0x10 [ice]
> [  540.942716]  napi_busy_loop+0xd7/0x320
> [  540.946537]  xsk_recvmsg+0x143/0x170
> [  540.950178]  sock_recvmsg+0x99/0xa0
> [  540.953729]  __sys_recvfrom+0xa8/0x120
> [  540.957543]  ? do_futex+0xbd/0x1d0
> [  540.961008]  ? __x64_sys_futex+0x73/0x1d0
> [  540.965083]  __x64_sys_recvfrom+0x20/0x30
> [  540.969155]  do_syscall_64+0x38/0x90
> [  540.972796]  entry_SYSCALL_64_after_hwframe+0x72/0xdc
> [  540.977934] RIP: 0033:0x7f6de5f27934
> 
> To fix this, compare next_to_clean with first_desc at the beginning of
> ice_clean_rx_irq(). In the case they are not the same, set cached_ntc to
> first_desc so that at the end, when freeing/recycling buffers,
> descriptors from first to ntc are not missed.
> 
> Fixes: 2fba7dc5157b ("ice: Add support for XDP multi-buffer on Rx side")
> Signed-off-by: Maciej Fijalkowski <[email-masked]>
> ---
>  drivers/net/ethernet/intel/ice/ice_txrx.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/drivers/net/ethernet/intel/ice/ice_txrx.c b/drivers/net/ethernet/intel/ice/ice_txrx.c
> index 4fcf2d07eb85..4d1fc047767f 100644
> --- a/drivers/net/ethernet/intel/ice/ice_txrx.c
> +++ b/drivers/net/ethernet/intel/ice/ice_txrx.c
> @@ -1162,6 +1162,9 @@ int ice_clean_rx_irq(struct ice_rx_ring *rx_ring, int budget)
>  	bool failure;
>  	u32 first;
>  
> +	if (ntc != rx_ring->first_desc)
> +		cached_ntc = rx_ring->first_desc;
> +

Above ntc is initialised as rx_ring->next_to_clean.
And cached_ntc is initialised as ntc, i.e. rx_ring->next_to_clean.

	u32 ntc = rx_ring->next_to_clean;
        ...
        u32 cached_ntc = ntc;

So in effect we have:

	if (rx_ring->next_to_clean != rx_ring->first_desc)
		cached_ntc = rx_ring->first_desc;
	else
		cached_ntc = rx_ring->next_to_clean;

I wonder if we can simplify this by simply initialising cached_ntc as:

	cached_ntc = rx_ring->first_desc;