Re: [Qemu-devel] [PATCH 2/2] hw/9pfs: fix alignment issue when host filesystem block size is larger than client msize

[Jevon Qiao]

[Removing ceph-devel alias]

Hi Aneesh,

Any further comment on my reply below?

Thanks,
Jevon
On 19/2/16 16:56, Jevon Qiao wrote:

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