linux-kernel - Re: [PATCH 1/2] mm, page_isolation: remove bogus tests for isolated pages

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Message-ID: <20150729135525.GG19352@techsingularity.net>
Date:	Wed, 29 Jul 2015 14:55:25 +0100
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,
	"minkyung88.kim" <minkyung88.kim@....com>, kmk3210@...il.com,
	Seungho Park <seungho1.park@....com>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Minchan Kim <minchan@...nel.org>,
	Michal Nazarewicz <mina86@...a86.com>,
	Laura Abbott <lauraa@...eaurora.org>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	Johannes Weiner <hannes@...xchg.org>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH 1/2] mm, page_isolation: remove bogus tests for isolated
 pages

On Tue, Jul 21, 2015 at 02:53:37PM +0200, Vlastimil Babka wrote:
> The __test_page_isolated_in_pageblock() is used to verify whether all pages
> in pageblock were either successfully isolated, or are hwpoisoned. Two of the
> possible state of pages, that are tested, are however bogus and misleading.
> 
> Both tests rely on get_freepage_migratetype(page), which however has no
> guarantees about pages on freelists. Specifically, it doesn't guarantee that
> the migratetype returned by the function actually matches the migratetype of
> the freelist that the page is on. Such guarantee is not its purpose and would
> have negative impact on allocator performance.
> 
> The first test checks whether the freepage_migratetype equals MIGRATE_ISOLATE,
> supposedly to catch races between page isolation and allocator activity. These
> races should be fixed nowadays with 51bb1a4093 ("mm/page_alloc: add freepage
> on isolate pageblock to correct buddy list") and related patches. As explained
> above, the check wouldn't be able to catch them reliably anyway. For the same
> reason false positives can happen, although they are harmless, as the
> move_freepages() call would just move the page to the same freelist it's
> already on. So removing the test is not a bug fix, just cleanup. After this
> patch, we assume that all PageBuddy pages are on the correct freelist and that
> the races were really fixed. A truly reliable verification in the form of e.g.
> VM_BUG_ON() would be complicated and is arguably not needed.
> 
> The second test (page_count(page) == 0 && get_freepage_migratetype(page)
> == MIGRATE_ISOLATE) is probably supposed (the code comes from a big memory
> isolation patch from 2007) to catch pages on MIGRATE_ISOLATE pcplists.
> However, pcplists don't contain MIGRATE_ISOLATE freepages nowadays, those are
> freed directly to free lists, so the check is obsolete. Remove it as well.
> 
> Signed-off-by: Vlastimil Babka <vbabka@...e.cz>

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

-- 
Mel Gorman
SUSE Labs
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