netdev - RE: [PATCH net 0/5] net: fix races accessing page->

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Date:	Fri, 10 Oct 2014 10:02:37 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Eric Dumazet' <edumazet@...gle.com>,
	"David S. Miller" <davem@...emloft.net>
CC:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	Alexander Duyck <alexander.h.duyck@...hat.com>,
	Andres Lagar-Cavilla <andreslc@...gle.com>,
	Greg Thelen <gthelen@...gle.com>,
	Hugh Dickins <hughd@...gle.com>,
	"David Rientjes" <rientjes@...gle.com>
Subject: RE: [PATCH net 0/5] net: fix races accessing page->_count

From: Eric Dumazet
> This is illegal to use atomic_set(&page->_count, ...) even if we 'own'
> the page. Other entities in the kernel need to use get_page_unless_zero()
> to get a reference to the page before testing page properties, so we could
> loose a refcount increment.

Plausibly the atomic_inc() could be avoided by allocating the page
with multiple references, or by gabbing a lot of extra references
and keeping a local count of when they are 'used' (by passing
ownership onwards).

	David



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