On Tuesday 14 March 2006 22:42, Alan Stern wrote: > On Tue, 14 Mar 2006, Pavel Machek wrote: > > > > > Even if you make sure *kernel* is consistent with changed filesystem, > > > > userland is going to be badly confused. Imagine what will happen with > > > > memory mapped files, for example. > > > > > > Something like what happens when you suspend with a mounted CD > > > and mmapped files from there, then you replace the CD while suspended > > > and resume. Not a wise thing to do, but I think people will do such things > > > from time to time and we'd better be prepared to handle them nicely. > > > > But... we can't prevent userspace from segfaulting in such case... can > > we? > > > > > > I'm not sure how it could work... > > > > > > IMO memory mapped files are the most difficult problem here, > > > but the rest seems to be doable in general. > > > > It seems to be the same problem with removable media... You could > > replace mounted CDrom while running. (Not all CDroms support door > > lock.) > > In theory, any SCSI device is supposed to realize when the medium has been > changed and notify the host by responding to the next command with a Unit > Attention sense key. That's how medium changes while the system _isn't_ > suspended get detected. > > The situation becomes much more problematic when the SCSI device itself is > powered off. In that case (which includes the "hardware doesn't support > USB power during suspend" problem discussed at great length earlier), the > kernel has only two choices. Either assume that the medium _has_ been > changed -- or more generally, that a new device is present -- or else try > to verify somehow that the current medium is the same as the old medium. > > This verification can't be certain, so there would always be a chance for > error. Besides, right now the kernel contains no code for making such > verifications. The question here, as far as I understand it, is whether we can write such a code and, if so, whether it's worth doing. I'm not sure of any of these now, but the problem is interesting. :-) Greetings, Rafael
