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

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Message-ID: <1412941022.9362.7.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Fri, 10 Oct 2014 04:37:02 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	David Laight <David.Laight@...LAB.COM>
Cc:	'Eric Dumazet' <edumazet@...gle.com>,
	"David S. Miller" <davem@...emloft.net>,
	"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

On Fri, 2014-10-10 at 10:02 +0000, David Laight wrote:
> 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).

Well, this is the idea already in place.

I assume you read __netdev_alloc_frag() ?

Note this is a fix, and a fix should be minimal and obvious.



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