Hi Trond, I'm back to this an would like a clarification on your comment. On Mon, Jan 13, 2020 at 4:51 PM Trond Myklebust <trondmy@xxxxxxxxxxxxxxx> wrote: > > On Mon, 2020-01-13 at 16:05 -0500, Olga Kornievskaia wrote: > > On Mon, Jan 13, 2020 at 1:24 PM Trond Myklebust < > > trondmy@xxxxxxxxxxxxxxx> wrote: > > > On Mon, 2020-01-13 at 13:09 -0500, Olga Kornievskaia wrote: > > > > On Mon, Jan 13, 2020 at 11:49 AM Trond Myklebust > > > > <trondmy@xxxxxxxxxxxxxxx> wrote: > > > > > On Mon, 2020-01-13 at 11:08 -0500, Olga Kornievskaia wrote: > > > > > > On Fri, Jan 10, 2020 at 4:03 PM Trond Myklebust < > > > > > > trondmy@xxxxxxxxxxxxxxx> wrote: > > > > > > > On Fri, 2020-01-10 at 14:29 -0500, Olga Kornievskaia wrote: > > > > > > > > Hi folks, > > > > > > > > > > > > > > > > We are having an issue with an interrupted RPCs again. > > > > > > > > Here's > > > > > > > > what I > > > > > > > > see when xfstests were ctrl-c-ed. > > > > > > > > > > > > > > > > frame 332 SETATTR call slot=0 seqid=0x000013ca (I'm > > > > > > > > assuming > > > > > > > > this > > > > > > > > is > > > > > > > > interrupted and released) > > > > > > > > frame 333 CLOSE call slot=0 seqid=0x000013cb (only way > > > > > > > > the > > > > > > > > slot > > > > > > > > could > > > > > > > > be free before the reply if it was interrupted, right? > > > > > > > > Otherwise > > > > > > > > we > > > > > > > > should never have the slot used by more than one > > > > > > > > outstanding > > > > > > > > RPC) > > > > > > > > frame 334 reply to 333 with SEQ_MIS_ORDERED (I'm assuming > > > > > > > > server > > > > > > > > received frame 333 before 332) > > > > > > > > frame 336 CLOSE call slot=0 seqid=0x000013ca (??? why did > > > > > > > > we > > > > > > > > decremented it. I mean I know why it's in the current > > > > > > > > code :- > > > > > > > > / ) > > > > > > > > frame 337 reply to 336 SEQUENCE with ERR_DELAY > > > > > > > > frame 339 reply to 332 SETATTR which nobody is waiting > > > > > > > > for > > > > > > > > frame 543 CLOSE call slot=0 seqid=0x000013ca (retry after > > > > > > > > waiting > > > > > > > > for > > > > > > > > err_delay) > > > > > > > > frame 544 reply to 543 with SETATTR (out of the cache). > > > > > > > > > > > > > > > > What this leads to is: file is never closed on the > > > > > > > > server. > > > > > > > > Can't > > > > > > > > remove it. Unmount fails with CLID_BUSY. > > > > > > > > > > > > > > > > I believe that's the result of commit > > > > > > > > 3453d5708b33efe76f40eca1c0ed60923094b971. > > > > > > > > We used to have code that bumped the sequence up when the > > > > > > > > slot > > > > > > > > was > > > > > > > > interrupted but after the commit "NFSv4.1: Avoid false > > > > > > > > retries > > > > > > > > when > > > > > > > > RPC calls are interrupted". > > > > > > > > > > > > > > > > Commit has this "The obvious fix is to bump the sequence > > > > > > > > number > > > > > > > > pre-emptively if an > > > > > > > > RPC call is interrupted, but in order to deal with > > > > > > > > the > > > > > > > > corner > > > > > > > > cases > > > > > > > > where the interrupted call is not actually received > > > > > > > > and > > > > > > > > processed > > > > > > > > by > > > > > > > > the server, we need to interpret the error > > > > > > > > NFS4ERR_SEQ_MISORDERED > > > > > > > > as a sign that we need to either wait or locate a > > > > > > > > correct > > > > > > > > sequence > > > > > > > > number that lies between the value we sent, and the > > > > > > > > last > > > > > > > > value > > > > > > > > that > > > > > > > > was acked by a SEQUENCE call on that slot." > > > > > > > > > > > > > > > > If we can't no longer just bump the sequence up, I don't > > > > > > > > think > > > > > > > > the > > > > > > > > correct action is to automatically bump it down (as per > > > > > > > > example > > > > > > > > here)? > > > > > > > > The commit doesn't describe the corner case where it was > > > > > > > > necessary to > > > > > > > > bump the sequence up. I wonder if we can return the > > > > > > > > knowledge > > > > > > > > of > > > > > > > > the > > > > > > > > interrupted slot and make a decision based on that as > > > > > > > > well as > > > > > > > > whatever > > > > > > > > the other corner case is. > > > > > > > > > > > > > > > > I guess what I'm getting is, can somebody (Trond) provide > > > > > > > > the > > > > > > > > info > > > > > > > > for > > > > > > > > the corner case for this that patch was created. I can > > > > > > > > see if > > > > > > > > I > > > > > > > > can > > > > > > > > fix the "common" case which is now broken and not break > > > > > > > > the > > > > > > > > corner > > > > > > > > case.... > > > > > > > > > > > > > > > > > > > > > > There is no pure client side solution for this problem. > > > > > > > > > > > > > > The change was made because if you have multiple > > > > > > > interruptions > > > > > > > of > > > > > > > the > > > > > > > RPC call, then the client has to somehow figure out what > > > > > > > the > > > > > > > correct > > > > > > > slot number is. If it starts low, and then goes high, and > > > > > > > the > > > > > > > server is > > > > > > > not caching the arguments for the RPC call that is in the > > > > > > > session > > > > > > > cache, then we will _always_ hit this bug because we will > > > > > > > always > > > > > > > hit > > > > > > > the replay of the last entry. > > > > > > > > > > > > > > At least if we start high, and iterate by low, then we > > > > > > > reduce > > > > > > > the > > > > > > > problem to being a race with the processing of the > > > > > > > interrupted > > > > > > > request > > > > > > > as it is in this case. > > > > > > > > > > > > > > However, as I said, the real solution here has to involve > > > > > > > the > > > > > > > server. > > > > > > > > > > > > Ok I see your point that if the server cached the arguments, > > > > > > then > > > > > > the > > > > > > server would tell that 2nd rpc using the same slot+seqid has > > > > > > different > > > > > > args and would not use the replay cache. > > > > > > > > > > > > However, I wonder if the client can do better. Can't we be > > > > > > more > > > > > > aware > > > > > > of when we are interrupting the rpc? For instance, if we are > > > > > > interrupted after we started to wait on the RPC, doesn't it > > > > > > mean > > > > > > the > > > > > > rpc is sent on the network and since network is reliable then > > > > > > server > > > > > > must have consumed the seqid for that slot (in this case > > > > > > increment > > > > > > seqid)? That's the case that's failing now. > > > > > > > > > > > > > > > > "Reliable transport" does not mean that a client knows what got > > > > > received and processed by the server and what didn't. All the > > > > > client > > > > > knows is that if the connection is still up, then the TCP layer > > > > > will > > > > > keep retrying transmission of the request. There are plenty of > > > > > error > > > > > scenarios where the client gets no information back as to > > > > > whether > > > > > or > > > > > not the data was received by the server (e.g. due to lost > > > > > ACKs). > > > > > > > > > > Furthermore, if a RPC call is interrupted on the client, either > > > > > due > > > > > to > > > > > a timeout or a signal, > > > > > > > > What timeout are you referring to here since 4.1 rcp can't > > > > timeout. I > > > > think it only leaves a signal. > > > > > > If you use 'soft' or 'softerr' mount options, then NFSv4.1 will > > > time > > > out when the server is being unresponsive. That behaviour is > > > different > > > to the behaviour under a signal, but has the same effect of > > > interrupting the RPC call without us being able to know if the > > > server > > > received the data. > > > > > > > > then it almost always ends up breaking the > > > > > connection in order to avoid corruption of the data stream (by > > > > > interrupting the transmission before the entire RPC call has > > > > > been > > > > > sent). You generally have to be lucky to see the timeout/signal > > > > > occur > > > > > only when all the RPC calls being cancelled