linux-kernel - Re: [PATCH 1/3] net

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Date:	Wed, 18 Dec 2013 10:05:51 -0800
From:	Dan Williams <dan.j.williams@...el.com>
To:	netdev@...r.kernel.org
Cc:	Alexander Duyck <alexander.h.duyck@...el.com>,
	Dave Jiang <dave.jiang@...el.com>,
	Vinod Koul <vinod.koul@...el.com>,
	David Whipple <whipple@...uredatainnovations.ch>,
	stable@...r.kernel.org,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"dmaengine@...r.kernel.org" <dmaengine@...r.kernel.org>,
	David Miller <davem@...emloft.net>
Subject: Re: [PATCH 1/3] net_dma: mark broken

[ adding davem directly to cc ]

Dave, looking for an ack to take this through the dmaengine tree, and
wondering if you want me to take patch 2 through dmaengine as well:

http://marc.info/?l=linux-netdev&m=138732577614066&w=2

On Tue, Dec 17, 2013 at 4:15 PM, Dan Williams <dan.j.williams@...el.com> wrote:
> net_dma can cause data to be copied to a stale mapping if a
> copy-on-write fault occurs during dma.  The application sees missing
> data.
>
> The following trace is triggered by modifying the kernel to WARN if it
> ever triggers copy-on-write on a page that is undergoing dma:
>
>  WARNING: CPU: 24 PID: 2529 at lib/dma-debug.c:485 debug_dma_assert_idle+0xd2/0x120()
>  ioatdma 0000:00:04.0: DMA-API: cpu touching an active dma mapped page [pfn=0x16bcd9]
>  Modules linked in: iTCO_wdt iTCO_vendor_support ioatdma lpc_ich pcspkr dca
>  CPU: 24 PID: 2529 Comm: linbug Tainted: G        W    3.13.0-rc1+ #353
>   00000000000001e5 ffff88016f45f688 ffffffff81751041 ffff88017ab0ef70
>   ffff88016f45f6d8 ffff88016f45f6c8 ffffffff8104ed9c ffffffff810f3646
>   ffff8801768f4840 0000000000000282 ffff88016f6cca10 00007fa2bb699349
>  Call Trace:
>   [<ffffffff81751041>] dump_stack+0x46/0x58
>   [<ffffffff8104ed9c>] warn_slowpath_common+0x8c/0xc0
>   [<ffffffff810f3646>] ? ftrace_pid_func+0x26/0x30
>   [<ffffffff8104ee86>] warn_slowpath_fmt+0x46/0x50
>   [<ffffffff8139c062>] debug_dma_assert_idle+0xd2/0x120
>   [<ffffffff81154a40>] do_wp_page+0xd0/0x790
>   [<ffffffff811582ac>] handle_mm_fault+0x51c/0xde0
>   [<ffffffff813830b9>] ? copy_user_enhanced_fast_string+0x9/0x20
>   [<ffffffff8175fc2c>] __do_page_fault+0x19c/0x530
>   [<ffffffff8175c196>] ? _raw_spin_lock_bh+0x16/0x40
>   [<ffffffff810f3539>] ? trace_clock_local+0x9/0x10
>   [<ffffffff810fa1f4>] ? rb_reserve_next_event+0x64/0x310
>   [<ffffffffa0014c00>] ? ioat2_dma_prep_memcpy_lock+0x60/0x130 [ioatdma]
>   [<ffffffff8175ffce>] do_page_fault+0xe/0x10
>   [<ffffffff8175c862>] page_fault+0x22/0x30
>   [<ffffffff81643991>] ? __kfree_skb+0x51/0xd0
>   [<ffffffff813830b9>] ? copy_user_enhanced_fast_string+0x9/0x20
>   [<ffffffff81388ea2>] ? memcpy_toiovec+0x52/0xa0
>   [<ffffffff8164770f>] skb_copy_datagram_iovec+0x5f/0x2a0
>   [<ffffffff8169d0f4>] tcp_rcv_established+0x674/0x7f0
>   [<ffffffff816a68c5>] tcp_v4_do_rcv+0x2e5/0x4a0
>   [..]
>  ---[ end trace e30e3b01191b7617 ]---
>  Mapped at:
>   [<ffffffff8139c169>] debug_dma_map_page+0xb9/0x160
>   [<ffffffff8142bf47>] dma_async_memcpy_pg_to_pg+0x127/0x210
>   [<ffffffff8142cce9>] dma_memcpy_pg_to_iovec+0x119/0x1f0
>   [<ffffffff81669d3c>] dma_skb_copy_datagram_iovec+0x11c/0x2b0
>   [<ffffffff8169d1ca>] tcp_rcv_established+0x74a/0x7f0:
>
> ...the problem is that the receive path falls back to cpu-copy in
> several locations and this trace is just one of the areas.  A few
> options were considered to fix this:
>
> 1/ sync all dma whenever a cpu copy branch is taken
>
> 2/ modify the page fault handler to hold off while dma is in-flight
>
> Option 1 adds yet more cpu overhead to an "offload" that struggles to compete
> with cpu-copy.  Option 2 adds checks for behavior that is already documented as
> broken when using get_user_pages().  At a minimum a debug mode is warranted to
> catch and flag these violations of the dma-api vs get_user_pages().
>
> Thanks to David for his reproducer.
>
> Cc: <stable@...r.kernel.org>
> Cc: Dave Jiang <dave.jiang@...el.com>
> Cc: Vinod Koul <vinod.koul@...el.com>
> Cc: Alexander Duyck <alexander.h.duyck@...el.com>
> Reported-by: David Whipple <whipple@...uredatainnovations.ch>
> Signed-off-by: Dan Williams <dan.j.williams@...el.com>
> ---
>  drivers/dma/Kconfig |    1 +
>  1 files changed, 1 insertions(+), 0 deletions(-)
>
> diff --git a/drivers/dma/Kconfig b/drivers/dma/Kconfig
> index 132a4fd375b2..c823daaf9043 100644
> --- a/drivers/dma/Kconfig
> +++ b/drivers/dma/Kconfig
> @@ -355,6 +355,7 @@ config NET_DMA
>         bool "Network: TCP receive copy offload"
>         depends on DMA_ENGINE && NET
>         default (INTEL_IOATDMA || FSL_DMA)
> +       depends on BROKEN
>         help
>           This enables the use of DMA engines in the network stack to
>           offload receive copy-to-user operations, freeing CPU cycles.
>
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