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

[Jevon Qiao <scaleqiao@gmail.com>]

[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
>>
>
>