makedumpfile: get_max_mapnr() from ELF header problem

[Michael Holzheu]

On Wed, 19 Mar 2014 07:14:25 +0000
Atsushi Kumagai <kumagai-atsushi at mxc.nes.nec.co.jp> wrote:

> >Hello Atsushi,
> >
> >I debugged my problem a bit further and tried to implement
> >a function that gets the maximum page frame number from the
> >Linux kernel memory management structures.
> >
> >I am no memory management expert, so the following patch probably
> >is not complete, but at least for my setup it worked.
> 
> The patch looks good for your case, but I don't think it's a proper
> approach for this problem.
> 
> Now, I think this is a problem of get_mm_sparsemem() in makedumpfile.
> To say in more detail, the problem is "wrong calculating the address
> of unused mem_map".
> 
> Looking at the log you sent, some addresses of mem_map corresponding
> to unused pages look invalid like below:
> 
> mem_map (256)
>   mem_map    : 80000c0002018
>   pfn_start  : 1000000
>   pfn_end    : 1010000
> mem_map (257)
>   mem_map    : 800001840400000
>   pfn_start  : 1010000
>   pfn_end    : 1020000
> ...
> mem_map (544)
>   mem_map    : a82400012f14fffc
>   pfn_start  : 2200000
>   pfn_end    : 2210000
> 
> ...(and more)
> 
> However, makedumpfile should calculate such unused mem_map addresses
> as 0(NOT_MEMMAP_ADDR). Actually it works as expected at least in my
> environment(x86_64):
> 
> ...
> mem_map (16)
>   mem_map    : 0
>   pfn_start  : 80000
>   pfn_end    : 88000
> mem_map (17)
>   mem_map    : 0
>   pfn_start  : 88000
>   pfn_end    : 90000
> ...
> 
> makedumpfile get the address from mem_section.section_mem_map,
> it will be initialized with zero: 
> 
> [CONFIG_SPARSEMEM_EXTREAM]
>   paging_init()
>     sparse_memory_present_with_active_regions()
>       memory_present()
>         sparse_index_init()
>           sparse_index_alloc()   // allocate mem_section with kzalloc()
> 
> makedumpfile assumes the value of unused mem_section will remain as 0,
> but I suspect this assumption may be broken in your environment.
> 

Hello Atshushi,

I noticed that my last patch was not complete. It only checked the
mem_section[] array for zero entries. But as you noticed, we also
have to check the section array that we get from the mem_section
entries.

So I updated the patch.

Michael
---
 makedumpfile.c |   22 +++++++++++++++++-----
 1 file changed, 17 insertions(+), 5 deletions(-)

--- a/makedumpfile.c
+++ b/makedumpfile.c
@@ -2690,11 +2690,14 @@ nr_to_section(unsigned long nr, unsigned
 {
 	unsigned long addr;
 
-	if (is_sparsemem_extreme())
+	if (is_sparsemem_extreme()) {
+		if (mem_sec[SECTION_NR_TO_ROOT(nr)] == 0)
+			return NOT_KV_ADDR;
 		addr = mem_sec[SECTION_NR_TO_ROOT(nr)] +
 		    (nr & SECTION_ROOT_MASK()) * SIZE(mem_section);
-	else
+	} else {
 		addr = SYMBOL(mem_section) + (nr * SIZE(mem_section));
+	}
 
 	if (!is_kvaddr(addr))
 		return NOT_KV_ADDR;
@@ -2778,10 +2781,19 @@ get_mm_sparsemem(void)
 	}
 	for (section_nr = 0; section_nr < num_section; section_nr++) {
 		section = nr_to_section(section_nr, mem_sec);
-		mem_map = section_mem_map_addr(section);
-		mem_map = sparse_decode_mem_map(mem_map, section_nr);
-		if (!is_kvaddr(mem_map))
+		if (section == NOT_KV_ADDR) {
 			mem_map = NOT_MEMMAP_ADDR;
+		} else {
+			mem_map = section_mem_map_addr(section);
+			if (mem_map == 0) {
+				mem_map = NOT_MEMMAP_ADDR;
+			} else {
+				mem_map = sparse_decode_mem_map(mem_map,
+								section_nr);
+				if (!is_kvaddr(mem_map))
+					mem_map = NOT_MEMMAP_ADDR;
+			}
+		}
 		pfn_start = section_nr * PAGES_PER_SECTION();
 		pfn_end   = pfn_start + PAGES_PER_SECTION();
 		if (info->max_mapnr < pfn_end)