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Date:   Tue, 10 Dec 2019 16:29:39 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc:     Arnaldo Carvalho de Melo <acme@redhat.com>,
        Ingo Molnar <mingo@redhat.com>, linux-kernel@vger.kernel.org,
        Jiri Olsa <jolsa@kernel.org>,
        Vince Weaver <vincent.weaver@maine.edu>
Subject: Re: [PATCH 1/2] perf/x86/intel/bts: Remove a silly warning
Message-ID: <20191210152939.GN2844@hirez.programming.kicks-ass.net>
References: <20191205142853.28894-1-alexander.shishkin@linux.intel.com>
 <20191205142853.28894-2-alexander.shishkin@linux.intel.com>
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On Thu, Dec 05, 2019 at 05:28:52PM +0300, Alexander Shishkin wrote:
> There is no comment or record anywhere that would explain the train of
> thought that went into this warning, it probably tried to make sure that
> the high order allocations indeed happened in the ring buffer code.
> 

> --- a/arch/x86/events/intel/bts.c
> +++ b/arch/x86/events/intel/bts.c
> @@ -83,8 +83,6 @@ bts_buffer_setup_aux(struct perf_event *event, void **pages,
>  	/* count all the high order buffers */
>  	for (pg = 0, nbuf = 0; pg < nr_pages;) {
>  		page = virt_to_page(pages[pg]);
> -		if (WARN_ON_ONCE(!PagePrivate(page) && nr_pages > 1))
> -			return NULL;
>  		pg += 1 << page_private(page);

I'm thinking that because ^^^^ uses page_private(), it wants to make
sure PagePrivate().

I haven't checked the current rules, but using page_private() without
PagePrivate() seems dodgy.

Also consider:

+               __nr_pages = PagePrivate(page) ? 1 << page_private(page) : 1;

>  		nbuf++;
>  	}
> -- 
> 2.24.0
>