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

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Message-ID: <20131105231310.GE20167@shutemov.name>
Date:	Wed, 6 Nov 2013 01:13:11 +0200
From:	"Kirill A. Shutemov" <kirill@...temov.name>
To:	Andrew Morton <akpm@...ux-foundation.org>
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 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.

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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