Re: [RFC PATCH] 9p: forbid use of mempool for TFLUSH

Dominique Martinet
2022-07-13T08:22:47Z

Kent Overstreet wrote on Wed, Jul 13, 2022 at 03:40:10AM -0400:
>> I don't think it really matters either way -- I'm much more worried
>> about the two points I gave in the commit comment section: mempools not
>> being shared leading to increased memory usage when many mostly-idle
>> mounts (I know users who need that), and more importantly that the
>> mempool waiting is uninterruptible/non-failible might be "nice" from the
>> using mempool side but I'd really prefer users to be able to ^C out of
>> a mount made on a bad server getting stuck in mempool_alloc at least.
>
> We should never get stuck allocating memory - if that happens, game over,
> system can no longer make forward progress.

Well, that boils down to how much you want to trust your server.
The qemu server has been working rather well but I just pulled out
nfs-ganesha back from its grave for trans_fd tests earlier and got it to
crash on fsstress in 5 minutes, because upstream hasn't had anyone
working on the 9p side of things lately.

In that particular case the socket closes and client gets out of
requests, but if the server just stops replying you'll get requests
stuck as 9p cannot get out of flushes -- another long standing bug, this
one is even worse as it's unkillable.
And as long as that waits it won't free up buffers.

Even if that gets fixed, the buffers won't be freed until the server
replies, which can be arbitrarily slow -- ^C on a process that waits
for a free buffer must work.

> > It looked good before I realized all the ways this could hang, but now I
> > just think for something like 9p it makes more sense to fail the
> > allocation and the IO than to bounce forever trying to allocate memory
> > we don't have.
>
> A filesystem that randomly fails IOs is, fundamentally, not a filesystem
> that _works_. This whole thing started because 9pfs failing IOs has been
> breaking my xfstests runs - and 9pfs isn't the thing I'm trying to test!

I can agree with that and I probably wasn't clear, what I'm talking
about here is interruptability.
If all works well I agree some amount of waiting makes sense, but for
networked stuff some fallback also is important.
(if I could be greedy, some timeout would also make sense to me, but we
can agree to disagree on this one and I'll live without one)

> Local filesystems and the local IO stack have always had this understanding
> - that IO needs to _just work_ and be able to make forward progress without
> allocating additional memory, otherwise everything falls over because memory
> reclaim requires doing IO. It's fundamentally no different with network
> filesystems, the cultural expectation just hasn't been there historically
> and not for any good technical reason - it's just that in -net land dropping
> packets is generally a fine thing to do when you have to - but it's really
> not in filesystem land, not if you want to make something that's reliable
> under memory pressure!

Well... I agree memory pressure isn't a good reason and am happy to
spend some precious free time improving this, but while network
reliability shouldn't be a problem (9p only works on reliable
transports), I wouldn't be trusting 9p with too much.
For virtio this isn't much of a problem but to give an example if the
connection drops (server restart, or if over wan some stateful firewall
restart) then the client gets lost and never recovers unless it's
remounted.
For this particular case, I know of a company that did an implementation
to remember rebuild state (reopen fids etc) from the dentry paths but
that was never made public nor would be applicable as is anyway.

Priorities also aren't the same for everyone: while it probably is
perfectly acceptable to put aside 2MB of memory in your case (single
mount, 4x2x256KB msize), I've seen a usecase which requires many mounts
(three per user logged in on the machine, possibly dozens) with large
RPCs for performance (msize=1MB); clogging up 24MB of memory per user
for filesystems they might not even use doesn't seem right to me.

Well, frankly I've just been advising to get away from 9p whenever I
could, it's simple so it's nice for testing, but I'm always surprised it
keeps getting new users... So I probably shouldn't be maintaining this :)

Anyway, for qemu it's probably possible to improve things some more:

> > Let's first see if you still see if you still see high order allocation
> > failures when these are made much less likely with Chritisan's patch.
>
> Which patch is that? Unless you're talking about my mempool patch?

You'll need his 4 patches to apply cleanly, it's what we were talking
about for not allocating such large buffers for all requests.

aa32658b1ca1 net/9p: split message size argument into 't_size' and 'r_size' pair
86b277a492be 9p: add P9_ERRMAX for 9p2000 and 9p2000.u
17c59311f84d net/9p: add p9_msg_buf_size()
4b9eca5829ed net/9p: allocate appropriate reduced message buffers

The first three prepare and last one enables it. Most requests will
allocate much smaller buffers, so if you have parallel IOs in particular
it should be quite less likely to hit allocation failures.
They'll still happen for the larger messages (read/readdir/write) if you
can't allocate a single contiguous segment, but even these would only
need one such buffer instead of the current two, so that still helps a
bit.

--
Dominique