Re: [PATCH v2 1/8] mm: dump_page: print head page's refcount, for compound pages

[Matthew Wilcox <willy@infradead.org>]

On Wed, Jan 29, 2020 at 02:26:06PM -0800, John Hubbard wrote:
> On 1/29/20 3:25 AM, Kirill A. Shutemov wrote:
> > On Tue, Jan 28, 2020 at 07:24:10PM -0800, John Hubbard wrote:
> >> When debugging a problem that involves compound pages, it is extremely
> >> helpful if dump_page() reports not only the page->_refcount, but also
> >> the refcount of the head page of the compound page. That's because the
> >> head page collects refcounts for the entire compound page.
> >>
> >> Therefore, enhance dump_page() so as to print out the refcount of the
> >> head page of a compound page.
> >>
> >> This approach (printing information about a struct page that is not the
> >> struct page that was passed into dump_page()) has a precedent:
> >> compound_mapcount is already being printed.
> > 
> > refcount on a tail must always be 0. I think we should only print it when
> > it is non-zero, emphasizing this fact with a standalone message.
> > 
> 
> Hi Kirill,
> 
> Yes, good point, that sounds like just the right balance. And it avoids adding 
> a new item to print in the common case, which is very nice. Here's what I've
> changed it to for the next version (I'll also rewrite the commit description 
> accordingly):
> 
> 
> diff --git a/mm/debug.c b/mm/debug.c
> index a90da5337c14..3a45e2b77de0 100644
> --- a/mm/debug.c
> +++ b/mm/debug.c
> @@ -75,12 +75,17 @@ 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,
> +	if (PageCompound(page)) {
> +		pr_warn("page:%px compound refcount:%d mapcount:%d mapping:%px "
> +			"index:%#lx compound_mapcount:%d\n",
> +			page, page_ref_count(compound_head(page)), mapcount,
>  			page->mapping, page_to_pgoff(page),
>  			compound_mapcount(page));
> +
> +		if (page != compound_head(page) && page_ref_count(page) != 0)
> +			pr_warn("page:%px PROBLEM: non-zero refcount (==%d) on "
> +				"this tail page\n", page, page_ref_count(page));
> +	}
>  	else
>  		pr_warn("page:%px refcount:%d mapcount:%d mapping:%px index:%#lx\n",
>  			page, page_ref_count(page), mapcount,

I have a hunk in my current tree which looks like this:

@@ -77,6 +77,11 @@ void __dump_page(struct page *page, const char *reason)
                pr_warn("page:%px refcount:%d mapcount:%d mapping:%px index:%#lx
\n",
                        page, page_ref_count(page), mapcount,
                        page->mapping, page_to_pgoff(page));
+       if (PageTail(page)) {
+               struct page *head = compound_head(page);
+               pr_warn("head:%px mapping:%px index:%#lx\n",
+                       head, head->mapping, page_to_pgoff(head));
+       }
        if (PageKsm(page))
                pr_warn("ksm flags: %#lx(%pGp)\n", page->flags, &page->flags);
        else if (PageAnon(page))

I wonder if we can combine these two patches in some more useful way?

I also think we probably want a sanity check that 'head' and 'page'
are within a sane range of each other (ie head < page and head +
MAX_ORDER_NR_PAGES > page) to protect against a struct page that contains
complete garbage.