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

[Andrew Morton]

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

> From: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
> 
> 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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