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.

    [root@localhost ~]# mount -t 9p -o trans=virtio,version=9p2000.L
    cephfs /mnt
    [root@localhost ~]# 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,

    [root@localhost ~]# 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
performance.

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

Thanks,
Jevon
> -aneesh
>