Re: [PATCH 01/15] mm: poison struct page for ptlock

[Ingo Molnar]

* Hugh Dickins <hugh@xxxxxxxxxxx> wrote:

> On Thu, 10 Nov 2005, Andrew Morton wrote:
> > Hugh Dickins <hugh@xxxxxxxxxxx> wrote:
> > > On Thu, 10 Nov 2005, Ingo Molnar wrote:
> > > > 
> > > > yuck. What is the real problem btw? AFAICS there's enough space for a 
> > > > 2-word spinlock in struct page for pagetables.
> > > 
> > > Yes.  There is no real problem.  But my patch offends good taste.
> > 
> > Isn't it going to overrun page.lru with CONFIG_DEBUG_SPINLOCK?
> 
> No.  There is just one case where it would,
> so in that case split ptlock is disabled by mm/Kconfig's
> # PA-RISC 7xxx's debug spinlock_t is too large for 32-bit struct page.
> 
> 	default "4096" if PARISC && DEBUG_SPINLOCK && !PA20
> 
> Of course, someone may extend spinlock debugging info tomorrow; but 
> when they do, presumably they'll try it out, and hit the BUILD_BUG_ON. 
> They'll then probably want to extend the suppression in mm/Kconfig.

why not do the union thing so that struct page grows automatically as 
new fields are added? It is quite bad design to introduce a hard limit 
like that. The only sizing concern is to make sure that the common 
.configs dont increase the size of struct page, but otherwise why not 
allow a larger struct page - it's for debugging only.

	Ingo
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