Re: QEMU dies on any attempt to load a Linux kernel module when using a 9P rootfs

From: Richard Yao <ryao@gentoo.org>
To: Christopher Covington <cov@codeaurora.org>
Cc: kernel@gentoo.org, v9fs-developer@lists.sourceforge.net,
	qemu-devel@nongnu.org, virtualization@lists.linux-foundation.org
Subject: Re: QEMU dies on any attempt to load a Linux kernel module when using a 9P rootfs
Date: Tue, 26 Nov 2013 10:38:04 -0500	[thread overview]
Message-ID: <5294C05C.20802@gentoo.org> (raw)
In-Reply-To: <5294BB66.3060608@codeaurora.org>


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Christopher,

It sounds like you disabled zero-copy entirely, which is not necessary.
As far as I recall, loading kernel modules is the only case in which
valloc() allocated buffers are used. In the worst case, we only need to
disable zero-copy on such buffers. I have been using a small patch to do
precisely that since yesterday. I have attached it to this email since
it sounds like the first version might be helpful to others while I take
the time to explore a few loose ends.

That being said, I would like to investigate a couple of things before I
send either this patch or some variant of it to the appropriate
subsystem maintainer. First, I need to review the valloc() routines to
ensure that range checking against [VMALLOC_START, VMALLOC_END) is the
correct way to identify valloc() generated buffers. Second, I want to
explore the feasibility of a suggestion by Alexander Graf to instead
rework the zero-copy to properly handle valloc() allocated buffers.

Yours truly,
Richard Yao

On 11/26/2013 10:16 AM, Christopher Covington wrote:
> Hi Richard,
> 
> On 11/25/2013 04:50 PM, Richard Yao wrote:
>> I figured out the problem. There is zerocopy IO is being done via DMA to
>> a buffer allocated with valloc(). Right now, I am running a hack-fix
>> locally so I can get some other stuff done first. I will propose a
>> proper fix to the list in a few days.
> 
> I've also encountered this issue on a non-QEMU simulator and have been
> carrying a disable-zero-copy hack for a few months. Let me know if there's
> anything I can help with.
> 
> Christopher
> 

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diff --git a/net/9p/client.c b/net/9p/client.c
index ee8fd6b..0adfcf5 100644
--- a/net/9p/client.c
+++ b/net/9p/client.c
@@ -1557,7 +1557,9 @@ p9_client_read(struct p9_fid *fid, char *data, char __user *udata, u64 offset,
 		rsize = count;
 
 	/* Don't bother zerocopy for small IO (< 1024) */
-	if (clnt->trans_mod->zc_request && rsize > 1024) {
+	if (clnt->trans_mod->zc_request && rsize > 1024 &&
+		!(udata >= (char __user *)VMALLOC_START &&
+			udata < (char __user *)VMALLOC_END)) {
 		char *indata;
 		if (data) {
 			kernel_buf = 1;

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