Am 11.01.2017 um 01:41 hat NeilBrown geschrieben:

On Wed, Jan 11 2017, Kevin Wolf wrote:

quoted

Hi all,

when I mentioned the I/O error handling problem especially with fsync()
we have in QEMU to Christoph Hellwig, he thought it would be great topic
for LSF/MM, so here I am. This came up a few months ago on qemu-devel [1]
and we managed to ignore it for a while, but it's a real and potentially
serious problem, so I think I agree with Christoph that it makes sense
to get it discussed at LSF/MM.


At the heart of it is the semantics of fsync(). A few years ago, fsync()
was fixed to actually flush data to the disk, so we now have a defined
and useful meaning of fsync() as long as all your fsync() calls return
success.

However, as soon as one fsync() call fails, even if the root problem is
solved later (network connection restored, some space freed for thin
provisioned storage, etc.), the state we're in is mostly undefined. As
Ric Wheeler told me back in the qemu-devel discussion, when a writeout
fails, you get an fsync() error returned (once), but the kernel page
cache simply marks the respective page as clean and consequently won't
ever retry the writeout. Instead, it can evict it from the cache even
though it isn't actually consistent with the state on disk, which means
throwing away data that was written by some process.

So if you do another fsync() and it returns success, this doesn't
currently mean that all of the data you wrote is on disk, but if
anything, it's just about the data you wrote after the failed fsync().
This isn't very helpful, to say the least, because you called fsync() in
order to get a consistent state on disk, and you still don't have that.

Essentially this means that once you got a fsync() failure, there is no
hope to recover for the application and it has to stop using the file.

This is not strictly correct.  The application could repeat all the
recent writes.  It might fsync after each write so it can find out
exactly where the problem is.
So it could a a lot of work to recover, but it is not intrinsically
impossible.

You are right, I probably overgeneralised from our situation. qemu
doesn't have the written data any more, and basically duplicating the
page cache by keeping a second copy of all data in qemu until the next
flush isn't really practicable and would both consume a considerable
amount of memory (if we don't add artificial flushes that the guest
didn't request, potentially unbounded) and impact performance because we
wouldn't be zero-copy any more.

So it is not intrinsically impossible, but practically impossible for at
least some applications. As you say, it's probably also too much extra
code to deal with an unlikely corner case for applications where it
would be possible, so it's still unlikely they will do this.

quoted

To give some context about my perspective as the maintainer for the QEMU
block subsystem: QEMU has a mode (which is usually enabled in
production) where I/O failure isn't communicated to the guest, which
would probably offline the filesystem, thinking its hard disk has died,
but instead QEMU pauses the VM and allows the administrator to resume
when the problem has been fixed. Often the problem is only temporary,
e.g. a network hiccup when a disk image is stored on NFS, so this is a
quite helpful approach.

If the disk image is stored over NFS, the write should hang, not cause
an error. (Of course if you mount with '-o soft' you will get an error,
but if you mount with '-o soft', then "you get to keep both halves").

Yes, bad example. (The hanging write is a problem of its own, and I
think one of the reasons why '-o soft' is bad is the behaviour of the
page cache if we let it fail, but while possibly related, it's a
separate problem.)

Is there a more realistic situation where you might get a write error
that might succeed if the write is repeated?

So where we noticed this problem in practice wasn't the kernel page
cache, but the userspace gluster implementation, which exposed a similar
behaviour: It threw away the cache contents on a failed fsync() and the
next fsync() would report success again.

In the following discussion we came to think of the kernel and that the
same problem exists there in theory. This was confirmed by Ric Wheeler
and Rik van Riel, who I trust to have some knowledge about this, and my
own superificial read of some kernel code didn't contradict. Neither did
anyone in this thread disagree, so I assume that the problem does exist
on the page cache level.

Now even if at the moment there were no storage backend where a write
failure can be temporary (which I find hard to believe, but who knows),
a single new driver is enough to expose the problem. Are you confident
enough that no single driver will ever behave this way to make data
integrity depend on the assumption?

Now to answer your question a bit more directly: The other example we
had in mind was ENOSPC in thin provisioned block devices, which can be
fixed by freeing up some space. I also still see potential for such
behaviour in things using the network, but I haven't checked them in
detail.

quoted

When QEMU is told to resume the VM, the request is just resubmitted.
This works fine for read/write, but not so much for fsync, because after
the first failure all bets are off even if a subsequent fsync()
succeeds.

So this is the aspect that directly affects me, even though the problem
is much broader and by far doesn't only affect QEMU.


This leads to a few invidivual points to be discussed:

1. Fix the data corruption problem that follows from the current
   behaviour. Imagine the following scenario:

   Process A writes to some file, calls fsync() and gets a failure. The
   data it wrote is marked clean in the page cache even though it's
   inconsistent with the disk. Process A knows that fsync() fails, so
   maybe it can deal with it, at least by stop using the file.

   Now process B opens the same file, reads the updated data that
   process A wrote, makes some additional changes based on that and
   calls fsync() again.  Now fsync() return success. The data written by
   B is on disk, but the data written by A isn't. Oops, this is data
   corruption, and process B doesn't even know about it because all its
   operations succeeded.

Can that really happen? I would expect the filesystem to call
SetPageError() if there was a write error, then I would expect a read to
report an error for that page if it were still in cache (or maybe flush
it out).  I admit that I haven't traced through the code in detail, but
I did find some examples for SetPageError after a write error.

To be honest, I kept the proposal intentionally on the high-level
userspace API semantics level because I'm not familiar with the
internals. I did have a look and could have been lucky enough to spot
something that contradicts the theoretical considerations (which I
didn't), but by far didn't spend enough time to make the opposite
statement, whether there isn't something that prevents it from
happening. I took Rik's word on this.

Anyway, it would probably be good if someone had a closer look.

quoted

2. Define fsync() semantics that include the state after a failure (this
   probably goes a long way towards fixing 1.).

   The semantics that QEMU uses internally (and which it needs to map)
   is that after a successful flush, all writes to the disk image that
   have successfully completed before the flush was issued are stable on
   disk (no matter whether a previous flush failed).

   A possible adaption to Linux, which considers that unlike QEMU
   images, files can be opened more than once, might be that a
   succeeding fsync() on a file descriptor means that all data that has
   been read or written through this file descriptor is consistent
   between the page cache and the disk (the read part is for avoiding
   the scenario from 1.; it means that fsync flushes data written on a
   different file descriptor if it has been seen by this one; hence, the
   page cache can't contain non-dirty pages which aren't consistent with
   the disk).

I think it would be useful to try to describe the behaviour of page
flags, particularly PG_error PG_uptodate PG_dirty in the different
scenarios.

For example, a successful read sets PG_uptodate and a successful write
clears PG_dirty.
A failed read doesn't set PG_uptodate, and maybe sets PG_error.
A failed read probably shouldn't clear PG_dirty but should set PG_error.

If background-write finds a PG_dirty|PG_error page, should it try to
write it out again?  Or should only a foreground (fsync) write?

That's a good question. I think a background write (if that includes
anything not coming from userspace) needs to be able to retry writing
out pages at least sometimes, specifically as the final attempt when we
need the memory and are about to throw the data away for good.

If we did this, PG_error|PG_dirty pages would be pinned in memory until
a write was successful.  We would need a way to purge these pages
without writing them.  We would also need a way to ensure they didn't
consume a large fraction of memory.

Yes, at some point throwing them away is unavoidable. If we do, a good
fsync() behaviour is important to communicate this to userspace.

It isn't clear to me that the behaviour can be different for different
file descriptors.  Once the data has been written to the page cache, it
belongs to the file, not to any particular fd.  So enabling
"keep-data-after-write-error" would need to be per-file rather than
per-fd, and would probably need to be a privileged operations due to the
memory consumption concerns.

Note that I didn't think of a "keep-data-after-write-error" flag,
neither per-fd nor per-file, because I assumed that everyone would want
it as long as there is some hope that the data could still be
successfully written out later.

The per-fd thing I envisioned was a flag that basically tells "this fd
has gone bad, fsync() won't ever return success for it again" and that
would be set for all open file descriptors for a file when we release
PG_error|PG_dirty pages in it without having written them.

I had assumed that there is a way to get back from the file to all file
descriptors that are open for it, but looking at the code I don't see
one indeed. Is this an intentional design decision or is it just that
nobody needed it?

You could still mark the whole file as "gone bad", but then this would
also affect new file descriptors that never saw the content that we
threw away. If I understand correctly, you would have to close all file
descriptors on the file first to get rid of the "gone bad" flag (is this
enough or are files kept around for longer than their fds?), and only
then you could get a working new one again. This sounds a bit too heavy
to me.

quoted

3. Actually make fsync() failure recoverable.

   You can implement 2. by making sure that a file descriptor for which
   pages have been thrown away always returns an error and never goes
   back to suceeding (it can't succeed according to the definition of 2.
   because the data that would have to be written out is gone). This is
   already a much better interface, but it doesn't really solve the
   actual problem we have.

   We also need to make sure that after a failed fsync() there is a
   chance to recover. This means that the pages shouldn't be thrown away
   immediately; but at the same time, you probably also don't want to
   keep pages indefinitely when there is a permanent writeout error.
   However, if we can make sure that these pages are only evicted in
   case of actual memory pressure, and only if there are no actually
   clean page to evict, I think a lot would be already won.

I think this would make behaviour unpredictable, being dependent on how
much memory pressure there is.  Predictability is nice!

Yes, predictability is nice. Recovering from errors and not losing data
is nice, too. I think I would generally value the latter higher, but I
see that there may be cases where a different tradeoff might make sense.
A sign that it should be an option?

On the other hand, I wouldn't really consider page cache writeouts
particularly predictable for userspace anyway.

quoted

   In the common case, you could then recover from a temporary failure,
   but if this state isn't maintainable, at least we get consistent
   fsync() failure telling us that the data is gone.


I think I've summarised most aspects here, but if something is unclear
or you'd like to see some more context, please refer to the qemu-devel
discussion [1] that I mentioned, or feel free to just ask.

Definitely an interesting question!

Thanks,
NeilBrown

Kevin