Re: [RFC] print symbolic page flag names in bad_page()

[Mel Gorman <mel@csn.ul.ie>]

On Sun, Dec 06, 2009 at 11:46:36AM +0800, Wu Fengguang wrote:
> Hi Alex,
> 
> On Sat, Dec 05, 2009 at 05:29:48AM +0800, Alex Chiang wrote:
> [...]
> > Teach page-types the -k mode, which parses and describes the bits in
> > the internal kernel order:
> > 
> >   # ./Documentation/vm/page-types -k 0x4000
> >   0x0000000000004000  ______________H_________  compound_head
> [...]
> > The implication is that attempting to use page-types -k on a kernel
> > with different CONFIG_* settings may lead to surprising and misleading
> > results. To retain sanity, always use the page-types built out of the
> > kernel tree you are actually testing.
> 
> This is useful feature, however not as convenient if the kernel can
> print its page flag names directly :)
> (especially when the dmesg comes from some end user)
> 
> So how about this patch?

There is a neat way of declaring strings that are used for the symbolic
printing of flags in a bitfield. It's orientated around ftrace so not
immediately usable for printk. Maybe the approaches can be merged but
minimally, it'd be nice to match the declaration style. An example is
show_gfp_flags in include/event/trace/kmem.h

> ---
> 
> mm: introduce dump_page()
> 
> - introduce dump_page() to print the page info for debugging some error condition.
> - print an extra field: the symbolic names of page->flags
> - convert three mm users: bad_page(), print_bad_pte() and memory offline failure. 
> 
> Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
> ---
>  mm/internal.h       |    2 +
>  mm/memory.c         |    8 +---
>  mm/memory_hotplug.c |    6 +--
>  mm/page_alloc.c     |   75 +++++++++++++++++++++++++++++++++++++++---
>  4 files changed, 78 insertions(+), 13 deletions(-)
> 
> --- linux-mm.orig/mm/page_alloc.c	2009-12-06 10:11:08.000000000 +0800
> +++ linux-mm/mm/page_alloc.c	2009-12-06 11:22:58.000000000 +0800
> @@ -48,6 +48,7 @@
>  #include <linux/page_cgroup.h>
>  #include <linux/debugobjects.h>
>  #include <linux/kmemleak.h>
> +#include <linux/kernel-page-flags.h>
>  #include <trace/events/kmem.h>
>  
>  #include <asm/tlbflush.h>
> @@ -262,10 +263,7 @@ static void bad_page(struct page *page)
>  
>  	printk(KERN_ALERT "BUG: Bad page state in process %s  pfn:%05lx\n",
>  		current->comm, page_to_pfn(page));
> -	printk(KERN_ALERT
> -		"page:%p flags:%p count:%d mapcount:%d mapping:%p index:%lx\n",
> -		page, (void *)page->flags, page_count(page),
> -		page_mapcount(page), page->mapping, page->index);
> +	dump_page(page);
>  
>  	dump_stack();
>  out:
> @@ -5106,3 +5104,72 @@ bool is_free_buddy_page(struct page *pag
>  	return order < MAX_ORDER;
>  }
>  #endif
> +
> +static char *page_flag_names[] = {
> +	[KPF_LOCKED]		= "L:locked",
> +	[KPF_ERROR]		= "E:error",
> +	[KPF_REFERENCED]	= "R:referenced",
> +	[KPF_UPTODATE]		= "U:uptodate",
> +	[KPF_DIRTY]		= "D:dirty",
> +	[KPF_LRU]		= "l:lru",
> +	[KPF_ACTIVE]		= "A:active",
> +	[KPF_SLAB]		= "S:slab",
> +	[KPF_WRITEBACK]		= "W:writeback",
> +	[KPF_RECLAIM]		= "I:reclaim",
> +	[KPF_BUDDY]		= "B:buddy",
> +
> +	[KPF_MMAP]		= "M:mmap",
> +	[KPF_ANON]		= "a:anonymous",
> +	[KPF_SWAPCACHE]		= "s:swapcache",
> +	[KPF_SWAPBACKED]	= "b:swapbacked",
> +	[KPF_COMPOUND_HEAD]	= "H:compound_head",
> +	[KPF_COMPOUND_TAIL]	= "T:compound_tail",
> +	[KPF_HUGE]		= "G:huge",
> +	[KPF_UNEVICTABLE]	= "u:unevictable",
> +	[KPF_HWPOISON]		= "X:hwpoison",
> +	[KPF_NOPAGE]		= "n:nopage",
> +	[KPF_KSM]		= "V:shared",
> +
> +	[KPF_RESERVED]		= "r:reserved",
> +	[KPF_MLOCKED]		= "m:mlocked",
> +	[KPF_MAPPEDTODISK]	= "d:mappedtodisk",
> +	[KPF_PRIVATE]		= "P:private",
> +	[KPF_PRIVATE_2]		= "p:private_2",
> +	[KPF_OWNER_PRIVATE]	= "O:owner_private",
> +	[KPF_ARCH]		= "h:arch",
> +	[KPF_UNCACHED]		= "c:uncached",
> +};
> +
> +static char *page_flags_longname(struct page *page, char *buf, int buflen)
> +{
> +	int i, n;
> +	u64 flags;
> +
> +	flags = stable_page_flags(page);
> +

I don't see where this is defined. I assume there is a patch dependency
I'm not seeing.

