linux-kernel - Re: [PATCH] vfs: allow stacked ->get

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Date:   Mon, 12 Apr 2021 11:27:33 +0200
From:   Miklos Szeredi <miklos@...redi.hu>
To:     Miklos Szeredi <mszeredi@...hat.com>
Cc:     Al Viro <viro@...iv.linux.org.uk>,
        overlayfs <linux-unionfs@...r.kernel.org>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        garyhuang <zjh.20052005@....com>
Subject: Re: [PATCH] vfs: allow stacked ->get_acl() in RCU lookup

On Tue, Mar 23, 2021 at 5:07 PM Miklos Szeredi <mszeredi@...hat.com> wrote:
>
> Overlayfs does not cache ACL's to avoid double caching with all its
> problems.  Instead it just calls the underlying filesystem's
> i_op->get_acl(), which will return the cached value, if possible.
>
> In rcu path walk, however, get_cached_acl_rcu() is employed to get the
> value from the cache, which will fail on overlayfs resulting in dropping
> out of rcu walk mode.  This can result in a big performance hit in certain
> situations.
>
> Add a flags argument to the ->get_acl() callback, and allow
> get_cached_acl_rcu() to call the ->get_acl() method with LOOKUP_RCU.
>
> Don't do this for the generic case of a cache miss, only in case of
> ACL_DONT_CACHE.
>
> Reported-by: garyhuang <zjh.20052005@....com>
> Signed-off-by: Miklos Szeredi <mszeredi@...hat.com>

Hi Al,

Could you please apply this patch?

It's fairly trivial, but unfortunately adds a fair bit of API churn.

Thanks,
Miklos