linux-kernel - Re: [RFC 2/2] page-flags: Catch the double setter of page flags

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Date:   Mon, 11 Feb 2019 14:47:50 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Chintan Pandya <chintan.pandya@...plus.com>
Cc:     Linux Upstream <linux.upstream@...plus.com>,
        "hughd@...gle.com" <hughd@...gle.com>,
        "jack@...e.cz" <jack@...e.cz>,
        "mawilcox@...rosoft.com" <mawilcox@...rosoft.com>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: [RFC 2/2] page-flags: Catch the double setter of page flags

On Mon, Feb 11, 2019 at 12:53:55PM +0000, Chintan Pandya wrote:
> Some of the page flags, like PG_locked is not supposed to
> be set twice. Currently, there is no protection around this
> and many callers directly tries to set this bit. Others
> follow trylock_page() which is much safer version of the
> same. But, for performance issues, we may not want to
> implement wait-until-set. So, at least, find out who is
> doing double setting and fix them.
> 
> Change-Id: I1295fcb8527ce4b54d5d11c11287fc7516006cf0
> Signed-off-by: Chintan Pandya <chintan.pandya@...plus.com>
> ---
>  include/linux/page-flags.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> index a56a9bd4bc6b..e307775c2b4a 100644
> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h
> @@ -208,7 +208,7 @@ static __always_inline int Page##uname(struct page *page)		\
>  
>  #define SETPAGEFLAG(uname, lname, policy)				\
>  static __always_inline void SetPage##uname(struct page *page)		\
> -	{ set_bit(PG_##lname, &policy(page, 1)->flags); }
> +	{ WARN_ON(test_and_set_bit(PG_##lname, &policy(page, 1)->flags)); }

You forgot to make this depend on CONFIG_DEBUG_VM. Also, I'm not
convinced this is always wrong, inefficient sure, but not wrong in
general.