netdev - Re: [PATCH 2/4] slub,rcu: don't assume the size of struct rcu

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Date:	Sun, 6 Mar 2011 11:39:04 -0800
From:	Hugh Dickins <hughd@...gle.com>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Pekka Enberg <penberg@...nel.org>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Ingo Molnar <mingo@...e.hu>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Eric Dumazet <eric.dumazet@...il.com>,
	"David S. Miller" <davem@...emloft.net>,
	Matt Mackall <mpm@...enic.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [PATCH 2/4] slub,rcu: don't assume the size of struct rcu_head

On Wed, Mar 2, 2011 at 4:32 AM, Christoph Lameter <cl@...ux.com> wrote:
> On Tue, 1 Mar 2011, Hugh Dickins wrote:
>
>> > Struct page may be larger for debugging purposes already because of the
>> > need for extended spinlock data.
>>
>> That was so for a long time, but I stopped it just over a year ago
>> with commit a70caa8ba48f21f46d3b4e71b6b8d14080bbd57a, stop ptlock
>> enlarging struct page.
>
> Strange. I just played around with in in January and the page struct size
> changes when I build kernels with full debugging. I have some
> cmpxchg_double patches here that depend on certain alignment in the page
> struct. Debugging causes all that stuff to get out of whack so that I had
> to do some special patches to make sure fields following the spinlock are
> properly aligned when the sizes change.

That puzzles me, it's not my experience and I don't have an
explanation: do you have time to investigate?

Uh oh, you're going to tell me you're working on an out-of-tree
architecture with a million cpus ;)  In that case, yes, I'm afraid
I'll have to update the SPLIT_PTLOCK_CPUS defaulting (for a million -
1 even).

>
>> If a union leads to "random junk" overwriting the page->mapping field
>> when the page is reused, and that junk could resemble the pointer in
>> question, then KSM would mistakenly think it still owned the page.
>> Very remote chance, and maybe it amounts to no more than a leak.  But
>> I'd still prefer we keep page->mapping for pointers (sometimes with
>> lower bits set as flags).
>
> DESTROY BY RCU uses the lru field which follows the mapping field in page
> struct. Why would random junk overwrite the mapping field?

Random junk does not overwrite the mapping field with the current
implementation of DESTROY_BY_RCU.  But you and Jiangshan were
discussing how to change it, so I was warning of this issue with
page->mapping.

But I would anyway agree with Jiangshan, that it's preferable not to
bloat struct page size just for this DESTROY_BY_RCU issue, even if it
is only an issue when debugging.

Hugh
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