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

[Christoph Lameter]

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.

> 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?

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