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

[Vivek Goyal]

On Mon, Apr 12, 2021 at 06:54:52PM +0100, Dr. David Alan Gilbert wrote:
> * 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.
> 
> The other one is that linkat(, AT_EMPTY_PATH) which can create the
> link from the existing O_PATH we have, can only be used with
> CAP_DAC_READ_SEARCH - which is a shame since otherwise it would seem to
> make life easier.

But that does not mitigate issues w.r.t submounts (links across
filesytem) and ctime updates?

So only thing this linkat(,AT_EMPTY_PATH) seems to buy is that link
source can be an fd instead of path, right? 

Vivek