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

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Message-Id: <20131105155619.021f32eba1ca8f15a73ed4c9@linux-foundation.org>
Date:	Tue, 5 Nov 2013 15:56:19 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	"Kirill A. Shutemov" <kirill@...temov.name>
Cc:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, linux-arch@...r.kernel.org
Subject: Re: [PATCH] mm: create a separate slab for page->ptl allocation

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

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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