Re: [PATCH] af_packet: flush complete kernel cache in packet_sendmsg

From: Phil Sutter <phil.sutter@viprinet.com>
To: Russell King - ARM Linux <linux@arm.linux.org.uk>
Cc: Ben Hutchings <bhutchings@solarflare.com>,
	netdev@vger.kernel.org, "David S. Miller" <davem@davemloft.net>,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH] af_packet: flush complete kernel cache in packet_sendmsg
Date: Tue, 6 Sep 2011 13:05:03 +0200	[thread overview]
Message-ID: <20110906110503.GE29025@philter> (raw)
In-Reply-To: <20110906095722.GK6619@n2100.arm.linux.org.uk>

On Tue, Sep 06, 2011 at 10:57:22AM +0100, Russell King - ARM Linux wrote:
> > The code in question uses __get_free_pages(), and if that fails uses
> > vmalloc() (see alloc_one_pg_vec_page() for reference). Both code paths
> > show result in the same faulty behaviour.
> 
> So, what you're wanting is cache coherency between vmalloc() and
> userspace.  There is no API in the kernel to do that, and you'll see
> the same failures of this interface not only on ARM but also other
> architectures with virtual caches.
> 
> It sounds like we need an API to flush the cache using both the
> userspace address, plus the kernel side address be that in the direct
> map or the vmalloc map areas.
> 
> Or maybe the right solution is to simply disable AF_PACKET MMAP support
> for virtual cached architectures - it may be that adding cache flushing
> calls makes the thing too expensive and the benefits of mmap over normal
> read/write are lost.

OK, that's horrible. Of course we depend on just this combination to
work flawlessly, i.e. PACKET_MMAP && VIVT. :(

Another userspace-interface I'm working on uses a different solution:
memory is allocated in userspace and accessed from kernelspace using
get_user_pages(). I did not explicitly search for the earlier described
fault pattern, but we didn't notice any problem with this approach on
the very same hardware either. I already see myself writing TPACKET_V3.
;)

What do you think?

Greetings, Phil

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