linux-kernel - Re: [PATCH 5/8] thp, mm: locking tail page is a bug

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Message-ID: <20130718020414.GA1786@redhat.com>
Date:	Wed, 17 Jul 2013 22:04:14 -0400
From:	Dave Jones <davej@...hat.com>
To:	Hugh Dickins <hughd@...gle.com>
Cc:	Dave Hansen <dave@...1.net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Wu Fengguang <fengguang.wu@...el.com>, Jan Kara <jack@...e.cz>,
	Mel Gorman <mgorman@...e.de>, linux-mm@...ck.org,
	Andi Kleen <ak@...ux.intel.com>,
	Matthew Wilcox <willy@...ux.intel.com>,
	"Kirill A. Shutemov" <kirill@...temov.name>,
	Hillf Danton <dhillf@...il.com>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 5/8] thp, mm: locking tail page is a bug

On Wed, Jul 17, 2013 at 05:58:13PM -0700, Hugh Dickins wrote:
 > On Wed, 17 Jul 2013, Dave Hansen wrote:
 > > On 07/17/2013 02:09 PM, Andrew Morton wrote:
 > > > lock_page() is a pretty commonly called function, and I assume quite a
 > > > lot of people run with CONFIG_DEBUG_VM=y.
 > > > 
 > > > Is the overhead added by this patch really worthwhile?
 > > 
 > > I always thought of it as a developer-only thing.  I don't think any of
 > > the big distros turn it on by default.
 > 
 > That's how I think of it too (and the problem is often that too few mm
 > developers turn it on); but Dave Jones did confirm last November that
 > Fedora turns it on.
 > 
 > I believe Fedora turns it on to help us all, and wouldn't mind a mere
 > VM_BUG_ON(PageTail(page)) in __lock_page() if it's helpful to Kirill.
 > 
 > But if VM_BUG_ONs become expensive, I do think it's for Fedora to
 > turn off CONFIG_DEBUG_VM, rather than for mm developers to avoid it.

I'm ambivalent about whether we keep it on or off, we have no shortage
of bugs to fix already, though I think as mentioned above, very few people
actually enable it, so we're going to lose a lot of testing.

Another idea, perhaps is an extra config option for more expensive debug options ?

	Dave


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