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Date:   Wed, 10 Oct 2018 15:26:55 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Keith Busch <keith.busch@intel.com>
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        Kirill Shutemov <kirill.shutemov@linux.intel.com>,
        Dave Hansen <dave.hansen@intel.com>,
        Dan Williams <dan.j.williams@intel.com>
Subject: Re: [PATCH 1/6] mm/gup_benchmark: Time put_page
Message-Id: <20181010152655.8510270e5db753f6666f12d3@linux-foundation.org>
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On Wed, 10 Oct 2018 13:56:00 -0600 Keith Busch <keith.busch@intel.com> wrote:

> We'd like to measure time to unpin user pages, so this adds a second
> benchmark timer on put_page, separate from get_page.
> 
> Adding the field will breaks this ioctl ABI, but should be okay since
> this an in-tree kernel selftest.
> 
> ...
>
> --- a/mm/gup_benchmark.c
> +++ b/mm/gup_benchmark.c
> @@ -8,7 +8,8 @@
>  #define GUP_FAST_BENCHMARK	_IOWR('g', 1, struct gup_benchmark)
>  
>  struct gup_benchmark {
> -	__u64 delta_usec;
> +	__u64 get_delta_usec;
> +	__u64 put_delta_usec;
>  	__u64 addr;
>  	__u64 size;
>  	__u32 nr_pages_per_call;

If we move put_delta_usec to the end of this struct, the ABI remains
back-compatible?