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

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Date:	Thu, 18 Jul 2013 01:45:18 +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>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Hugh Dickins <hughd@...gle.com>,
	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>, Dave Hansen <dave@...1.net>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 5/8] thp, mm: locking tail page is a bug

Andrew Morton wrote:
> On Mon, 15 Jul 2013 13:47:51 +0300 "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com> wrote:
> 
> > From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
> > 
> > Locking head page means locking entire compound page.
> > If we try to lock tail page, something went wrong.
> > 
> > ..
> >
> > --- a/mm/filemap.c
> > +++ b/mm/filemap.c
> > @@ -639,6 +639,7 @@ void __lock_page(struct page *page)
> >  {
> >  	DEFINE_WAIT_BIT(wait, &page->flags, PG_locked);
> >  
> > +	VM_BUG_ON(PageTail(page));
> >  	__wait_on_bit_lock(page_waitqueue(page), &wait, sleep_on_page,
> >  							TASK_UNINTERRUPTIBLE);
> >  }
> > @@ -648,6 +649,7 @@ int __lock_page_killable(struct page *page)
> >  {
> >  	DEFINE_WAIT_BIT(wait, &page->flags, PG_locked);
> >  
> > +	VM_BUG_ON(PageTail(page));
> >  	return __wait_on_bit_lock(page_waitqueue(page), &wait,
> >  					sleep_on_page_killable, TASK_KILLABLE);
> >  }
> 
> 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 found it useful, especially, when I was starting experiments with THP
for pagecache. But feel free to drop it if think that it adds to much
overhead.

> I'm thinking I might leave it in -mm indefinitely but not send it
> upstream.

Works for me too.

-- 
 Kirill A. Shutemov
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