Re: [PATCH -V14 0/11] Generic name to handle and open by handle syscalls

["Aneesh Kumar K. V" <aneesh.kumar@linux.vnet.ibm.com>]

On Wed, 7 Jul 2010 11:02:47 -0600, Andreas Dilger <andreas.dilger@oracle.com> wrote:
> On 2010-07-07, at 09:05, J. Bruce Fields wrote:
> > On Wed, Jul 07, 2010 at 01:40:53AM -0600, Andreas Dilger wrote:
> >> On 2010-07-06, at 11:09, Aneesh Kumar K. V wrote:
> >>> Since we know that system wide file handle should include a file system
> >>> identifier and a file identifier my plan was to retrieve both in the
> >>> same syscall.
> >> 
> >> Won't having it be in a separate system call be racy w.r.t. doing the pathname lookup twice?
> > 
> > It'll be rare that a server will want to *just* get a filehandle;
> > normally it will at least want to get some attributes at the same time.
> > So I think it will always need to open the file first and then do the
> > rest of the operations on the returned filehandle.
> 
> I think you are assuming too much about the use of the file handle.
> What I'm interested in is not a userspace file server, but rather a
> more efficient way to have 10000's to millions of clients to be able
> to open the same regular file, without having to do full path
> traversal for each one.


With the suggested syscall approach we can do on the client that does
the path traversal.

fd = open(name)
file_identifier = fd_to_handle(fd);
fs_identifier  = fd_to_fshandle(fd);
close(fd);


> 
> >>> That still leaves the problem that there isn't always an underlying
> >>> block device, and/or when there is it doesn't always uniquely specify
> >>> the filesystem.
> >> 
> >> And for this reason we would need this as a syscall right ?
> > 
> > That's the only solution I see.  (Or use an xattr?)
> 
> Or... return the UUID as part of the file handle in the first place.
> That avoids races, avoids adding more syscalls that have to be called
> for each file handle, or IMNSHO the worst proposal that requires
> applications to parse a text file in some obscure path for each file
> handle (requiring a stat() to find the major/minor device of the file,
> walking through /proc or /sys, and other nastiness).

I would also like to get both file system identifier and file identifier 
in a single call.

That would also imply instead of the above sequence of 4 calls, we can
do

file_handle = name_to_handle(name);

-aneesh