From: Al Viro <viro@zeniv.linux.org.uk>
To: Miklos Szeredi <mszeredi@redhat.com>
Cc: linux-unionfs@vger.kernel.org, linux-fsdevel@vger.kernel.org,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH] ovl: make private mounts longterm
Date: Fri, 22 May 2020 17:08:15 +0100 [thread overview]
Message-ID: <20200522160815.GT23230@ZenIV.linux.org.uk> (raw)
In-Reply-To: <20200522085723.29007-1-mszeredi@redhat.com>
On Fri, May 22, 2020 at 10:57:23AM +0200, Miklos Szeredi wrote:
> Overlayfs is using clone_private_mount() to create internal mounts for
> underlying layers. These are used for operations requiring a path, such as
> dentry_open().
>
> Since these private mounts are not in any namespace they are treated as
> short term, "detached" mounts and mntput() involves taking the global
> mount_lock, which can result in serious cacheline pingpong.
>
> Make these private mounts longterm instead, which trade the penalty on
> mntput() for a slightly longer shutdown time due to an added RCU grace
> period when putting these mounts.
>
> Introduce a new helper kern_unmount_many() that can take care of multiple
> longterm mounts with a single RCU grace period.
Umm...
1) Documentation/filesystems/porting - something along the lines
of "clone_private_mount() returns a longterm mount now, so the proper
destructor of its result is kern_unmount()"
2) the name kern_unmount_many() has an unfortunate clash with
fput_many(), with arguments that look similar and mean something
entirely different. How about kern_unmount_array()?
3)
> - mntput(ofs->upper_mnt);
> - for (i = 1; i < ofs->numlayer; i++) {
> - iput(ofs->layers[i].trap);
> - mntput(ofs->layers[i].mnt);
> +
> + if (!ofs->layers) {
> + /* Deal with partial setup */
> + kern_unmount(ofs->upper_mnt);
> + } else {
> + /* Hack! Reuse ofs->layers as a mounts array */
> + struct vfsmount **mounts = (struct vfsmount **) ofs->layers;
> +
> + for (i = 0; i < ofs->numlayer; i++) {
> + iput(ofs->layers[i].trap);
> + mounts[i] = ofs->layers[i].mnt;
> + }
> + kern_unmount_many(mounts, ofs->numlayer);
> + kfree(ofs->layers);
That's _way_ too subtle. AFAICS, you rely upon ->upper_mnt == ->layers[0].mnt,
->layers[0].trap == NULL, without even mentioning that. And the hack you do
mention... Yecchhh... How many layers are possible, again?
next prev parent reply other threads:[~2020-05-22 16:08 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-05-22 8:57 [PATCH] ovl: make private mounts longterm Miklos Szeredi
2020-05-22 16:08 ` Al Viro [this message]
2020-05-22 16:32 ` Miklos Szeredi
2020-05-22 17:02 ` Amir Goldstein
2020-05-22 18:53 ` Miklos Szeredi
2020-05-22 19:56 ` Al Viro
2020-05-23 4:07 ` Miklos Szeredi
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