linux-kernel - Re: overlayfs non-persistent inodes

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Date:   Sat, 13 Jan 2018 06:38:48 +0200
From:   Amir Goldstein <amir73il@...il.com>
To:     Niklas Cassel <niklas.cassel@...s.com>
Cc:     Miklos Szeredi <mszeredi@...hat.com>,
        overlayfs <linux-unionfs@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: overlayfs non-persistent inodes

On Sat, Jan 13, 2018 at 2:19 AM, Niklas Cassel <niklas.cassel@...s.com> wrote:
> Hello Miklos, Amir
>
> We are having some problems with inotify + overlayfs.
>
> If we start to monitor a directory on an overlayfs,
> and get into a low memory situation, or if
> /proc/sys/vm/drop_caches is called explicitly,
> our inotify stops reporting events.
>
> This appears to be caused by non-persistent inodes.

Not "non-persistent" inodes, but "non-unique" inodes.
inotify pins overlay inode in memory, but because overlay
directory inodes are not hashed in inode cache, after drop
caches, if nothing holds a reference to overlay dentry,
overlay lookup will allocate a new dentry with a new inode
instead of re-using the inotify pinned inode.

>
> Looking at the commits, it looked like inode numbers
> were persistent since commit
> b7a807dc2010 ("ovl: persistent inode number for directories").
>

Persistent inode *number*, meaning that all non-unique overlay
directory inodes have the same inode number - doesn't help
inotify.

> However, this doesn't seem to take different file systems
> into account:
>

Are you saying that inotify issue is not happening when layers
are on same fs? I doubt that.

Please try attached patch.

Thanks,
Amir.

View attachment "0001-ovl-hash-directory-inodes.patch" of type "text/x-patch" (5180 bytes)