From mboxrd@z Thu Jan  1 00:00:00 1970
Date: Mon, 12 Apr 2021 18:54:52 +0100
From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
Message-ID: <YHSJbL8in1BhMuyu@work-vm>
References: <20210408175150.GA1053917@redhat.com>
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Subject: Re: [Virtio-fs] [RFC] Using hard links instead of lo_inode->fd
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To: Vivek Goyal <vgoyal@redhat.com>
Cc: virtio-fs-list <virtio-fs@redhat.com>, Max Reitz <mreitz@redhat.com>, Miklos Szeredi <miklos@szeredi.hu>

* Vivek Goyal (vgoyal@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.

Dave

> Thanks
> Vivek
-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK