linux-kernel - Re: [PATCH 3/3] mm: tail page refcounting optimization for slab and hugetlbfs

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Message-ID: <528BD7BC.4010205@oracle.com>
Date:	Tue, 19 Nov 2013 14:27:24 -0700
From:	Khalid Aziz <khalid.aziz@...cle.com>
To:	Andrea Arcangeli <aarcange@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>
CC:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Pravin Shelar <pshelar@...ira.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Ben Hutchings <bhutchings@...arflare.com>,
	Christoph Lameter <cl@...ux.com>,
	Johannes Weiner <jweiner@...hat.com>,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	Andi Kleen <andi@...stfloor.org>,
	Minchan Kim <minchan@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 3/3] mm: tail page refcounting optimization for slab and
 hugetlbfs

On 11/15/2013 10:47 AM, Andrea Arcangeli wrote:
> This skips the _mapcount mangling for slab and hugetlbfs pages.
>
> The main trouble in doing this is to guarantee that PageSlab and
> PageHeadHuge remains constant for all get_page/put_page run on the
> tail of slab or hugetlbfs compound pages. Otherwise if they're set
> during get_page but not set during put_page, the _mapcount of the tail
> page would underflow.
>
> PageHeadHuge will remain true until the compound page is released and
> enters the buddy allocator so it won't risk to change even if the tail
> page is the last reference left on the page.
>
> PG_slab instead is cleared before the slab frees the head page with
> put_page, so if the tail pin is released after the slab freed the
> page, we would have a problem. But in the slab case the tail pin
> cannot be the last reference left on the page. This is because the
> slab code is free to reuse the compound page after a
> kfree/kmem_cache_free without having to check if there's any tail pin
> left. In turn all tail pins must be always released while the head is
> still pinned by the slab code and so we know PG_slab will be still set
> too.
>
> Signed-off-by: Andrea Arcangeli <aarcange@...hat.com>

Reviewed-by: Khalid Aziz <khalid.aziz@...cle.com>

--
Khalid
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