From: Matthew Wilcox <willy@infradead.org>
To: Al Viro <viro@ZenIV.linux.org.uk>
Cc: linux-fsdevel@vger.kernel.org, samba-technical@lists.samba.org,
Eric Biggers <ebiggers@kernel.org>
Subject: Re: Streams support in Linux
Date: Sat, 25 Aug 2018 13:57:16 -0700 [thread overview]
Message-ID: <20180825205716.GA2664@bombadil.infradead.org> (raw)
In-Reply-To: <20180825180026.GR6515@ZenIV.linux.org.uk>
On Sat, Aug 25, 2018 at 07:00:26PM +0100, Al Viro wrote:
> > > Where do dentries of these things belong, anyway?
> >
> > Do streams have to have dentries? They don't have names in the hierarchy.
> > Can they share their parent's dentry? Can they have a NULL dentry? Or
> > a magic dentry of some kind?
>
> If you want them as opened files - yes, they have to, no, they can't and no,
> they can't.
I was hoping for something like alloc_file_pseudo() rather than having
real dentries in the dentry tree.
> > > > unlinkat()
> > > > This is how you remove an individual named stream
> > >
> > > Yeah... and what happens to access via other hardlinks to the same parent
> > > file?
> >
> > I'm not sure I understand the problem here. You have, say /a/b/c
> > and /d which refer to the same inode. This inode has streams x and y.
> > If you unlinkat() /a/b/c::x then it is no longer openable. Any currently
> > existing open fds for x keep x alive until the last one is closed, no
> > matter which path was used to open them.
>
> Again, where it the tree would the dentries live? And I really hope you are
> not suggesting to reserve '::' - not when we have files like
> /usr/share/man/man3/Params::Check.3perl.gz
> just to pull a random line out of locate :: output here...
I was just using '::' as shorthand for this email. Just to be clear,
there's no intent to have these streams in the filesystem namespace.
> > > > Unlinking a file with named streams removes all named streams from that
> > > > file and then unlinks the file. Open streams will continue to exist in
> > > > the filesystem until they are closed, just as unlinked files do.
> > >
> > > Can they be looked up by openat() from such a starting point?
> >
> > No, you can't use openat() on a stream fd. I should have been clearer
> > on that above; this proposal should have split the syscall list in two;
> > one for how the syscall behaves on a fd-for-a-file-with-streams and one
> > for how the syscall behaves on a stream-fd.
>
> I'm not suggesting passing a stream fd as the starting point; just a normal
> opened-and-then-unlinked regular file.
Ah, I see your point. I think that it makes most sense to define the
semantics of an open-but-unlinked file that you can create new streams,
delete existing streams and rename streams until the last reference to
that file are gone, and all references to streams within a file count
as a reference on that file. Does that make sense?
> > Right, but that code also assumes it's dealing with files.
>
> I'm looking forward to your analysis of all code related to dentry tree,
> with an eye towards the places where such asserts are quietly made...
I really was hoping not to change the dentry tree at all.
next prev parent reply other threads:[~2018-08-26 0:37 UTC|newest]
Thread overview: 36+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-08-25 13:51 Streams support in Linux Matthew Wilcox
2018-08-25 14:47 ` Al Viro
2018-08-25 15:51 ` Matthew Wilcox
2018-08-25 18:00 ` Al Viro
2018-08-25 20:57 ` Matthew Wilcox [this message]
2018-08-25 22:36 ` Al Viro
2018-08-26 1:03 ` Steve French
2018-08-27 17:05 ` Jeremy Allison
2018-08-27 17:41 ` Jeremy Allison
2018-08-27 18:21 ` Matthew Wilcox
2018-08-27 18:45 ` Al Viro
2018-08-27 19:06 ` Jeremy Allison
2018-08-28 0:45 ` Theodore Y. Ts'o
2018-08-28 1:07 ` Steve French
2018-08-28 18:12 ` Jeremy Allison
2018-08-28 18:32 ` Steve French
2018-08-28 18:40 ` Jeremy Allison
2018-08-28 19:43 ` Steve French
2018-08-28 19:47 ` Jeremy Allison
2018-08-28 20:43 ` Steve French
2018-08-28 20:47 ` Jeremy Allison
2018-08-28 20:51 ` Steve French
2018-08-28 21:19 ` Stefan Metzmacher
2018-08-28 21:22 ` Jeremy Allison
2018-08-28 21:23 ` Steve French
2018-08-29 5:13 ` Ralph Böhme
2018-08-29 13:46 ` Tom Talpey
2018-08-29 13:54 ` Aurélien Aptel
2018-08-29 15:02 ` Tom Talpey
2018-08-29 16:00 ` Jeremy Allison
2018-08-29 15:59 ` Jeremy Allison
2018-08-29 18:52 ` Andreas Dilger
2018-08-26 20:30 ` Matthew Wilcox
2018-08-25 16:25 ` Theodore Y. Ts'o
2018-08-27 16:33 ` Jeremy Allison
2018-09-20 2:06 Shahbaz Youssefi
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