have exactly fit > > > > > into > > > > > the > > > > > socket buffer. > > > > > > > > Wouldn't a retransmission (due to a connection reset for whatever > > > > reason) be different and doesn't involve reprocessing of the > > > > slot. > > > > > > I'm not talking about retransmissions here. I'm talking only about > > > NFSv4.x RPC calls that suffer a fatal interruption (i.e. no > > > retransmission). > > > > > > > > Finally, just because the server's TCP layer ACKed receipt of > > > > > the > > > > > RPC > > > > > call data, that does not mean that it will process that call. > > > > > The > > > > > connection could break before the call is read out of the > > > > > receiving > > > > > socket, or the server may later decide to drop it on the floor > > > > > and > > > > > break the connection. > > > > > > > > > > IOW: the RPC protocol here is not that "reliable transport > > > > > implies > > > > > processing is guaranteed". It is rather that "connection is > > > > > still > > > > > up > > > > > implies processing may eventually occur". > > > > > > > > "eventually occur" means that its process of the rpc is > > > > guaranteed > > > > "in > > > > time". Again unless the client is broken, we can't have more than > > > > an > > > > interrupted rpc (that has nothing waiting) and the next rpc (both > > > > of > > > > which will be re-transmitted if connection is dropped) going to > > > > the > > > > server. > > > > > > > > Can we distinguish between interrupted due to re-transmission and > > > > interrupted due to ctrl-c of the thread? If we can't, then I'll > > > > stop > > > > arguing that client can do better. > > > > > > There is no "interrupted due to re-transmission" case. We only > > > retransmit NFSv4 requests if the TCP connection breaks. > > > > > > As far as I'm concerned, this discussion is only about > > > interruptions > > > that cause the RPC call to be abandoned (i.e. fatal timeouts and > > > signals). > > > > > > > But right now we are left in a bad state. Client leaves opened > > > > state > > > > on the server and will not allow for files to be deleted. I think > > > > in > > > > case the "next rpc" is the write that will never be completed it > > > > would > > > > leave the machine in a hung state. I just don't see how can you > > > > justify that having the current code is any better than having > > > > the > > > > solution that was there before. > > > > > > That's a general problem with allowing interruptions that is > > > largely > > > orthogonal to the question of which strategy we choose when > > > resynchronising the slot numbers after an interruption has > > > occurred. > > > > > > > I'm re-reading the spec and in section 2.10.6.2 we have "A requester > > MUST wait for a reply to a request before using the slot for another > > request". Are we even legally using the slot when we have an > > interrupted slot? > > > > You can certainly argue that. However the fact that the spec fails to > address the issue doesn't imply lack of need. I have workloads on my > own systems that would cause major disruption if I did not allow them > to time out when the server is unavailable (e.g. with memory filling up > with dirty pages that can't be cleaned). > > IOW: I'm quite happy to make a best effort attempt to meet that > requirement, by making 'hard' mounts the default, and by making > signalling be a fatal operation. However I'm unwilling to make it > impossible to fix up my system when the server is unresponsive just > because the protocol is lazy about providing for that ability. When you say: "I'm quite happy to make a best effort attempt to meet that requirement, by making 'hard' mounts the default, and by making signalling be a fatal operation". Do you mean that if somebody is doing a default hard mount, then you are OK with making the code to always wait for the response instead of leaving it interrupted? Somehow I think I'm still misreading what you wrote there. Thanks. > > -- > Trond Myklebust > Linux NFS client maintainer, Hammerspace > trond.myklebust@xxxxxxxxxxxxxxx > >
