[PATCH] Fix broken zero-copy on vmalloc() buffers (4th and hopefully final submission)

[PATCH] Fix broken zero-copy on vmalloc() buffers (4th and hopefully final submission)

[Richard Yao]

This patch has been submitted for a few times.

The first time was my first time doing any sort of Linux patch
submission. At the time, I was unaware of ./scripts/get_maintainer.pl
and sent the patch to only a subset of the correct people. Consequently,
it was not submitted properly for acceptance by the subsystem maintainer.

The second time was a week ago. I had taken advice from Greg Koah-Hartman to
use ./scripts/get_maintainer.pl to determine the correct recipients. It was
initially accepted by the subsystem maintainer and then rejected. This patch
uses is_vmalloc_or_module_addr(), which is not exported for use in kernel
modules. Using it causes a build failure when CONFIG_NET_9P_VIRTIO=m is set in
.config.

The third time was earlier today, when I sent it straight to Linus Torvalds
because merging it required exporting is_vmalloc_or_module_addr(), which he
wrote. A brief correspondence with Linus revealed that my earlier belief that
it would be better to use is_vmalloc_or_module_addr() instead of
is_vmalloc_addr() was incorrect.

I expect this submission to be the last. I have changed the patch to use
is_vmalloc_addr() as Linus Torvalds suggested. This resolves the build
regression the problem David S. Miller found when CONFIG_NET_9P_VIRTIO=m was
set and should resolve all criticism.

Richard Yao (1):
  9p/trans_virtio.c: Fix broken zero-copy on vmalloc() buffers

 net/9p/trans_virtio.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

-- 
1.8.3.2

[PATCH] 9p/trans_virtio.c: Fix broken zero-copy on vmalloc() buffers

[Richard Yao]

The 9p-virtio transport does zero copy on things larger than 1024 bytes
in size. It accomplishes this by returning the physical addresses of
pages to the virtio-pci device. At present, the translation is usually a
bit shift.

That approach produces an invalid page address when we read/write to
vmalloc buffers, such as those used for Linux kernel modules. Any
attempt to load a Linux kernel module from 9p-virtio produces the
following stack.

[<ffffffff814878ce>] p9_virtio_zc_request+0x45e/0x510
[<ffffffff814814ed>] p9_client_zc_rpc.constprop.16+0xfd/0x4f0
[<ffffffff814839dd>] p9_client_read+0x15d/0x240
[<ffffffff811c8440>] v9fs_fid_readn+0x50/0xa0
[<ffffffff811c84a0>] v9fs_file_readn+0x10/0x20
[<ffffffff811c84e7>] v9fs_file_read+0x37/0x70
[<ffffffff8114e3fb>] vfs_read+0x9b/0x160
[<ffffffff81153571>] kernel_read+0x41/0x60
[<ffffffff810c83ab>] copy_module_from_fd.isra.34+0xfb/0x180

Subsequently, QEMU will die printing:

qemu-system-x86_64: virtio: trying to map MMIO memory

This patch enables 9p-virtio to correctly handle this case. This not
only enables us to load Linux kernel modules off virtfs, but also
enables ZFS file-based vdevs on virtfs to be used without killing QEMU.

Special thanks to both Avi Kivity and Alexander Graf for their
interpretation of QEMU backtraces. Without their guidence, tracking down
this bug would have taken much longer. Also, special thanks to Linus
Torvalds for his insightful explanation of why this should use
is_vmalloc_addr() instead of is_vmalloc_or_module_addr():

https://lkml.org/lkml/2014/2/8/272

Signed-off-by: Richard Yao <ryao@gentoo.org>
---
 net/9p/trans_virtio.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/net/9p/trans_virtio.c b/net/9p/trans_virtio.c
index cd1e1ed..ac2666c 100644
--- a/net/9p/trans_virtio.c
+++ b/net/9p/trans_virtio.c
@@ -340,7 +340,10 @@ static int p9_get_mapped_pages(struct virtio_chan *chan,
 		int count = nr_pages;
 		while (nr_pages) {
 			s = rest_of_page(data);
-			pages[index++] = kmap_to_page(data);
+			if (is_vmalloc_addr(data))
+				pages[index++] = vmalloc_to_page(data);
+			else
+				pages[index++] = kmap_to_page(data);
 			data += s;
 			nr_pages--;
 		}
-- 
1.8.3.2