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Date: Mon, 17 Jul 2017 07:09:34 -0700
From: Matthew Wilcox <willy@infradead.org>
To: Waiman Long <longman@redhat.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>,
        Jonathan Corbet <corbet@lwn.net>, linux-kernel@vger.kernel.org,
        linux-doc@vger.kernel.org, linux-fsdevel@vger.kernel.org,
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        Andrew Morton <akpm@linux-foundation.org>,
        Ingo Molnar <mingo@kernel.org>,
        Miklos Szeredi <mszeredi@redhat.com>
Subject: Re: [PATCH 2/4] fs/dcache: Report negative dentry number in
 dentry-state
Message-ID: <20170717140934.GA14983@bombadil.infradead.org>
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 <1500298773-7510-3-git-send-email-longman@redhat.com>
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On Mon, Jul 17, 2017 at 09:39:31AM -0400, Waiman Long wrote:
> @@ -63,9 +63,10 @@ struct qstr {
>  struct dentry_stat_t {
>  	long nr_dentry;
>  	long nr_unused;
> -	long age_limit;          /* age in seconds */
> -	long want_pages;         /* pages requested by system */
> -	long dummy[2];
> +	long nr_negative;	/* # of negative dentries */
> +	long age_limit;		/* age in seconds */
> +	long want_pages;	/* pages requested by system */
> +	long dummy;
>  };
>  extern struct dentry_stat_t dentry_stat;

You can't just insert a field in the middle like that.  It'll break any code
parsing /proc/sys/fs/dentry-state.  You have to put it at the end:

 	long age_limit;          /* age in seconds */
 	long want_pages;         /* pages requested by system */
-	long dummy[2];
+	long nr_negative;	/* # of negative dentries */
+	long dummy;
 };