linux-kernel - Re: [PATCH 1/2] mm, page_alloc: don't convert pfn to idx when merging

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Date:   Fri, 9 Dec 2016 17:26:58 +0000
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Joonsoo Kim <iamjoonsoo.kim@....com>,
        Michal Hocko <mhocko@...nel.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Johannes Weiner <hannes@...xchg.org>
Subject: Re: [PATCH 1/2] mm, page_alloc: don't convert pfn to idx when merging

On Fri, Dec 09, 2016 at 10:37:53AM +0100, Vlastimil Babka wrote:
> In __free_one_page() we do the buddy merging arithmetics on "page/buddy index",
> which is just the lower MAX_ORDER bits of pfn. The operations we do that affect
> the higher bits are bitwise AND and subtraction (in that order), where the
> final result will be the same with the higher bits left unmasked, as long as
> these bits are equal for both buddies - which must be true by the definition of
> a buddy.

Ok, other than the kbuild warning, both patchs look ok. I expect the
benefit is marginal but every little bit helps.

> 
> We can therefore use pfn's directly instead of "index" and skip the zeroing of
> >MAX_ORDER bits. This can help a bit by itself, although compiler might be
> smart enough already. It also helps the next patch to avoid page_to_pfn() for
> memory hole checks.
> 

I expect this benefit only applies to a few archiectures and won't be
visible on x86 but it still makes sense so for both patches;

Acked-by: Mel Gorman <mgorman@...hsingularity.net>

As a slight aside, I recently spotted that one of the largest overhead
in the bulk free path was in the page_is_buddy() checks so pretty much
anything that helps that is welcome.

-- 
Mel Gorman
SUSE Labs