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

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Tue, Nov 05, 2013 at 03:56:19PM -0800, Andrew Morton wrote:
> On Wed, 6 Nov 2013 00:42:17 +0200 "Kirill A. Shutemov" <kirill@shutemov.name> 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.

Okay, I will tomorrow.

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

I would like to get rid of __ptlock_alloc()/__ptlock_free() too, but I
don't see a way within C: we need to know sizeof(spinlock_t) on
preprocessor stage.

We can have a hack on kbuild level: write small helper program to find out
sizeof(spinlock_t) before start building and turn it into define.
But it's overkill from my POV. And cross-compilation will be a fun.

-- 
 Kirill A. Shutemov

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