Re: [nfsv4] RFC 7530: Filehandle of opened file after the REMOVE

[David Noveck <[e-mail]>]

> You need a guarantee that the user holds the same file removal
permissions in that directory; that’s something the client cannot enforce.

The client needs that to do the eventual remove at last close.  To do the
rename you also need ADD_FILE permissions on the target directory.

Within the NFSV4 ACL model, you can have a directory for which a user has
DELETE_CHILD permissions without having ADD_FILE, so that the silly rename
(within the same directory) would fail with a permission error.  Not sure
what existing clients do about this.  My best guess is that the user
process gets a permission error on his remove, despite having DELETE_CHILD
permissions on the directory.  Such directories are unlikely to exist with
current clients and servers but the NFSv4 RFCs allow them to be created.

On Wed, Dec 28, 2016 at 7:59 AM, Trond Myklebust <[e-mail]> wrote:

>
> On Dec 28, 2016, at 07:08, Benny Halevy <[e-mail]> wrote:
>
>
>
> On Dec 27, 2016 11:00 PM, "Trond Myklebust" <[e-mail]> wrote:
>
>
> On Dec 27, 2016, at 21:24, Trond Myklebust <[e-mail]> wrote:
>
>
> On Dec 27, 2016, at 19:13, David Noveck <[e-mail]> wrote:
>
> Bruce wrote:
>
> > I think this is what OPEN4_RESULT_PRESERVE_UNLINKED in 5661 was meant to
> > do.
>
> In fact it was.  See the sixth bullet in section 1.8 of RFC5661.
>
> The problem I see is that it doesn't solve the problem that Marcel has
> pointed out.  If client A
> opens the file and client B removes it, there is no way it can decide
> whether or not to do a
> silly rename.  Since he hasn't opened, the file he doesn't see the flag
> indicating it is not
> needed.  Also, since the file is not open he has no indication that it is
> needed either.  So client
> B will do a REMOVE in all cases.
>
>
> As far as I can tell, Marcel is mainly interested in NFSv4.0. RFC5661 does
> not address any of the cases he was describing.
>
>
> Server implementations may or may not preserve the file until last close.
> For those that do,
> everything will work out OK, while for those that don't, there isn't much
> that the client can
> do.  If he did find out the server did not have the support, he has o way
> to defer the remove
> until last close because he has no way of finding out when this happens,
>
> Trond wrote:
>
> > There are 4 main problems that are inadequately discussed in any of the
> existing RFCs,
> > and that you need to address before we can consider replacing
> sillyrename
>
> It appears it has already been considered and spec'd but the spec may not
> be
> adequate.  Sigh!
>
> > (which is well established today, and well understood by most users).
>
> Most people understand why it works when it does but don't understand why
> it doesn't
> work in some cases.  So they are happy until they aren’t.
>
>
> Umm…. If the solution works with unlink-on-close then you all do realize
> that it MUST always also work with sillyrename, right? Having a third party
> client remove a file named ‘.nfsXXXXX’ is just a special case of having it
> remove a file with a generic name.
>
> IOW: I could argue that the problem here can be considered to be purely a
> server problem and irrelevant to the actual delete strategy chosen by the
> client.
>
>
> Correction: most of the problem is orthogonal to the client delete
> strategy. The one place where the client can put its foot in its mouth is
> ‘rm -rf’, where the sillyrename does prevent the removal of the directory
> from succeeding.
>
>
> A different silly rename strategy could be to a root-based directory, e.g.
> /trashcan. That would prevent the rm -rf issue, no?
>
>
> You need a guarantee that the user holds the same file removal permissions
> in that directory; that’s something the client cannot enforce.
>
>
> Benny
>
>
>
> > 1) How does the client identify that the server supports this
> functionality?
>
> The problem is that the client who doesn't open file, the troublesome
> case, is exactly the one in which
> he receives no information.  The client who does the open finds out silly
> rename is not needed, but, in
> this case, silly rename seems to work OK.
>
> > 2) What functionality is needed on the underlying filesystems on the
