Re: [PATCH v3 01/12] mm: dump_page(): better diagnostics for compound pages

[Kirill A. Shutemov]

On Fri, Jan 31, 2020 at 07:40:18PM -0800, John Hubbard wrote:
> A compound page collects the refcount in the head page, while leaving
> the refcount of each tail page at zero. Therefore, when debugging a
> problem that involves compound pages, it's best to have diagnostics that
> reflect that situation. However, dump_page() is oblivious to these
> points.
> 
> Change dump_page() as follows:
> 
> 1) For tail pages, print relevant head page information: refcount, in
>    particular. But only do this if the page is not corrupted so badly
>    that the pointer to the head page is all wrong.
> 
> 2) Do a separate check to catch any (rare) cases of the tail page's
>    refcount being non-zero, and issue a separate, clear pr_warn() if
>    that ever happens.
> 
> Suggested-by: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> Suggested-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> Signed-off-by: John Hubbard <jhubbard@xxxxxxxxxx>

Acked-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>

Few nit-picks below.

> ---
>  mm/debug.c | 34 ++++++++++++++++++++++++++++------
>  1 file changed, 28 insertions(+), 6 deletions(-)
> 
> diff --git a/mm/debug.c b/mm/debug.c
> index ecccd9f17801..beb1c59d784b 100644
> --- a/mm/debug.c
> +++ b/mm/debug.c
> @@ -42,6 +42,32 @@ const struct trace_print_flags vmaflag_names[] = {
>  	{0, NULL}
>  };
>  
> +static void __dump_tail_page(struct page *page, int mapcount)
> +{
> +	struct page *head = compound_head(page);
> +
> +	if ((page < head) || (page >= head + MAX_ORDER_NR_PAGES)) {

I'm not sure if we want to use compound_nr() here instead of
MAX_ORDER_NR_PAGES. Do you have any reasonaing about it?

> +		/*
> +		 * Page is hopelessly corrupted, so limit any reporting to
> +		 * information about the page itself. Do not attempt to look at
> +		 * the head page.
> +		 */
> +		pr_warn("page:%px refcount:%d mapcount:%d mapping:%px "
> +			"index:%#lx (corrupted tail page case)\n",
> +			page, page_ref_count(page), mapcount, page->mapping,
> +			page_to_pgoff(page));
> +	} else {
> +		pr_warn("page:%px compound refcount:%d mapcount:%d mapping:%px "
> +			"index:%#lx compound_mapcount:%d\n",
> +			page, page_ref_count(head), mapcount, head->mapping,
> +			page_to_pgoff(head), compound_mapcount(page));
> +	}
> +
> +	if (page_ref_count(page) != 0)
> +		pr_warn("page:%px PROBLEM: non-zero refcount (==%d) on this "
> +			"tail page\n", page, page_ref_count(page));

Wrap into {}, please.

> +}
> +
>  void __dump_page(struct page *page, const char *reason)
>  {
>  	struct address_space *mapping;
> @@ -75,12 +101,8 @@ void __dump_page(struct page *page, const char *reason)
>  	 */
>  	mapcount = PageSlab(page) ? 0 : page_mapcount(page);
>  
> -	if (PageCompound(page))
> -		pr_warn("page:%px refcount:%d mapcount:%d mapping:%px "
> -			"index:%#lx compound_mapcount: %d\n",
> -			page, page_ref_count(page), mapcount,
> -			page->mapping, page_to_pgoff(page),
> -			compound_mapcount(page));
> +	if (PageTail(page))
> +		__dump_tail_page(page, mapcount);
>  	else
>  		pr_warn("page:%px refcount:%d mapcount:%d mapping:%px index:%#lx\n",
>  			page, page_ref_count(page), mapcount,
> -- 
> 2.25.0
> 

-- 
 Kirill A. Shutemov