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

[Miklos Szeredi]

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)
- needless ctime time update (hard to correct)

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