[changing the subject and adding more NFS guys so they can shoot my
idea down if it is too dumb to live]
On Fri, Apr 7, 2017 at 4:03 PM, Miklos Szeredi <miklos@xxxxxxxxxx> wrote:
> On Fri, Apr 7, 2017 at 12:47 PM, Amir Goldstein <amir73il@xxxxxxxxx> wrote:
>
>> Come to think about it, NFS export of regular file don't need to
>> follow renames at all:
>> - The handle for a regular file is always the handle for the real
>> lower or upper inode
>> - To decode a handle, create an O_TMPFILE style overlay dentry, which
>> is not linked
>> to any path in overlay, but has the _upperdentry/lowerstack setup
>
> I don't think nfs will allow such a scheme. NFS3 server is stateless,
> which means there's no open/close in the protocol. Hence we can't
> copy-up on open(O_WR*) and return a different file handle for writing.
> If client looks up a file currently on lower and we return file handle
> based on lower file, then we must be able to decode that handle after
> the file has been copied up and even after rename. And this must work
> reliably even if the overlay dentry is no longer in the dcache.
>
> So there's no option, other than to have a reverse mapping somewhere.
>
Either I am missing something or you are.
Consider this scenario:
On server:
- touch a
- ln a b
On NFS client:
- rofd = open("a", O_RDONLY)
On server
- rm a
- reboot
NFS client must be able to continue to work with rofd
even after reboot and even after original file was unlinked.
Furthermore, on server:
- rm b
NFS client must continue to work with rofd even though
NFS server is stateless and even though inode is now
nlink = 0.
That is possible because fs will instantiate a disconnected
dentry when decoding the file handle is there is no dentry
already in cache.
So what I am saying is that when nfsd tries to decode
a handle from overlay mount, and there is no mathcing
overlay dentry in cache (with the lower ino of course)
then we instantiate a new disconnected dentry without
lookup and set its _upperdenry or lowerstack according
to the knowledge that we found the handle in underlying
fs and we checked if it is a decedent of lower_mnt[i] or
upper_mnt.
When NFS client opens a new rwfd it WILL get a different
handle, but it WILL really be a different file then the rofd,
so that sounds like a good thing?
I realize it may sound complicated, but redirect_fh patch
has already all the needed parts in place for this, so the
proof if what I am saying is right or wrong will be in whether
or not I am able to present a working POC...
Amir.
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