linux-kernel - Re: [PATCH 2/6] thp: optimize away unnecessary page table locking

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Date:	Thu,  2 Feb 2012 00:27:58 -0500
From:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>, linux-mm@...ck.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Rientjes <rientjes@...gle.com>,
	Andi Kleen <andi@...stfloor.org>,
	Wu Fengguang <fengguang.wu@...el.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/6] thp: optimize away unnecessary page table locking

On Mon, Jan 30, 2012 at 03:22:12PM +0900, KAMEZAWA Hiroyuki wrote:
> On Fri, 27 Jan 2012 18:02:49 -0500
> Naoya Horiguchi <n-horiguchi@...jp.nec.com> wrote:
>
> > Currently when we check if we can handle thp as it is or we need to
> > split it into regular sized pages, we hold page table lock prior to
> > check whether a given pmd is mapping thp or not. Because of this,
> > when it's not "huge pmd" we suffer from unnecessary lock/unlock overhead.
> > To remove it, this patch introduces a optimized check function and
> > replace several similar logics with it.
> >
> > Signed-off-by: Naoya Horiguchi <n-horiguchi@...jp.nec.com>
> > Cc: David Rientjes <rientjes@...gle.com>
> >
> > Changes since v3:
> >   - Fix likely/unlikely pattern in pmd_trans_huge_stable()
> >   - Change suffix from _stable to _lock
> >   - Introduce __pmd_trans_huge_lock() to avoid micro-regression
> >   - Return 1 when wait_split_huge_page path is taken
> >
> > Changes since v2:
> >   - Fix missing "return 0" in "thp under splitting" path
> >   - Remove unneeded comment
> >   - Change the name of check function to describe what it does
> >   - Add VM_BUG_ON(mmap_sem)
>
>
> > +/*
> > + * Returns 1 if a given pmd maps a stable (not under splitting) thp,
> > + * -1 if the pmd maps thp under splitting, 0 if the pmd does not map thp.
> > + *
> > + * Note that if it returns 1, this routine returns without unlocking page
> > + * table locks. So callers must unlock them.
> > + */
>
>
> Seems nice clean up but... why you need to return (-1, 0, 1) ?
>
> It seems the caller can't see the difference between -1 and 0.
>
> Why not just return 0 (not locked) or 1 (thp found and locked) ?

Sorry, I changed wrongly from v3.
We can do fine without return value of -1 if we remove else-if (!err)
{...} block after move_huge_pmd() call in move_page_tables(), right?
(split_huge_page_pmd() after wait_split_huge_page() do nothing...)
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