Re: [Qemu-block] [PATCH v7 12/35] util: let qemu_fd_getlength support block device

[Eduardo Habkost]

On Mon, Nov 09, 2015 at 01:58:27PM +0800, Xiao Guangrong wrote:
> 
> 
> On 11/06/2015 11:54 PM, Eduardo Habkost wrote:
> >On Mon, Nov 02, 2015 at 05:13:14PM +0800, Xiao Guangrong wrote:
> >>lseek can not work for all block devices as the man page says:
> >>| Some devices are incapable of seeking and POSIX does not specify
> >>| which devices must support lseek().
> >>
> >>This patch tries to add the support on Linux by using BLKGETSIZE64
> >>ioctl
> >>
> >>Signed-off-by: Xiao Guangrong <address@hidden>
> >
> >On which cases is this patch necessary? Do you know any examples of
> >Linux block devices that won't work with lseek(SEEK_END)?
> 
> To be honest, i have not checked all block device, this patch was made
> based on the man page. However, i do not mind drop this patch (and maybe
> other patches) to make this pachset smaller. BLKGETSIZE64 can be added
> in the future if we meet such device.

By looking at the Linux source code implementing BLKGETSIZE64, it looks
like it should work for all block devices where SEEK_END works:

* BLKGETSIZE64 returns i_size_read(bdev->bd_inode)
  (block/ioctl.c:blkdev_ioctl())
* llseek(SEEK_END) uses i_size_read(bd_inode) as the offset
  (fs/block_dev.c:block_llseek())

That's probably why raw_getlength() never needed a Linux-specific
BLKGETSIZE call.

-- 
Eduardo