On 04/29/2014 01:34 PM, Jeff Layton wrote: > On Tue, 29 Apr 2014 11:53:40 +0200 > "Michael Kerrisk (man-pages)" <mtk.manpages@xxxxxxxxx> wrote: > >> On 04/29/2014 11:24 AM, NeilBrown wrote: >>> On Tue, 29 Apr 2014 11:07:16 +0200 "Michael Kerrisk (man-pages)" >>> <mtk.manpages@xxxxxxxxx> wrote: >>> >>>> On 04/27/2014 11:28 PM, NeilBrown wrote: >>>>> On Sun, 27 Apr 2014 13:11:33 +0200 "Michael Kerrisk (man-pages)" >>>>> <mtk.manpages@xxxxxxxxx> wrote: >>>>> >>>>>> On Sun, Apr 27, 2014 at 12:04 PM, NeilBrown <neilb@xxxxxxx> wrote: >>>>>>> On Sun, 27 Apr 2014 11:16:02 +0200 "Michael Kerrisk (man-pages)" >>>>>>> <mtk.manpages@xxxxxxxxx> wrote: >>>>>>> >>>>>>>> [Trimming some folk from CC, and adding various NFS people] >>>>>>>> >>>>>>>> On 04/27/2014 06:51 AM, NeilBrown wrote: >>>>>>>> >>>>>>>> [...] >>>>>>>> >>>>>>>>> Note to Michael: The text >>>>>>>>> flock() does not lock files over NFS. >>>>>>>>> in flock(2) is no longer accurate. The reality is ... complex. >>>>>>>>> See nfs(5), and search for "local_lock". >>>>>>>> >>>>>>>> Ahhh -- I see: >>>>>>>> http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=5eebde23223aeb0ad2d9e3be6590ff8bbfab0fc2 >>>>>>>> >>>>>>>> Thanks for the heads up. >>>>>>>> >>>>>>>> Just in general, it would be great if the flock(2) and fcntl(2) man pages >>>>>>>> contained correct details for NFS, of course. So, for example, if there >>>>>>>> are any current gotchas for NFS and fcntl() byte-range locking, I'd like >>>>>>>> to add those to the fcntl(2) man page. >>>>>>> >>>>>>> The only peculiarities I can think of are: >>>>>>> - With NFS, locking or unlocking a region forces a flush of any cached data >>>>>>> for that file (or maybe for the region of the file). I'm not sure if this >>>>>>> is worth mentioning. >>>>>> >>>>>> I agree that it's probably not necessary to mention. >>>>>> >>>>>>> - With NFSv4 the client can lose a lock if it is out of contact with the >>>>>>> server for a period of time. When this happens, any IO to the file by a >>>>>>> process which "thinks" it holds a lock will fail until that process closes >>>>>>> and re-opens the file. >>>>>>> This behaviour is since 3.12. Prior to that the client might lose and >>>>>>> regain the lock without ever knowing thus potentially risking corruption >>>>>>> (but only if client and server lost contact for an extended period). >>>>>> >>>>>> Do you have a pointer for that commit to 3.12? >>>>>> >>>>> >>>>> http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=ef1820f9be27b6ad158f433ab38002ab8131db4d >>>>> >>>>> did most of the work while the subsequent commit >>>>> >>>>> http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=f6de7a39c181dfb8a2c534661a53c73afb3081cd >>>>> >>>>> changed some details, added some documentation, and inverted the default >>>>> behaviour. >>>> >>>> Thanks for that detail. What do you think of the following text for the >>>> fcntl(2) man page: >>>> >>>> Before Linux 3.12, if an NFS client is out of contact with the >>>> server for a period of time, it might lose and regain a lock >>>> without ever being aware of the fact. This scenario poten‐ >>>> tially risks data corruption, since another process might >>>> acquire a lock in the intervening period and perform file I/O. >>>> Since Linux 3.12, if the client loses contact with the server, >>>> any I/O to the file by a process which "thinks" it holds a lock >>>> will fail until that process closes and reopens the file. A >>>> kernel parameter, nfs.recover_lost_locks, can be set to 1 to >>>> obtain the pre-3.12 behavior, whereby the client will attempt >>>> to recover lost locks when contact is reestablished with the >>>> server. Because of the attendant risk of data corruption, this >>>> parameter defaults to 0 (disabled). >>>> >>> >>> Mostly good. >>> >>> I'm just a little concerned about "if the client loses contact with the >>> server" in the middle there. It is no longer qualified and it isn't clear >>> that the "for a period of time" qualification