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Re: [Qemu-devel] [PATCH 2/2] hw/9pfs: fix alignment issue when host file

From: Jevon Qiao
Subject: Re: [Qemu-devel] [PATCH 2/2] hw/9pfs: fix alignment issue when host filesystem block size is larger than client msize
Date: Fri, 19 Feb 2016 16:56:00 +0800
User-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.10; rv:38.0) Gecko/20100101 Thunderbird/38.5.1

Hi Aneesh,
I am not sure I understand the details correctly. iounit is the size
that we use in client_read to determine the  size in which
we should request I/O from the client. But we still can't do I/O in size
larger than s->msize. If you look at the client side (kernel 9p fs), you
will find

	rsize = fid->iounit;
	if (!rsize || rsize > clnt->msize-P9_IOHDRSZ)
		rsize = clnt->msize - P9_IOHDRSZ;
Yes, I know this.
if your iounit calculation ends up zero, that should be handled
correctly by

    if (!iounit) {
        iounit = s->msize - P9_IOHDRSZ;
    return iounit;

So what is the issue here. ?
This will result in an alignment issue while mapping the I/O requested by
client into pages in the function of p9_nr_pages().
int p9_nr_pages(char *data, int len)
        unsigned long start_page, end_page;
        start_page =  (unsigned long)data >> PAGE_SHIFT;
        end_page = ((unsigned long)data + len + PAGE_SIZE - 1) >> PAGE_SHIFT;
        return end_page - start_page;
Please see the following experiment I did without the fix.

1) Start qemu with cephfs,
$ qemu-system-x86_64 /root/CentOS---6.6-64bit---2015-03-06-a.qcow2 -smp 4 -m 4096 -fsdev cephfs,security_model=passthrough,id=fsdev0,path=/ -device virtio-9p-pci,id=fs0,fsdev=fsdev0,mount_tag=cephfs --enable-kvm -nographic -net nic -net tap,ifname=tap0,script=no,downscript=no

2) Mount the fs in the guest.
address@hidden ~]# mount -t 9p -o trans=virtio,version=9p2000.L cephfs /mnt
address@hidden ~]# ls -lah /mnt/8kfile
-rw-r--r-- 1 root root 8.0K 2016-02-19 09:37 /mnt/8kfile
In this case, I used the default msize which is 8192(in Byte). Since cephfs
is using 4M as the f_bsize, the iounit will be 8168 as P9_IOHDRSZ is
equal to 24.

3) Run the following systemtap script to trace the paging result,
address@hidden ~]# cat p9_read.stp
probe kernel.function("p9_virtio_zc_request").call
    printf("p9_virtio_zc_request: inlen size is %d\n", int_arg(5));

probe kernel.function("p9_nr_pages").call
    printf("p9_nr_pages: start_page = %ld\n", int_arg(1) >> 12);
    printf("p9_nr_pages: end_age = %ld\n", (int_arg(1) + 8168 + 4096 -1) >> 12);
4) The output I got when I copied out the file /mnt/8kfile to /tmp/ directory,
p9_virtio_zc_request: inlen size is 8168
p9_nr_pages: start_page = 34293757815
p9_nr_pages: end_age = 34293757818

Per the text in red(start_page = 34293757815, end_page = 34293757818),
it turns out 8k data will be mapped into three pages. This could hurt the

Actually, I enabled the cephfs debug functionality added by me to see
how the data is distributed in this case, the result is as follows,
CEPHFS_DEBUG: cephfs_preadv iov_len=4096
CEPHFS_DEBUG: cephfs_preadv iov_len=4072
CEPHFS_DEBUG: cephfs_preadv iov_len=24
This patch aims to fix this. And the result turns out it works quite well, all the
data is well aligned.
p9_virtio_zc_request: inlen size is 4096
p9_nr_pages: start_page = 34203171814
p9_nr_pages: end_age = 34203171815
p9_virtio_zc_request: inlen size is 4096
p9_nr_pages: start_page = 34203171815
p9_nr_pages: end_age = 34203171816

CEPHFS_DEBUG: cephfs_preadv iov_len=4096
CEPHFS_DEBUG: cephfs_preadv iov_len=4096


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