Re: [PATCH v6 12/12] mm: dump_page(): additional diagnostics for huge pinned pages

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

On Mon, Feb 10, 2020 at 04:15:36PM -0800, John Hubbard wrote:
> As part of pin_user_pages() and related API calls, pages are
> "dma-pinned". For the case of compound pages of order > 1, the per-page
> accounting of dma pins is accomplished via the 3rd struct page in the
> compound page. In order to support debugging of any pin_user_pages()-
> related problems, enhance dump_page() so as to report the pin count
> in that case.
> 
> Documentation/core-api/pin_user_pages.rst is also updated accordingly.
> 
> Cc: Jan Kara <jack@suse.cz>
> Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> Cc: Matthew Wilcox <willy@infradead.org>
> Signed-off-by: John Hubbard <jhubbard@nvidia.com>
> ---
>  Documentation/core-api/pin_user_pages.rst |  7 +++++++
>  mm/debug.c                                | 21 ++++++++++++++++-----
>  2 files changed, 23 insertions(+), 5 deletions(-)
> 
> diff --git a/Documentation/core-api/pin_user_pages.rst b/Documentation/core-api/pin_user_pages.rst
> index 5c8a5f89756b..2e939ff10b86 100644
> --- a/Documentation/core-api/pin_user_pages.rst
> +++ b/Documentation/core-api/pin_user_pages.rst
> @@ -238,6 +238,13 @@ long-term [R]DMA pins in place, or during pin/unpin transitions.
>  (...unless it was already out of balance due to a long-term RDMA pin being in
>  place.)
>  
> +Other diagnostics
> +=================
> +
> +dump_page() has been enhanced slightly, to handle these new counting fields, and
> +to better report on compound pages in general. Specifically, for compound pages
> +with order > 1, the exact (hpage_pinned_refcount) pincount is reported.
> +
>  References
>  ==========
>  
> diff --git a/mm/debug.c b/mm/debug.c
> index f5ffb0784559..2189357f0987 100644
> --- a/mm/debug.c
> +++ b/mm/debug.c
> @@ -85,11 +85,22 @@ void __dump_page(struct page *page, const char *reason)
>  	mapcount = PageSlab(head) ? 0 : page_mapcount(page);
>  
>  	if (compound)
> -		pr_warn("page:%px refcount:%d mapcount:%d mapping:%p "
> -			"index:%#lx head:%px order:%u compound_mapcount:%d\n",
> -			page, page_ref_count(head), mapcount,
> -			mapping, page_to_pgoff(page), head,
> -			compound_order(head), compound_mapcount(page));
> +		if (hpage_pincount_available(page)) {
> +			pr_warn("page:%px refcount:%d mapcount:%d mapping:%p "
> +				"index:%#lx head:%px order:%u "
> +				"compound_mapcount:%d compound_pincount:%d\n",
> +				page, page_ref_count(head), mapcount,
> +				mapping, page_to_pgoff(page), head,
> +				compound_order(head), compound_mapcount(page),
> +				compound_pincount(page));
> +		} else {
> +			pr_warn("page:%px refcount:%d mapcount:%d mapping:%p "
> +				"index:%#lx head:%px order:%u "
> +				"compound_mapcount:%d\n",
> +				page, page_ref_count(head), mapcount,
> +				mapping, page_to_pgoff(page), head,
> +				compound_order(head), compound_mapcount(page));
> +		}

Have you considered using pr_cont() here. I guess it would be easier to
read.

You can use my Ack anyway.


>  	else
>  		pr_warn("page:%px refcount:%d mapcount:%d mapping:%p index:%#lx\n",
>  			page, page_ref_count(page), mapcount,
> -- 
> 2.25.0
> 

-- 
 Kirill A. Shutemov