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

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Date:	Wed, 9 May 2012 12:25:24 -0500 (CDT)
From:	Christoph Lameter <cl@...ux.com>
To:	Pravin Shelar <pshelar@...ira.com>
cc:	Eric Dumazet <eric.dumazet@...il.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 Wed, 9 May 2012, Pravin Shelar wrote:

> On Tue, May 8, 2012 at 12:22 PM, Christoph Lameter <cl@...ux.com> wrote:
> > 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.
> >
>
> I agree this would impact performance. I am not sure how else we can
> fix this issue. As far as slab_unlock in hot path case is concerned,
> it is more likely to corrupt page->flags in that case.

Dont modify any page flags from THP logic if its a slab page? THP cannot
break up or merge slab pages anyways.


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