On Thu, 17 Mar 2016, Trond Myklebust wrote: > On Thu, Mar 17, 2016 at 6:11 AM, Benjamin Coddington > <bcodding@xxxxxxxxxx> wrote: > > On Thu, 17 Mar 2016, Mkrtchyan, Tigran wrote: > >> I agree with Trond, that returning zero bytes without setting eof > >> with a high probability a server side issue. We had that situation > >> with dCache server, where eof flag was set only if you read beyond > >> file size, e.q. READ with count=0 at the offset=file size, we returned > >> zero bytes with no eof set. The pynfs test, actually, do retry such > >> request and there was an infinite loop. > > > > But that isn't a short read.. if the request is with count=0 and the > > response is 0 without eof, there shouldn't be a retry, and the current > > client won't retry in that case, nor fail with EIO. > > > >> I think, if we (you) add retry on zero byte short-reads > >> without eof we may have applications/client hangs in case of > >> misbehaving servers. But failing with EIO is not the best > >> option. May be it makes sense to query file size in such > >> situations? As this is a rare corner case, performance > >> penalty will by negligible. > > > > I do see now that the resistance to this change is because it seems > > important to ensure that we make forward progress. This change would lose > > that guarantee for a server that gets "stuck". And to Trond's point: if the > > zero-length short read really was due to eof at read time, then eof should > > have been set. > > > > It seems tricky for the linux server to determine when to set eof. The best > > way might be to adopt the convention that a local short read means we should > > set eof and accept the small penalty that a read that completes fully (not > > short) up to the end of the file will require another read to detect eof. > > That should be a fairly rare case except where file sizes end up being > > multiples of rsize. > > It should be trivial for the linux server to determine when eof is to > be set, because it is running in a POSIX environment. As I pointed out > earlier, the POSIX spec says that all reads to a regular file that > return less than the number of requested bytes are hitting eof. I agree. I meant to say it seems tricky to do it they way we are doing it now: by checking i_size afterward. So, thanks for pointing out how specific POSIX is about this. I'll send a patch for the server. > > > > Ben > > > >> > >> Tigran. > >> > >> ----- Original Message ----- > >> > From: "Trond Myklebust" <trond.myklebust@xxxxxxxxxxxxxxx> > >> > To: "J. Bruce Fields" <bfields@xxxxxxxxxxxx> > >> > Cc: "Benjamin Coddington" <bcodding@xxxxxxxxxx>, "Anna Schumaker" <anna.schumaker@xxxxxxxxxx>, "Linux NFS Mailing List" > >> > <linux-nfs@xxxxxxxxxxxxxxx> > >> > Sent: Wednesday, March 16, 2016 9:02:49 PM > >> > Subject: Re: [PATCH] NFS: Retry a zero-length short read > >> > >> > On Wed, Mar 16, 2016 at 3:56 PM, J. Bruce Fields <bfields@xxxxxxxxxxxx> wrote: > >> >> On Wed, Mar 16, 2016 at 03:46:28PM -0400, Benjamin Coddington wrote: > >> >>> So, sounds like fixing this is a good idea on the server. I hope Trond will > >> >>> let us know if he still feels that the client ought not to be changed since > >> >>> it seems an easy enough fix to avoid a similar problem on another server. > >> >>> Perhaps there's a downside I'm not seeing on the client. > >> >> > >> >> My worry would just be ensuring forward progress--if the client gets > >> >> some data back, then at least the next read can start at a later > >> >> offset.... With zero reads, we can set a maximum number of retries, I > >> >> guess, but that makes it little messy. > >> >> > >> >>> Or maybe the > >> >>> convention of read() returning 0 meaning eof is global enough to cause it to > >> >>> be acceptible behavior -- we really should treat a zero-length read response > >> >>> without eof as an error. My lack of experience is showing.. :) > >> >> > >> >> Eh, I think it's legitimately more confusing than it should be. > >> >> > >> > > >> > POSIX is very specific about the cases where you are allowed to return > >> > a short read: > >> > > >> > See http://pubs.opengroup.org/onlinepubs/9699919799/functions/read.html > >> > > >> > "The value returned may be less than nbyte if the number of bytes left > >> > in the file is less than nbyte, if the read() request was interrupted > >> > by a signal, or if the file is a pipe or FIFO or special file and has > >> > fewer than nbyte bytes immediately available for reading. For example, > >> > a read() from a file associated with a terminal may return one typed > >> > line of data." > >> > > >> > So I'm guessing most POSIX based server implementations should have no > >> > trouble working out exactly when to set the eof flag. However the > >> > client has no clue as to what OS your server is based on, which is > >> > presumably the main reason why NFS has an eof flag in the first place. > >> > -- > >> > To unsubscribe from this list: send the line "unsubscribe linux-nfs" in > >> > the body of a message to majordomo@xxxxxxxxxxxxxxx > >> > More majordomo info at http://vger.kernel.org/majordomo-info.html > >> > -- To unsubscribe from this list: send the line "unsubscribe linux-nfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