> +	for (i = 0, n = 0; i < ARRAY_SIZE(page_flag_names); i++) {
> +		if (!page_flag_names[i])
> +			continue;

How it is possible to get flags that are not recognised? Surely that
that spark some sort of warning.

> +		if ((flags >> i) & 1)
> +			n += snprintf(buf + n, buflen - n, "%s,",
> +					page_flag_names[i] + 2);

Should that be | instead of , ?
Why page_flag_names[i] + 2?

> +	}
> +	if (n)
> +		n--;
> +	buf[n] = '\0';
> +
> +	return buf;
> +}

As the symbolic long name is always going to printk(), why buffer it?
It's slower to call printk() multiple times but it's hardly a
performance-critical path and it avoids the declaration of buf.

> +
> +void dump_page(struct page *page)
> +{
> +	char buf[1024];
> +

Ingo flagged this already.

> +	printk(KERN_ALERT
> +	"page:%p flags:%p(%s) count:%d mapcount:%d mapping:%p index:%lx\n",
> +		page, (void *)page->flags,
> +		page_flags_longname(page, buf, sizeof(buf)),

This can push things like count, mapcount and the like beyond the edge
of 80 columns. While it rarely matters, it can get truncated on serial
consoles depending on how they are being recorded. Can the string go to
a line of it's own?

> +		page_count(page), page_mapcount(page),
> +		page->mapping, page->index);
> +}
> +EXPORT_SYMBOL(dump_page);

Ingo pointed out that this is exported GPL but I'm confused as to why
it's exported at all. What module needs it?

> --- linux-mm.orig/mm/memory.c	2009-12-06 10:37:47.000000000 +0800
> +++ linux-mm/mm/memory.c	2009-12-06 10:57:25.000000000 +0800
> @@ -430,12 +430,8 @@ static void print_bad_pte(struct vm_area
>  		"BUG: Bad page map in process %s  pte:%08llx pmd:%08llx\n",
>  		current->comm,
>  		(long long)pte_val(pte), (long long)pmd_val(*pmd));
> -	if (page) {
> -		printk(KERN_ALERT
> -		"page:%p flags:%p count:%d mapcount:%d mapping:%p index:%lx\n",
> -		page, (void *)page->flags, page_count(page),
> -		page_mapcount(page), page->mapping, page->index);
> -	}
> +	if (page)
> +		dump_page(page);
>  	printk(KERN_ALERT
>  		"addr:%p vm_flags:%08lx anon_vma:%p mapping:%p index:%lx\n",
>  		(void *)addr, vma->vm_flags, vma->anon_vma, mapping, index);
> --- linux-mm.orig/mm/internal.h	2009-12-06 10:47:37.000000000 +0800
> +++ linux-mm/mm/internal.h	2009-12-06 11:03:53.000000000 +0800
> @@ -264,6 +264,8 @@ int __get_user_pages(struct task_struct 
>  #define ZONE_RECLAIM_SUCCESS	1
>  #endif
>  
> +void dump_page(struct page *page);
> +
>  extern int hwpoison_filter(struct page *p);
>  
>  extern u32 hwpoison_filter_dev_major;
> --- linux-mm.orig/mm/memory_hotplug.c	2009-12-06 11:01:20.000000000 +0800
> +++ linux-mm/mm/memory_hotplug.c	2009-12-06 11:01:23.000000000 +0800
> @@ -678,9 +678,9 @@ do_migrate_range(unsigned long start_pfn
>  			if (page_count(page))
>  				not_managed++;
>  #ifdef CONFIG_DEBUG_VM
> -			printk(KERN_INFO "removing from LRU failed"
> -					 " %lx/%d/%lx\n",
> -				pfn, page_count(page), page->flags);
> +			printk(KERN_INFO "removing pfn %lx from LRU failed\n",
> +				pfn);
> +			dump_page(page);

The message is printed at KERN_INFO and the dump at KERN_ALERT. Is this
intentional?

>  #endif
>  		}
>  	}
> 

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>