Re: [PATCH] mm: create a separate slab for page->ptl allocation

[Andrew Morton]

On Wed, 6 Nov 2013 00:42:17 +0200 "Kirill A. Shutemov" <kirill@xxxxxxxxxxxxx> wrote:

> > >  #if USE_SPLIT_PTE_PTLOCKS
> > > +struct kmem_cache *page_ptl_cachep;
> > > +void __init ptlock_cache_init(void)
> > > +{
> > > +	if (sizeof(spinlock_t) > sizeof(long))
> > > +		page_ptl_cachep = kmem_cache_create("page->ptl",
> > > +				sizeof(spinlock_t), 0, SLAB_PANIC, NULL);
> > > +}
> > 
> > Confused.  If (sizeof(spinlock_t) > sizeof(long)) happens to be false
> > then the kernel will later crash.  It would be better to use BUILD_BUG_ON()
> > here, if that works.  Otherwise BUG_ON.
> 
> if (sizeof(spinlock_t) > sizeof(long)) is false, we don't need dynamicly
> allocate page->ptl. It's embedded to struct page itself. __ptlock_alloc()
> never called in this case.

OK.  Please add a comment explaining this so the next reader doesn't get
tripped up like I was.

Really the function shouldn't exist in this case.  It is __init so the
sin is not terrible, but can this be arranged?
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