On Sun, Apr 28, 2019 at 8:09 AM Jeff Layton <jlayton@xxxxxxxxxx> wrote: > > On Sat, 2019-04-27 at 16:16 -0400, Amir Goldstein wrote: > > [adding back samba/nfs and fsdevel] > > > > cc'ing Pavel too -- he did a bunch of work in this area a few years ago. > > > On Fri, Apr 26, 2019 at 6:22 PM Jeff Layton <jlayton@xxxxxxxxxx> wrote: > > > On Fri, 2019-04-26 at 10:50 -0400, J. Bruce Fields wrote: > > > > On Fri, Apr 26, 2019 at 04:11:00PM +0200, Amir Goldstein wrote: > > > > > On Fri, Apr 26, 2019, 4:00 PM J. Bruce Fields <bfields@xxxxxxxxxxxx> wrote: > > > > > > > > > > > On Fri, Apr 26, 2019 at 03:50:46PM +0200, Amir Goldstein wrote: > > > > > > > On Fri, Feb 8, 2019, 5:03 PM Jeff Layton <jlayton@xxxxxxxxxx> wrote: > > > > > > > > Share/deny open semantics are pretty similar across NFS and SMB (by > > > > > > > > design, really). If you intend to solve that use-case, what you really > > > > > > > > want is whole-file, shared/exclusive locks that are set atomically with > > > > > > > > the open call. O_EXLOCK and O_SHLOCK seem like a reasonable fit there. > > > > > > > > > > > > > > > > Then you could have SMB and NFS servers set these flags when opening > > > > > > > > files, and deal with the occasional denial at open time. Other > > > > > > > > applications won't be aware of them of course, but that's probably fine > > > > > > > > for most use-cases where you want this sort of protocol interop. > > > > > > > > > > > > > > Sorry for posting off list. Airport emails... > > > > > > > I looked at implemeting O_EXLOCK and O_SHLOCK and it looks doable. > > > > > > > > > > > > > > I was wondering if there is an inherent reason not to allow an exclusive > > > > > > > lock on a file that is open read-only. > > > > > > > > > > > > > > Samba seems to need it and currently flock and ofd locks won't allow it. > > > > > > > Do you thing it will be ok to allow it with O_EXLOCK? > > > > > > > > > > > > Somebody could deny everyone access to a shared resource that everyone > > > > > > needs to make progress, like /etc/passwd or a shared library. > > > > > > > > > > > > Have you looked at Pavel Shilovsky's O_DENY patches? He had the feature > > > > > > off by default, with a mount option provided to turn it on. > > > > > > > > > > > > > > > > O_EXLOCK is advisory. It only aquired flock or ofd lock atomically with > > > > > open. > > > > > > > > Whoops, got it. > > > > > > > > Is that really adequate for open share locks, though? > > > > > > > > I assumed that Windows apps depend on the assumption that they're > > > > mandatory. So e.g. if you can get a DENY_READ open on a shared library > > > > then you know you can update it without the risk of making someone else > > > > crash. > > > > > > > > > > I think this is (slightly) better than doing it internally like we do > > > today and would give you coherent locking between NFS and SMB. Other > > > applications wouldn't see them, but for a NAS-style deployment, that's > > > probably ok. > > > > > > > We can do a little bit better. > > We can make sure that O_DENY_WRITE (named for convenience) fails > > if file is currently open for write by anyone and similarly for O_DENY_READ. > > But if we cannot deny future non-cooperative opens what's the point?.... > > > > As you said in another mail, the main interest here is in getting > NFS+SMB semantics right. If the exported filesystem is _only_ available > via NFS+SMB, then do we need to deny non-cooperative opens? > We do not. > > > Any open by samba or nfsd would need to start setting O_SHLOCK, and deny > > > mode opens would have to set O_EXLOCK. We would actually need 2 per > > > inode though (one for read and one for write). > > > > > > > ...the point is that O_DENY_NONE does not need to be implemented with > > a new type of lock object (O_WR_SHLOCK) its enough that it checks there > > are no relevant exclusive locks and the then inode->i_writecount and > > inode->i_readcount already provide enough context to cooperate with > > O_DENY_WRITE and O_DENY_READ. > > > > That would work, if the goal is to have deny modes affect all opens. We > could also do this on the opt-in basis that I was suggesting with a new > set of counters in struct file_lock_context. > Ok. > > I need to see if incrementing inode->i_readcount on O_RDWR opens is > > possible (right now it only counts O_RDONLY opens). > > > > > I think these should probably be in their own "namespace" too. They > > > could use the same semantics as flock, but should sit on their own list > > > in file_lock_context. > > > > > > > I would much rather that they didn't. The reason is that new open flags > > are a backward compat problem. The way I want to solve it is this API: > > > > // On new kernel this will acquire OFD F_WRLCK atomically... > > fd = open(..., O_RDWR | O_EXLOCK); > > // ...check if it did acquire OFD lock > > fcntl(fd, F_OFD_GETLK, ...); > > > > We'd need at least one new l_type F_EX_RDLCK and maybe also a new > > semantic F_EX_RDWRLCK, although similar in conflicts to F_WRLCK it can be > > acquired without FMODE_WRITE. Though I personally thing we can do without > > it if the only way to acquire F_WRLCK on readonly file is via new open flag. > > > > I don't think that will work at all. Share/deny modes are entirely > orthogonal to byte-range locks in both NFS and SMB. Consider: > > Two clients open a file with O_RDWR | | O_SHARE_WRITE | O_SHARE_READ. > One of them now wants to set byte-range write lock on the file. That > should be allowed, but now it'll be denied, because the other client > will effectively hold a whole-file readlock on it. > Got it. flock semantics (as Pavel chose) are a better fit. It only does not support O_SHARE_WRITE | O_DENY_READ naively, but easy to add. > There is also the problem that read and write deny modes are orthogonal > to one other, so you have to have a way to deal with them independently. > > I'd suggest an API like this: > > // open read/write and deny read/write > fd = open(..., O_RDWR | O_DENY_READ | O_DENY_WRITE); > // test for flags with F_GETFL > flags = fcntl(fd, F_GETFL); > > That would also allow you to use F_SETFL to change those flags on an > existing fd. > Nice. If only old kernel wouldn't give out in F_GETFL any garbage flags you piled on open. That's why I wanted a different way to check if lock is taken and thought of F_OFD_GETLK as a natural candidate. We can play this game: // New kernel doesn't copy O_TEST to f_flags #define O_DENY_READ O_TEST | __O_DENY_READ fd = open(..., O_RDWR | O_DENY_READ); flags = fcntl(fd, F_GETFL); if ((flags & O_DENY_READ) && !(flags & O_TEST)) A bit ugly, but if its wrapped in a library function get_open_flags() who cares... > > > That said, we could also look at a vfs-level mount option that would > > > make the kernel enforce these for any opener. That could also be useful, > > > and shouldn't be too hard to implement. Maybe even make it a vfsmount- > > > level option (like -o ro is). > > > > > > > Yeh, I am humbly going to leave this struggle to someone else. > > Not important enough IMO and completely independent effort to the > > advisory atomic open&lock API. > > Having the kernel allow setting deny modes on any open call is a non- > starter, for the reasons Bruce outlined earlier. This _must_ be > restricted in some fashion or we'll be opening up a ginormous DoS > mechanism. > > My proposal was to make this only be enforced by applications that > explicitly opt-in by setting O_SH*/O_EX* flags. It wouldn't be too > difficult to also allow them to be enforced on a per-fs basis via mount > option or something. Maybe we could expand the meaning of '-o mand' ? > > How would you propose that we restrict this? > Our communication channel is broken. I did not intend to propose any implicit locking. If samba and nfsd can opt-in with O_SHARE flags, I do not understand why a mount option is helpful for the cause of samba/nfsd interop. If someone else is interested in samba/local interop than yes, a mount option like suggested by Pavel could be a good option, but it is an orthogonal effort IMO. > > > If you're denied, what error should you get back when you try to open > > > it? It should be something distinct. We may even want to add new error > > > codes for this. > > > > IMO EBUSY does the job. Its distinct because open is not expected > > to return EBUSY for regular files/dirs and when open is expected to > > return EBUSY for blockdev its for the exact same use case (i.e. > > exclusive write open is acquired by userspace tools). > > That works for me. >From Pavel's v6 cover letter: "Make nfs code return -EBUSY for share conflicts (was -EACCESS)." ;-) > > We should probably have a close look at the work that Pavel did several > years ago too. It has almost certainly bitrotted by now, but it may > serve as a starting point (and he may he may have valuable input here). I looked at the patches. There's good stuff in there. Once we agree on the specifications I can rip some code off ;-) A lot of the work in Pavel's patches evolves around making the mount option work and respecting O_DENYDELETE. IMO, that is not a good use of up-streaming effort, because: - NFS won't ask for deny delete - IMO, Windows applications should be used to being denied a DENY_DELETE and fall back to SHARE_DELETE So while implementing DENYDELETE may fall into a category of making samba server behave more like Windows server, I don't think it falls into the category of better samba/nfs interop. It is something that we can add later if anyone really cares about. Thanks, Amir.
