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Subject: Re: [PATCH v5 01/12] mm: dump_page(): better diagnostics for compound
 pages
To:     Matthew Wilcox <willy@infradead.org>
CC:     Andrew Morton <akpm@linux-foundation.org>,
        Al Viro <viro@zeniv.linux.org.uk>,
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From:   John Hubbard <jhubbard@nvidia.com>
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On 2/7/20 9:27 AM, Matthew Wilcox wrote:
...
> 
> A definite improvement, but I think we could do better.  For example,
> you've changed PageCompound to PageTail here, whereas we really do want
> to dump some more information for PageHead pages than the plain vanilla
> order-0 page has.  Another thing is that page_mapping() calls compound_head(),
> so if the page is corrupted, we're going to get a funky pointer dereference.
> 
> I spent a bit of time on this reimplementation ... what do you think?
> 

It looks fine to me. I gave it a quick spin, here's the output for a normal
and a huge page, and it has everything we want to see:

page:ffffea0010f0d640 refcount:1025 mapcount:1 mapping:0000000021857089 index:0xed
anon flags: 0x17ffe0000080036(referenced|uptodate|lru|active|swapbacked)
raw: 017ffe0000080036 ffffea0011731f08 ffffea0011730008 ffff8884777272c1
raw: 00000000000000ed 0000000000000000 0000040100000000 0000000000000000
page dumped because: testing dump_page()

page:ffffea0010ef1b80 head:ffffea0010ef0000 refcount:0 mapcount:1 mapping:00000000a8e1c7fa index:0xed order:9 compound_mapcount: 1
anon flags: 0x17ffe0000000000()
raw: 017ffe0000000000 ffffea0010ef0001 ffffea0010ef1b88 dead000000000400
raw: 0000000000000000 0000000000000000 00000000ffffffff 0000000000000000
head: 017ffe0000090036 ffffea0011734548 ffffea0010ef8008 ffff8884777271b9
head: 000000000000007f 0000000000000000 00000201ffffffff 0000000000000000
page dumped because: testing dump_page()


>  - Print the mapping pointer using %p insted of %px.  The actual value of
>    the pointer can be read out of the raw page dump and using %p gives a
>    chance to correlate it to earlier printk of the mapping pointer.
>  - Add the order of the page for compound pages
>  - Dump the raw head page as well as the raw page being dumped
> 
> diff --git a/mm/debug.c b/mm/debug.c
> index ecccd9f17801..0564d4cb8233 100644
> --- a/mm/debug.c
> +++ b/mm/debug.c
> @@ -44,8 +44,10 @@ const struct trace_print_flags vmaflag_names[] = {
>  
>  void __dump_page(struct page *page, const char *reason)
>  {
> +	struct page *head = compound_head(page);
>  	struct address_space *mapping;
>  	bool page_poisoned = PagePoisoned(page);
> +	bool compound = PageCompound(page);
>  	/*
>  	 * Accessing the pageblock without the zone lock. It could change to
>  	 * "isolate" again in the meantime, but since we are just dumping the
> @@ -66,25 +68,32 @@ void __dump_page(struct page *page, const char *reason)
>  		goto hex_only;
>  	}
>  
> -	mapping = page_mapping(page);
> +	if (page < head || (page >= head + MAX_ORDER_NR_PAGES)) {
> +		/* Corrupt page, cannot call page_mapping */
> +		mapping = page->mapping;
> +		head = page;
> +		compound = false;
> +	} else {
> +		mapping = page_mapping(page);
> +	}
>  
>  	/*
>  	 * Avoid VM_BUG_ON() in page_mapcount().
>  	 * page->_mapcount space in struct page is used by sl[aou]b pages to
>  	 * encode own info.
>  	 */
> -	mapcount = PageSlab(page) ? 0 : page_mapcount(page);
> +	mapcount = PageSlab(head) ? 0 : page_mapcount(head);
>  
> -	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 (compound)
> +		pr_warn("page:%px head:%px refcount:%d mapcount:%d mapping:%p "
> +			"index:%#lx order:%u compound_mapcount: %d\n",
> +			page, head, page_ref_count(page), mapcount,
>  			page->mapping, page_to_pgoff(page),
> -			compound_mapcount(page));
> +			compound_order(head), compound_mapcount(page));
>  	else
> -		pr_warn("page:%px refcount:%d mapcount:%d mapping:%px index:%#lx\n",
> +		pr_warn("page:%px refcount:%d mapcount:%d mapping:%p index:%#lx\n",
>  			page, page_ref_count(page), mapcount,
> -			page->mapping, page_to_pgoff(page));
> +			mapping, page_to_pgoff(page));
>  	if (PageKsm(page))
>  		type = "ksm ";
>  	else if (PageAnon(page))
> @@ -106,6 +115,10 @@ void __dump_page(struct page *page, const char *reason)
>  	print_hex_dump(KERN_WARNING, "raw: ", DUMP_PREFIX_NONE, 32,
>  			sizeof(unsigned long), page,
>  			sizeof(struct page), false);
> +	if (!page_poisoned && compound)
> +		print_hex_dump(KERN_WARNING, "head: ", DUMP_PREFIX_NONE, 32,
> +			sizeof(unsigned long), head,
> +			sizeof(struct page), false);


Good thought to get the hex dump of the head page in this case, yes.


>  
>  	if (reason)
>  		pr_warn("page dumped because: %s\n", reason);
> 


Seeing as how I want to further enhance dump_page() slightly for this series (to 
include the 3rd struct page's hpage_pincount), would you care to send this as a 
formal patch that I could insert into this series, to replace patch 5?


thanks,
-- 
John Hubbard
NVIDIA