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

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Message-ID: <20131106103403.GB21074@mudshark.cambridge.arm.com>
Date:	Wed, 6 Nov 2013 10:34:03 +0000
From:	Will Deacon <will.deacon@....com>
To:	"Kirill A. Shutemov" <kirill@...temov.name>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>
Subject: Re: [PATCH] mm: create a separate slab for page->ptl allocation

On Tue, Nov 05, 2013 at 11:13:11PM +0000, Kirill A. Shutemov wrote:
> 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@...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.
> 
> 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.

FWIW: if the architecture selects ARCH_USE_CMPXCHG_LOCKREF, then a spinlock_t
is 32-bit (assuming that unsigned int is also 32-bit).

Will
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