linux-kernel - Re: [PATCH] mm: sl[auo]b: Use atomic bit operations to update page-flags.

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Date:	Tue, 8 May 2012 14:22:13 -0500 (CDT)
From:	Christoph Lameter <cl@...ux.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
cc:	Pravin B Shelar <pshelar@...ira.com>, penberg@...nel.org,
	mpm@...enic.com, linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	jesse@...ira.com, abhide@...ira.com
Subject: Re: [PATCH] mm: sl[auo]b: Use atomic bit operations to update
 page-flags.

On Tue, 8 May 2012, Eric Dumazet wrote:

> On Tue, 2012-05-08 at 11:55 -0700, Pravin B Shelar wrote:
> > Transparent huge pages can change page->flags (PG_compound_lock)
> > without taking Slab lock. So sl[auo]b need to use atomic bit
> > operation while changing page->flags.
> > Specificly this patch fixes race between compound_unlock and slab
> > functions which does page-flags update. This can occur when
> > get_page/put_page is called on page from slab object.
>
>
> But should get_page()/put_page() be called on a page own by slub ?

Can occur in slab allocators if the slab memory is used for DMA. I dont
like the performance impact of the atomics. In particular slab_unlock() in
slub is or used to be a hot path item. It is still hot on arches that do
not support this_cpu_cmpxchg_double. With the cmpxchg_double only the
debug mode is affected.

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