linux-kernel - Re: [PATCH 2/2] perf/x86/intel/bts: Fix the use of page

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Message-ID: <20191210153546.GO2844@hirez.programming.kicks-ass.net>
Date:   Tue, 10 Dec 2019 16:35:46 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Alexander Shishkin <alexander.shishkin@...ux.intel.com>
Cc:     Arnaldo Carvalho de Melo <acme@...hat.com>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        Jiri Olsa <jolsa@...nel.org>
Subject: Re: [PATCH 2/2] perf/x86/intel/bts: Fix the use of page_private()

On Thu, Dec 05, 2019 at 05:28:53PM +0300, Alexander Shishkin wrote:
> Commit
> 
>   8062382c8dbe2 ("perf/x86/intel/bts: Add BTS PMU driver")
> 
> uses page_private(page) without checking the PagePrivate(page) first,

Well, arguably it did check it, but you didn't like that WARN ;-)

> which seems like a potential bug, considering that page->private aliases
> with other stuff in struct page.
> 
> Fix this by checking PagePrivate() first.
> 
> Signed-off-by: Alexander Shishkin <alexander.shishkin@...ux.intel.com>
> Fixes: 8062382c8dbe2 ("perf/x86/intel/bts: Add BTS PMU driver")
> ---
>  arch/x86/events/intel/bts.c | 14 +++++++++++---
>  1 file changed, 11 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/events/intel/bts.c b/arch/x86/events/intel/bts.c
> index d53b4fb86d87..9e4da1c5a129 100644
> --- a/arch/x86/events/intel/bts.c
> +++ b/arch/x86/events/intel/bts.c
> @@ -63,9 +63,17 @@ struct bts_buffer {
>  
>  static struct pmu bts_pmu;
>  
> +static int buf_nr_pages(struct page *page)
> +{
> +	if (!PagePrivate(page))
> +		return 1;
> +
> +	return 1 << page_private(page);
> +}

Yes, that seems like a sensible helper.