[Virtio-fs] [RFC] Using hard links instead of lo_inode->fd

[Vivek Goyal]

On Fri, Apr 09, 2021 at 11:03:47AM +0200, Miklos Szeredi wrote:
> On Thu, Apr 8, 2021 at 7:52 PM Vivek Goyal <vgoyal at redhat.com> wrote:
> >
> > Hi All,
> >
> > Max has been trying to solve the problem of not having to keep an O_PATH
> > fd open for every file (until inode goes away). Lot of inodes can get
> > cached in guest inode cache and soon we can hit number of max fd limit.
> >
> > Today Christophe de Dinechin mentioned that can we maintain hardlinks
> > for these files and open these hardlinks instead whenever we need
> > lo_inode->fd.
> >
> > So say there is a notion of "workdir" which is passed to virtiofs where
> > virtiofsd can create hardlinks. Once inode comes into existence, we
> > create this link in workdir/<file-link>. Once last reference to inode
> > is dropped, workdir/<file-link> is removed.
> >
> > Now any operation which needs the lo_inode->fd, can do operation on
> > workdir/<file-link>.
> >
> > How does this proposal sound. What are the issues where. Some of the
> > which came to surface during discussion with david gilbert are.
> >
> > 1. links can't be created for directories. So this will only help with
> >   regular files.
> >
> > 2. There might be a performance penalty. We don't know yet.
> >
> > 3. More overhead of managing workdir by the user/tools.
> >
> >
> > Anything else? Is this a viable idea even if we can implement only
> > for regular files.
> 
> - nlink offset (this could be corrected)

Oh yes, extra nlink count will be there which will have to be corrected
in getattr().

> - needless ctime time update (hard to correct)

This can pose bigger problems. Sepecially if some operation does not
result in ctime update and it gets updated as a side affect of operating
on this link. In that case a user or a test might complain and that's
sort of hard to fix.  All this ctime/mtime/atime logic is already
pretty complicated to handle.

Thanks
Vivek

> 
> I guess this could be a fallback in case the filehandle stuff isn't
> working (e.g. because of an old host kernel).
> 
> Thanks.
> Miklos
>