> server?
>
> Almost all filesystems have the ability to defer deletion until the last
> close.  If they don't, they
> are not very usable.  Servers which don't support this are not suffering
> from a lack of filesystem
> functionality.  Instead, the problem seems to be that some servers have an
> NFSv4 open which
> either doesn't connect the to the underlying open functionality or there
> is no underlying open functionality.
>
>
> Sure, but in order to be reboot safe, you need to go well beyond the
> functionality that you describe. As far as I can tell, you need to defer
> the garbage collection that would normally occur when the server comes back
> up again until after the state reclaim has occurred (as you describe below).
>
>
> > 3) How does the server function in the case of a reboot? What can the
> client expect in terms of recoverability?
>
> This is addressed b section 18.16.3 of rfc5661.  The third bullet on page
>  449 says:
>
> Furthermore, the server promises to preserve the file through the grace period after server restart, thereby giving the client the opportunity to reclaim its open.
>
>
> Again, this is specified for NFSv4.x (x>0) but not for x=0. It is not
> clear to me that older NFSv4 servers will have any of this functionality.
>
>
> > 4) How does the client in practice perform recovery?
> >   a) In the case of server reboots.
>
> I think he just recovers this opens within the grace period.
>
> >   b) In the case of lease timeouts/network partitions
>
> This case is not addressed RFC5661, so far as I can see.  The typical
> handling, in which
> the locks are all dropped would cause the file to go away.  Courtesy locks
> could address the
> problem, but the spec doesn't that have to be implemented.
>
> > Note that the lack of open-by-filehandle in NFSv4.0 makes 4.b) more
> difficult than it should otherwise be.
>
> True. In v4.1, a client can try an open-by-filehandle and take advantage
> of any courtesy locks that the server
> has retained.
>
>
> On Tue, Dec 27, 2016 at 9:44 AM, J. Bruce Fields <[e-mail]>
> wrote:
>
>> On Wed, Dec 14, 2016 at 09:28:41AM -0500, Trond Myklebust wrote:
>> > On Wed, Dec 14, 2016 at 6:21 AM, Marcel Telka <[e-mail]> wrote:
>> >
>> > > Citát David Noveck <[e-mail]>:
>> > >
>> > >> It appears that you want an informational document saying, more or
>> less:
>> > >>
>> > >>    - If the server does not want clients to be discomfited by open
>> files
>> > >>    being removed, since such behavior is disallowed by typical OS
>> > >> (e.g.Unix)
>> > >>    semantics, the server can avoid this situation by delaying the
>> actual
>> > >>    removal of the file until last close, as allowed by RFC7530.
>> > >>    - The use of rename by clients as a substitute for remove,
>> normally
>> > >>    known as "silly rename", has significant problems, since removes
>> can
>> > >> happen
>> > >>    on nodes that do not have the file open.
>> > >>
>> > >> If this is what you want, then you can write an I-D and submit it.
>> > >>
>> > >
>> > > Yes, this is exactly what I want to see.
>> > >
>> > >
>> > There are 4 main problems that are inadequately discussed in any of the
>> > existing RFCs, and that you need to address before we can consider
>> > replacing sillyrename (which is well established today, and well
>> understood
>> > by most users).
>> >
>> > 1) How does the client identify that the server supports this
>> functionality?
>>
>> I think this is what OPEN4_RESULT_PRESERVE_UNLINKED in 5661 was meant to
>> do.  It'd be interesting to try an implementation and see if your other
>> points are addressed, but I haven't thought about it in a long time.
>>
>> --b.
>>
>> > 2) What functionality is needed on the underlying filesystems on the
>> server?
>> > 3) How does the server function in the case of a reboot? What can the
>> > client expect in terms of recoverability?
>> > 4) How does the client in practice perform recovery?
>> >    a) In the case of server reboots.
>> >    b) In the case of lease timeouts/network partitions
>> >
>> > Note that the lack of open-by-filehandle in NFSv4.0 makes 4.b) more
>> > difficult than it should otherwise be.
>>
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