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

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Message-ID: <CALnjE+pExzAS4bk89RD4XJtHtSyB2g0qMsqdrGWPuD27axiNBw@mail.gmail.com>
Date:	Wed, 9 May 2012 09:41:05 -0700
From:	Pravin Shelar <pshelar@...ira.com>
To:	Christoph Lameter <cl@...ux.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 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.
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