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From: Tony Luck <tony.luck@gmail.com>
Date: Tue, 7 Mar 2017 11:36:36 -0800
Message-ID: <CA+8MBbJNWT5LmekxdLz96_L62qHRJF3_PztjYVBikgv5goorbA@mail.gmail.com>
Subject: Re: [PATCH v2 2/2] mm, page_alloc: avoid page_to_pfn() when merging buddies
To: Vlastimil Babka <vbabka@suse.cz>
Cc: Andrew Morton <akpm@linux-foundation.org>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>, Michal Hocko <mhocko@kernel.org>,
        Mel Gorman <mgorman@techsingularity.net>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Johannes Weiner <hannes@cmpxchg.org>
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On Tue, Mar 7, 2017 at 10:40 AM, Tony Luck <tony.luck@gmail.com> wrote:
> The commit messages talks about the "only caller" of page_is_buddy().
> But grep shows two call sites:
>
> mm/page_alloc.c:816:            if (!page_is_buddy(page, buddy, order))
> mm/page_alloc.c:876:            if (page_is_buddy(higher_page,

and it looks like the second one is the problem:

        if ((order < MAX_ORDER-2) && pfn_valid_within(buddy_pfn)) {
                struct page *higher_page, *higher_buddy;
                combined_pfn = buddy_pfn & pfn;
                higher_page = page + (combined_pfn - pfn);
                buddy_pfn = __find_buddy_pfn(combined_pfn, order + 1);
                higher_buddy = higher_page + (buddy_pfn - combined_pfn);
                if (page_is_buddy(higher_page, higher_buddy, order + 1)) {
                        list_add_tail(&page->lru,
                                &zone->free_area[order].free_list[migratetype]);
                        goto out;
                }
        }

Although outer "if" checked for pfn_valid_within(buddy_pfn),
we actually pass "higher_buddy" to this call of page_is_buddy().

-Tony