Re: POSIX violation by writeback error

Jeff Layton
2018-09-05T11:42:55Z

On Wed, 2018-09-05 at 09:37 +0200, Martin Steigerwald wrote:
> Jeff Layton - 04.09.18, 17:44:
> > > - If the following read() could be served by a page in memory, just
> > > returns the data. If the following read() could not be served by a
> > > page in memory and the inode/address_space has a writeback error
> > > mark, returns EIO. If there is a writeback error on the file, and
> > > the request data could not be served
> > > by a page in memory, it means we are reading a (partically)
> > > corrupted
> > > (out-of-data)
> > > file. Receiving an EIO is expected.
> >
> > No, an error on read is not expected there. Consider this:
> >
> > Suppose the backend filesystem (maybe an NFSv3 export) is really r/o,
> > but was mounted r/w. An application queues up a bunch of writes that
> > of course can't be written back (they get EROFS or something when
> > they're flushed back to the server), but that application never calls
> > fsync.
> >
> > A completely unrelated application is running as a user that can open
> > the file for read, but not r/w. It then goes to open and read the file
> > and then gets EIO back or maybe even EROFS.
> >
> > Why should that application (which did zero writes) have any reason to
> > think that the error was due to prior writeback failure by a
> > completely separate process? Does EROFS make sense when you're
> > attempting to do a read anyway?
> >
> > Moreover, what is that application's remedy in this case? It just
> > wants to read the file, but may not be able to even open it for write
> > to issue an fsync to "clear" the error. How do we get things moving
> > again so it can do what it wants?
> >
> > I think your suggestion would open the floodgates for local DoS
> > attacks.
>
> I wonder whether a new error for reporting writeback errors like this
> could help out of the situation. But from all I read here so far, this
> is a really challenging situation to deal with.
>
> I still remember how AmigaOS dealt with this case and from an usability
> point of view it was close to ideal: If a disk was removed, like a
> floppy disk, a network disk provided by Envoy or even a hard disk, it
> pops up a dialog "You MUST insert volume <name of volume> again". And if
> you did, it continued writing. That worked even with networked devices.
> I tested it. I unplugged the ethernet cable and replugged it and it
> continued writing.
>
> I can imagine that this would be quite challenging to implement within
> Linux. I remember there has been a Google Summer of Code project for
> NetBSD at least been offered to implement this, but I never got to know
> whether it was taken or even implemented. If so it might serve as an
> inspiration. Anyway AmigaOS did this even for stationary hard disks. I
> had the issue of a flaky connection through IDE to SCSI and then SCSI to
> UWSCSI adapter. And when the hard disk had connection issues that dialog
> popped up, with the name of the operating system volume for example.
>
> Every access to it was blocked then. It simply blocked all processes
> that accessed it till it became available again (usually I rebooted in
> case of stationary device cause I had to open case or no hot plug
> available or working).
>
> But AFAIR AmigaOS also did not have a notion of caching writes for
> longer than maybe a few seconds or so and I think just within the device
> driver. Writes were (almost) immediate. There have been some
> asynchronous I/O libraries and I would expect an delay in the dialog
> popping up in that case.
>
> It would be challenging to implement for Linux even just for removable
> devices. You have page dirtying and delayed writeback – which is still
> an performance issue with NFS of 1 GBit, rsync from local storage that
> is faster than 1 GBit and huge files, reducing dirty memory ratio may
> help to halve the time needed to complete the rsync copy operation. And
> you would need to communicate all the way to userspace to let the user
> know about the issue.
>

You may be interested in Project Banbury:

http://www.wil.cx/~willy/banbury.html

> Still, at least for removable media, this would be almost the most
> usability friendly approach. With robust filesystems (Amiga Old
> Filesystem and Fast Filesystem was not robust in case of sudden write
> interruption, so the "MUST" was mean that way) one may even offer
> "Please insert device <name of device> again to write out unwritten data
> or choose to discard that data" in a dialog. And for removable media it
> may even work as blocking processes that access it usually would not
> block the whole system. But for the operating system disk? I know how
> Plasma desktop behaves during massive I/O operations. It usually just
> completely stalls to a halt. It seems to me that its processes do some
> I/O almost all of the time … or that the Linux kernel blocks other
> syscalls too during heavy I/O load.
>
> I just liked to mention it as another crazy idea. But I bet it would
> practically need to rewrite the I/O subsystem in Linux to a great
> extent, probably diminishing its performance in situations of write
> pressure. Or maybe a genius finds a way to implement both. :)
>
> What I do think tough is that the dirty page caching of Linux with its
> current standard settings is excessive. 5% / 10% of available memory
> often is a lot these days. There has been a discussion reducing the
> default, but AFAIK it was never done. Linus suggested in that discussion
> to about what the storage can write out in 3 to 5 seconds. That may even
> help with error reporting as reducing dirty memory ratio will reduce the
> memory pressure and so you may choose to add some memory allocations for
> error handling. And the time till you know its not working may be less.
>
--
Jeff Layton <jlayton@redhat.com>