Comment 65 for bug 2056461

> Can someone ping the mailing list again? I think this issue might have
> dropped off the radar.

I've now subscribed to this bz so no need to ping the list again (and David and Eric have been there from the start and should be getting the mails too... But at this point you won't get more people from the list anyway)

As I said on the list I don't think the fix is appropriate -- and I still haven't had the time to dig into this properly.

My understanding so far is that the guest is reading a file that is being modified from the host, and then for some reason the file size isn't coherent internally and the read falls short despite userspace knowing the file is bigger.
Is that correct?

We don't have cache invalidation in 9p so for cache=loose/fscache we just don't support files being modified from the host and I'd understand this behaviour, but for cache=none (the default you're using here) we should just not limit the read sizes to whatever the inode's i_size is.

So the fix really isn't to do a new stat after short reads, but to ignore the size attribute in the first place.

Since I haven't had time to check the traces properly I also obviously haven't had time to properly try to reproduce either, but given there is no cache invalidation I'd expect this shouldn't be difficult... I'd also expect the fix to not be too hard to do once we've been able to trace where the size gets cuts off, which is probably easy enough with qemu/GDB or for someone familiar with the netfs read helpers.

So, I'm sorry after a first fix has been made -- the analysis up till now already is great help! -- but my free time has gotten very short and I'll try to get to it when possible but if someone can help with a more appropriate fix in that direction it'd be great.
(Kernel work unfortunately isn't easy unless your work pays for it or you have a lot of free time, and both stopped being true for me a while ago...)

At a higher level, cache=loose/fscache could also perhaps use some invalidation logic like NFS has (iirc NFS v3 re-checks the ctime every 60s or so?), but that's not needed here, cache=none (and possibly writeback) should not trust server attributes at all and should not have this problem.