linux-kernel - Re: [PATCHv4 09/10] mm: implement split page table lock for PMD level

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Message-Id: <20131003233800.8A003E0090@blue.fi.intel.com>
Date:	Fri,  4 Oct 2013 02:38:00 +0300 (EEST)
From:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Alex Thorlton <athorlton@....com>,
	Ingo Molnar <mingo@...hat.com>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	"Eric W . Biederman" <ebiederm@...ssion.com>,
	"Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Andi Kleen <ak@...ux.intel.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Dave Hansen <dave.hansen@...el.com>,
	Dave Jones <davej@...hat.com>,
	David Howells <dhowells@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Kees Cook <keescook@...omium.org>,
	Mel Gorman <mgorman@...e.de>,
	Michael Kerrisk <mtk.manpages@...il.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Rik van Riel <riel@...hat.com>,
	Robin Holt <robinmholt@...il.com>,
	Sedat Dilek <sedat.dilek@...il.com>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCHv4 09/10] mm: implement split page table lock for PMD level

Andrew Morton wrote:
> On Fri, 27 Sep 2013 16:16:26 +0300 "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com> wrote:
> 
> > The basic idea is the same as with PTE level: the lock is embedded into
> > struct page of table's page.
> > 
> > We can't use mm->pmd_huge_pte to store pgtables for THP, since we don't
> > take mm->page_table_lock anymore. Let's reuse page->lru of table's page
> > for that.
> > 
> > pgtable_pmd_page_ctor() returns true, if initialization is successful
> > and false otherwise. Current implementation never fails, but assumption
> > that constructor can fail will help to port it to -rt where spinlock_t
> > is rather huge and cannot be embedded into struct page -- dynamic
> > allocation is required.
> 
> spinlock_t is rather large when lockdep is enabled.  What happens?

The same as with PTE split lock: CONFIG_SPLIT_PTLOCK_CPUS set to 999999
if DEBUG_SPINLOCK || DEBUG_LOCK_ALLOC. It effectively blocks split locks
usage if spinlock_t is too big.

-- 
 Kirill A. Shutemov
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