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Date: Wed, 17 Jul 2013 14:09:53 -0700
From: Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH 5/8] thp, mm: locking tail page is a bug
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To: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>, Al Viro <viro@zeniv.linux.org.uk>, Hugh Dickins <hughd@google.com>, Wu Fengguang <fengguang.wu@intel.com>, Jan Kara <jack@suse.cz>, Mel Gorman <mgorman@suse.de>, linux-mm@kvack.org, Andi Kleen <ak@linux.intel.com>, Matthew Wilcox <willy@linux.intel.com>, "Kirill A. Shutemov" <kirill@shutemov.name>, Hillf Danton <dhillf@gmail.com>, Dave Hansen <dave@sr71.net>, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org

On Mon, 15 Jul 2013 13:47:51 +0300 "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> wrote:

> From: "Kirill A. Shutemov" <kirill.shutemov@linux.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'm thinking I might leave it in -mm indefinitely but not send it
upstream.

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