Date: Fri, 4 Oct 2013 09:21:32 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Alex Thorlton <athorlton@sgi.com>, Ingo Molnar <mingo@redhat.com>,
        Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>,
        "Eric W . Biederman" <ebiederm@xmission.com>,
        "Paul E . McKenney" <paulmck@linux.vnet.ibm.com>,
        Al Viro <viro@zeniv.linux.org.uk>, Andi Kleen <ak@linux.intel.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Dave Hansen <dave.hansen@intel.com>, Dave Jones <davej@redhat.com>,
        David Howells <dhowells@redhat.com>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Kees Cook <keescook@chromium.org>, Mel Gorman <mgorman@suse.de>,
        Michael Kerrisk <mtk.manpages@gmail.com>,
        Oleg Nesterov <oleg@redhat.com>, Rik van Riel <riel@redhat.com>,
        Robin Holt <robinmholt@gmail.com>, Sedat Dilek <sedat.dilek@gmail.com>,
        Srikar Dronamraju <srikar@linux.vnet.ibm.com>,
        Thomas Gleixner <tglx@linutronix.de>, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org
Subject: Re: [PATCHv4 09/10] mm: implement split page table lock for PMD level
Message-ID: <20131004072132.GT28601@twins.programming.kicks-ass.net>
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 <1380287787-30252-10-git-send-email-kirill.shutemov@linux.intel.com>
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On Thu, Oct 03, 2013 at 04:11:09PM -0700, Andrew Morton wrote:
> On Fri, 27 Sep 2013 16:16:26 +0300 "Kirill A. Shutemov" <kirill.shutemov@linux.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?

I could go fix all the arch code and pgtable ctor thingies and do the
same thing we do on -rt if anybody cares.

Hugh thought the single pagetable lock would catch the more interesting
locking scenarios, but its of course sad to have an entire locking
scheme not covered by lockdep -- that's just waiting for a bug to sneak
in there.
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