linux-kernel - Re: [rfc 5/5] mincore: transparent huge page support

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Message-ID: <20100325012316.GB27304@cmpxchg.org>
Date:	Thu, 25 Mar 2010 02:23:16 +0100
From:	Johannes Weiner <hannes@...xchg.org>
To:	Andrea Arcangeli <aarcange@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [rfc 5/5] mincore: transparent huge page support

On Wed, Mar 24, 2010 at 11:48:58PM +0100, Andrea Arcangeli wrote:
> On Tue, Mar 23, 2010 at 03:35:02PM +0100, Johannes Weiner wrote:
> > +static int mincore_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
> > +			unsigned long addr, unsigned long end,
> > +			unsigned char *vec)
> > +{
> > +	int huge = 0;
> > +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> > +	spin_lock(&vma->vm_mm->page_table_lock);
> > +	if (likely(pmd_trans_huge(*pmd))) {
> > +		huge = !pmd_trans_splitting(*pmd);
> 
> Under mmap_sem (read or write) a hugepage can't materialize under
> us. So here the pmd_trans_huge can be lockless and run _before_ taking
> the page_table_lock. That's the invariant I used to keep identical
> performance for all fast paths.

Wait, there _is_ an unlocked fast-path pmd_trans_huge()
in mincore_pmd_range(), maybe you missed it?

This function is never called if the pmd is not huge.

So the above is the _second check_ under lock to get a stable
read on the entry that could be splitting or already have been
split while we checked locklessly.
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