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

[Lai Jiangshan <laijs@cn.fujitsu.com>]

On 03/07/2011 03:39 AM, Hugh Dickins wrote:
> 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.
> 

A union with rcu_head does not cause overwriting, But the problem is
only one minority use of the page (as a DESTROY_BY_RCU slab) needs to
fit a rcu_head and to bloat the struct page size.

Except for preparing for debugging or adding priority information for rcu_head,
this patch also does a de-coupling work.