On (Fri) 08 Apr 2011 [09:52:07], Tejun Heo wrote: > Hello, > > On Fri, Apr 08, 2011 at 05:43:16PM +0100, Stefan Hajnoczi wrote: > > >> I think it would make sense to refresh the inode size on media change > > >> so that even open file descriptors see the new size and a single > > >> process cannot force a stale value for all other userspace processes > > >> on the system. > > > > > > Hmmm... I don't know. Maybe we can but I'm not sure whether there's a > > > good reason for it. cdrom is locked while opened after all. Are > > > there actual problems? > > > > Yeah, sorry I didn't explain what the use case was. With QEMU you can > > pass through the physical CD-ROM into the virtual machine. > > > > QEMU opens /dev/cdrom with O_NONBLOCK | O_RDONLY. The guest can test > > if the medium is present and QEMU will do ioctl(fd, > > CDROM_DRIVE_STATUS, CDSL_CURRENT). The guest can also lock the tray > > and eject, again using the respective ioctls. Read operations are > > serviced by performing a read on the file descriptor in QEMU. And > > finally the medium size is queried by QEMU using lseek(fd, 0, > > SEEK_END). > > > > Today QEMU cannot keep /dev/cdrom open across media change because it > > will have an outdated inode size returned from lseek(fd, 0, SEEK_END). > > But if the cdrom driver (or sr) refresh the inode size on media > > change then there is no need to work around this from userspace. > > Hmmm... ISTR there was some discussion about changing inode size on > the fly quite a while ago. I didn't follow the discussion but it > seemed to have rather nasty/delicate implications. I don't necessarily agree with having to modify inode sizes on the fly, but the main bug here is that the inode doesn't get invalidated if a CDROM is ejected while a process has an fd to the CDROM device opened. So as in the original case, if a CD is swapped with one having more data, lseek continues to report the original media's size in the process that keeps the fd open across eject/insert. I haven't tried this on physical systems, so can't count out it being a qemu bug as well. Amit -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
