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From: "Ramsay, Lincoln" <Lincoln.Ramsay@digi.com>
To: Igor Russkikh <irusskikh@marvell.com>,
	"David S. Miller" <davem@davemloft.net>,
	Jakub Kicinski <kuba@kernel.org>,
	"netdev@vger.kernel.org" <netdev@vger.kernel.org>,
	Dmitry Bogdanov <dbogdanov@marvell.com>
Subject: Re: [EXT] [PATCH] aquantia: Reserve space when allocating an SKB
Date: Thu, 19 Nov 2020 05:19:56 +0000	[thread overview]
Message-ID: <CY4PR1001MB231125B16A35324A79270373E8E00@CY4PR1001MB2311.namprd10.prod.outlook.com> (raw)
In-Reply-To: <CY4PR1001MB2311C0DA2840AFC20AE6AEB5E8E10@CY4PR1001MB2311.namprd10.prod.outlook.com>

Hi Igor,

> > With your solution, packets of size (AQ_CFG_RX_FRAME_MAX - AQ_SKB_PAD) up to
> > size of AQ_CFG_RX_FRAME_MAX will overwrite the area of page they designated
> > to. Ultimately, HW will do a memory corruption of next page.
> 
> The code in aq_get_rxpages seems to suggest that multiple frames can fit in a rxpage, so
> maybe the logic there prevents overwriting? (at the expense of not fitting as many
> frames into the page before it has to get a new one?)

I am not terribly experienced with such low-level code, but it looks to me looks like a page is allocated (4k) and then DMA mapped to the device. Frames are 2k, so only 2 can fit into a single mapped page. If the mapping was done with an offset of AQ_SKB_PAD, that'd leave space for the SKB headroom but it would mean only a single frame could fit into that mapped page. Since this is the "I only have 1 fragment less than 2k" code path, maybe that's ok? I'm not sure if the hardware side can know that it's only allowed to write 1 frame into the buffer...

I noticed on my device that aq_ring_rx_clean always hits the "fast" codepath. I guess that just means I am not pushing it hard enough?

> > I think the only acceptable solution here would be removing that optimized
> > path of build_skb, and keep only napi_alloc_skb. Or, we can think of keeping
> > it under some configuration condition (which is also not good).
> 
> I'll attempt to confirm that this works too, at least for our tests :)

FWIW: This does work. I notice that this has to copy data into an allocated skb rather than building the skb around the data. I guess avoiding that copy is why the fast path existed in the first place?

Would you like me to post this patch (removing the fast path)?

Lincoln

  reply	other threads:[~2020-11-19  5:20 UTC|newest]

Thread overview: 28+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-11-18  1:52 [PATCH] aquantia: Reserve space when allocating an SKB Ramsay, Lincoln
2020-11-18 14:02 ` [EXT] " Igor Russkikh
2020-11-19  0:14   ` Ramsay, Lincoln
2020-11-19  5:19     ` Ramsay, Lincoln [this message]
2020-11-19 22:01       ` [PATCH] aquantia: Remove the build_skb path Ramsay, Lincoln
2020-11-19 22:07         ` [PATCH v2] " Ramsay, Lincoln
2020-11-19 22:15           ` Florian Westphal
2020-11-19 22:24             ` Ramsay, Lincoln
2020-11-19 22:28               ` Florian Westphal
2020-11-19 22:34                 ` [PATCH v3] " Ramsay, Lincoln
2020-11-19 22:49                   ` Maciej Fijalkowski
2020-11-20  8:18                     ` [EXT] " Igor Russkikh
2020-11-23 19:28                       ` Maciej Fijalkowski
2020-11-24 15:26                         ` Igor Russkikh
2020-11-19 22:58                   ` Florian Westphal
2020-11-19 23:52                     ` [PATCH v4] " Ramsay, Lincoln
2020-11-20  0:17                       ` Florian Westphal
2020-11-20  0:23                         ` Ramsay, Lincoln
2020-11-21 21:22                       ` Jakub Kicinski
2020-11-21 21:23                         ` Jakub Kicinski
2020-11-22 22:36                           ` Ramsay, Lincoln
2020-11-23 16:42                             ` Jakub Kicinski
2020-11-23 21:40                               ` [PATCH net v5] " Ramsay, Lincoln
2020-11-24 19:02                                 ` Jakub Kicinski
2020-11-22 21:55                         ` [PATCH v4] " Ramsay, Lincoln
2020-11-20  7:52         ` [EXT] [PATCH] " Igor Russkikh
2020-11-23  4:20           ` Ramsay, Lincoln
2020-11-24 14:29             ` Igor Russkikh

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