still applied. And we should >>> probably quantify the period of time - which defaults to 90 seconds. >>> I don't remember just now the difference between >>> /proc/fs/nfsd/nfsv4{lease,grace}time >>> but this 90 seconds is one of those. >>> >>> Also this is NFSv4 specific. With NFSv3 the failure mode is the reverse. If >>> the server loses contact with a client then any lock stays in place >>> indefinitely ("why can't I read my mail"... I remember it well). >>> >>> Before Linux 3.12, if an NFSv4 client loses contact with the server >>> (defined as more than 90 seconds with no communication), it might lose >>> and regain .... >> >> Thanks, Neil. Changed as you suggest. I'd quite like to mention >> which of /proc/fs/nfsd/nfsv4{lease,grace}time is relevant here. I had a >> quick scan, but could not determine it with complete confidence. My suspicion, >> looking at fs/lockd/svcproc.c and fs/lockd/grace.c::locks_in_grace() >> is that it is /proc/fs/nfsd/nfsv4gracetime that is relevant here. Can anyone >> confirm? >> > > The difference here is subtle. The gracetime is how long after a reboot > should knfsd allow clients to reclaim state (and deny the creation of > new locks and opens). The leasetime is how long the NFSv4 lease period > is. There is a relationship between the two that's illustrated in the > comments above write_gracetime: > > /** > * write_gracetime - Set or report current NFSv4 grace period time > * > * As above, but sets the time of the NFSv4 grace period. > * > * Note this should never be set to less than the *previous* > * lease-period time, but we don't try to enforce this. (In the common > * case (a new boot), we don't know what the previous lease time was > * anyway.) > */ > > The value you're interested in here is the nfsv4leasetime. If the > client doesn't renew its lease within that period, then it's subject to > the server giving up on it and dropping any state that it holds on that > clients' behalf. > > Note that this is not a firm timeout. The server runs a job > periodically to clean out expired stateful objects, and it's likely > that there is some time (maybe even up to another whole lease period) > between when the timeout expires and the job actually runs. If the > client gets a RENEW in there within that window, its lease will be > renewed and its state preserved. > > Also note that all of the above just applies to the Linux knfsd. There > are many other servers in the field and they have different rules for > dropping state held by clients that have gone AWOL. Thanks for the detailed explanation, Jeff. I've updated the draft text to mention nfsv4gracetime. I won't add the subtleties you mention above (but they'll go into the commit message). The text is now: Record locking and NFS Before Linux 3.12, if an NFSv4 client loses contact with the server for a period of time (defined as more than 90 seconds with no communication), it might lose and regain a lock without ever being aware of the fact. (The period of time after which contact is assumed lost is defined by /proc/fs/nfsd/nfsv4lease‐ time, which expresses the period in seconds. The default value for this file is 90.) This scenario potentially risks data corruption, since another process might acquire a lock in the intervening period and perform file I/O. Since Linux 3.12, if an NFSv4 client loses contact with the server, any I/O to the file by a process which "thinks" it holds a lock will fail until that process closes and reopens the file. A kernel parameter, nfs.recover_lost_locks, can be set to 1 to obtain the pre-3.12 behavior, whereby the client will attempt to recover lost locks when contact is reestab‐ lished with the server. Because of the attendant risk of data corruption, this parameter defaults to 0 (disabled). Cheers, Michael -- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ Linux/UNIX System Programming Training: http://man7.org/training/ -- 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
