2.5.55/.56 instant reboot problem on 486

My '94 vintage 486 has problems booting 2.5.55 and 2.5.56. When it fails, the boot gets to loading the kernel and printing "Ok, booting the kernel.". Then there is a short pause (line a tenth of a second) and the machine reboots.

After doing a binary search with "for(;;);" statements (printk doesn't work this early) I found that the reboot occurs in arch/i386/mm/init.c:kernel_physical_mapping_init():
(start_kernel() -> setup_arch() -> paging_init() -> pagetable_init() -> kernel_physical_mapping_init())

diff -ruNp linux-2.5.55/arch/i386/mm/init.c linux-2.5.55.hack/arch/i386/mm/init.c
@@ -134,6 +134,7 @@ static void __init kernel_physical_mappi
 	pgd = pgd_base + pgd_ofs;
 	pfn = 0;
 
+	//for(;;);
 	for (; pgd_ofs < PTRS_PER_PGD; pgd++, pgd_ofs++) {
 		pmd = one_md_table_init(pgd);
 		if (pfn >= max_low_pfn)

@@ -151,6 +152,7 @@ static void __init kernel_physical_mappi
 			}
 		}
 	}	
+	for(;;);
 }
 
 static inline int page_kills_ppro(unsigned long pagenr)

If I uncomment the first "//for(;;);" the kernel hangs, but if I keep it commented out, the kernel reboots -- i.e. it doesn't get to the final "for(;;);" at the end of the function.

The problem is apparently related to the size of the kernel. With gcc-2.95.3 and my normal config for this machine, size vmlinux is

text data bss dec hex filename
1330953 109008 125656 1565617 17e3b1 vmlinux

and the kernel reboots. If I alter the size by changing some irrelevant config option (like disabling INPUT_MOUSEDEV or enabling KALLSYMS), the reboot problem doesn't occur.

With gcc-3.2 the bug disappears, but only because gcc-3.2 generates a much larger code segment. If I remove some driver & fs config options, the vmlinux size becomes almost the same as above, and the reboot bug appears again.

The same kernel that fails on the 486 boots Ok on my newer test boxes, so the problem is either 486-specific, related to the actual memory size, or the BIOS memory size reporting method (the 486 uses int 15 0x88); here's what 2.5.54 says:

BIOS-provided physical RAM map:
BIOS-88: 0000000000000000 - 000000000009f000 (usable)
BIOS-88: 0000000000100000 - 0000000001c00000 (usable)

The 486 has no known HW problems, and it survives memtest86.

/Mikael

Should I take it that 2.5.54 works? Or you haven't tested?

Ho humm.. Sounds like the non-PSE case is broken. Which should probably mean that even newer CPU's should show the same thing if we boot with "mem=nopentium". Can you verify that with your other machine that otherwise boots the same kernel fine?

That's bizarre. Especially the fact that a _smaller_ kernel has problems, but a biger one does not.

That looks like a perfectly fine memory map, even if 28MB or memory sounds a bit strange.

Linus

The problem is that one_page_table_init() pulls the rug out from under the kernel by installing a new page table before setting it up. A 486 has a small TLB so any miss will cause a triple fault and reset. Try this patch and see if it fixes it.

--
Brian Gerst

diff -urN linux-2.5.56/arch/i386/mm/init.c linux/arch/i386/mm/init.c
@@ -71,12 +71,16 @@
  */
 static pte_t * __init one_page_table_init(pmd_t *pmd)
 {
-	pte_t *page_table = (pte_t *) alloc_bootmem_low_pages(PAGE_SIZE);
-	set_pmd(pmd, __pmd(__pa(page_table) | _PAGE_TABLE));
-	if (page_table != pte_offset_kernel(pmd, 0))
-		BUG();	
+	if (pmd_none(*pmd)) {
+		pte_t *page_table = (pte_t *) alloc_bootmem_low_pages(PAGE_SIZE);
+		set_pmd(pmd, __pmd(__pa(page_table) | _PAGE_TABLE));
+		if (page_table != pte_offset_kernel(pmd, 0))
+			BUG();	
 
-	return page_table;
+		return page_table;
+	}
+	
+	return pte_offset_kernel(pmd, 0);
 }
 
 /*

As someone who uses 486's for routers on occasion, thank you!

bill davidsen <[email protected]>
 CTO, TMR Associates, Inc
Doing interesting things with little computers since 1979.

diff -urN linux-2.5.56/arch/i386/mm/init.c linux/arch/i386/mm/init.c
@@ -71,12 +71,16 @@
  */
 static pte_t * __init one_page_table_init(pmd_t *pmd)
 {
-	pte_t *page_table = (pte_t *) alloc_bootmem_low_pages(PAGE_SIZE);
-	set_pmd(pmd, __pmd(__pa(page_table) | _PAGE_TABLE));
-	if (page_table != pte_offset_kernel(pmd, 0))
-		BUG();	
+	if (pmd_none(*pmd)) {
+		pte_t *page_table = (pte_t *) alloc_bootmem_low_pages(PAGE_SIZE);
+		set_pmd(pmd, __pmd(__pa(page_table) | _PAGE_TABLE));
+		if (page_table != pte_offset_kernel(pmd, 0))
+			BUG();	
 
-	return page_table;
+		return page_table;
+	}
+	
+	return pte_offset_kernel(pmd, 0);
 }
 
 /*