From: Hugh Dickins <hughd@google.com>
To: Christoph Lameter <cl@linux.com>
Cc: Pekka Enberg <penberg@kernel.org>,
Lai Jiangshan <laijs@cn.fujitsu.com>, Ingo Molnar <mingo@elte.hu>,
"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
Eric Dumazet <eric.dumazet@gmail.com>,
"David S. Miller" <davem@davemloft.net>,
Matt Mackall <mpm@selenic.com>,
linux-mm@kvack.org, linux-kernel@vger.kernel.org,
netdev@vger.kernel.org
Subject: Re: [PATCH 2/4] slub,rcu: don't assume the size of struct rcu_head
Date: Sun, 6 Mar 2011 11:39:04 -0800 [thread overview]
Message-ID: <AANLkTikk02f6kLiPFqqAGroJErQkHbJFfHzpHy4Y5P8Y@mail.gmail.com> (raw)
In-Reply-To: <alpine.DEB.2.00.1103020625290.10180@router.home>
On Wed, Mar 2, 2011 at 4:32 AM, Christoph Lameter <cl@linux.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
--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org. For more info on Linux MM,
see: http://www.linux-mm.org/ .
Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>
next prev parent reply other threads:[~2011-03-06 19:39 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-03-01 8:03 [PATCH 2/4] slub,rcu: don't assume the size of struct rcu_head Lai Jiangshan
2011-03-01 13:00 ` Pekka Enberg
2011-03-01 15:11 ` Christoph Lameter
2011-03-02 2:55 ` Lai Jiangshan
2011-03-02 4:31 ` Hugh Dickins
2011-03-02 12:32 ` Christoph Lameter
2011-03-06 19:39 ` Hugh Dickins [this message]
2011-03-08 2:17 ` Lai Jiangshan
2011-03-17 15:16 ` Christoph Lameter
2011-03-19 3:58 ` Hugh Dickins
2011-03-08 18:25 ` Christoph Lameter
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=AANLkTikk02f6kLiPFqqAGroJErQkHbJFfHzpHy4Y5P8Y@mail.gmail.com \
--to=hughd@google.com \
--cc=cl@linux.com \
--cc=davem@davemloft.net \
--cc=eric.dumazet@gmail.com \
--cc=laijs@cn.fujitsu.com \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-mm@kvack.org \
--cc=mingo@elte.hu \
--cc=mpm@selenic.com \
--cc=netdev@vger.kernel.org \
--cc=paulmck@linux.vnet.ibm.com \
--cc=penberg@kernel.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).