Re: [patch 5/6] mm-introduce-do_shared_fault-and-drop-do_fault-fix-fix

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Mon, Dec 15, 2014 at 03:03:43PM -0800, akpm@linux-foundation.org wrote:
> From: Andrew Morton <akpm@linux-foundation.org>
> Subject: mm-introduce-do_shared_fault-and-drop-do_fault-fix-fix
> 
> add comment which may not be true :(
> 
> Cc: Andi Kleen <ak@linux.intel.com>
> Cc: Bob Liu <lliubbo@gmail.com>
> Cc: Dave Hansen <dave.hansen@linux.intel.com>
> Cc: "Kirill A. Shutemov" <kirill@shutemov.name>
> Cc: Matthew Wilcox <matthew.r.wilcox@intel.com>
> Cc: Mel Gorman <mgorman@suse.de>
> Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
> Cc: Rik van Riel <riel@redhat.com>
> Cc: Sasha Levin <sasha.levin@oracle.com>
> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> ---
> 
>  mm/memory.c |    6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff -puN mm/memory.c~mm-introduce-do_shared_fault-and-drop-do_fault-fix-fix mm/memory.c
> --- a/mm/memory.c~mm-introduce-do_shared_fault-and-drop-do_fault-fix-fix
> +++ a/mm/memory.c
> @@ -3009,6 +3009,12 @@ static int do_shared_fault(struct mm_str
>  
>  	if (set_page_dirty(fault_page))
>  		dirtied = 1;
> +	/*
> +	 * Take a local copy of the address_space - page.mapping may be zeroed
> +	 * by truncate after unlock_page().   The address_space itself remains
> +	 * pinned by vma->vm_file's reference.  We rely on unlock_page()'s
> +	 * release semantics to prevent the compiler from undoing this copying.
> +	 */

Looks correct to me.

We need the same comment or reference to this one in do_wp_page().

>  	mapping = fault_page->mapping;

BTW, I noticed that fault_page here can be a tail page: sound subsytem
allocates its pages with GFP_COMP and maps them with ptes. The problem is
that we never set ->mapping for tail pages and the check below is always
false. It seems doesn't cause any problems right now (looks like ->mapping
is NULL also for head page sound case), but logic is somewhat broken.

I only triggered the problem when tried to reuse ->mapping in first tail
page for compound_mapcount in my thp refcounting rework.

If it sounds right, I will prepare patch to replace the line above and the
same case in do_wp_page() with

	mapping = compound_head(fault_page)->mapping;

Ok?

>  	unlock_page(fault_page);
>  	if ((dirtied || vma->vm_ops->page_mkwrite) && mapping) {
> _
> 
-- 
 Kirill A. Shutemov

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