Re: Bootmem allocator broken

[David Miller <davem@davemloft.net>]

From: Mikulas Patocka <mpatocka@redhat.com>
Date: Thu, 14 Aug 2008 19:11:19 -0400 (EDT)

[ Adding Alexander back to the CC: as he is seeing this same
  exact bug too, please keep him in the loop for testing. ]

> So I tried the patch and found out that the corruption happens in 
> setup_command_line --- the first strcpy call corrupted the migratetype 
> map.
> 
> Examining the problem further, it turned out that Johannes Weiner 
> committed new bootmem allocator to 2.6.27-rc1 and the allocator is broken.

Ok, I was just looking at Alexander's debugging dump from my patch
and in his case it pointed to kernel_physical_mapping_init() and
I couldn't find any obvious problems there.

But with the bug you found, it makes perfect sense, nice work!

> This is the minimal sequence that jams the allocator:
> 
> void *p, *q, *r;
> p = alloc_bootmem(PAGE_SIZE);
> q = alloc_bootmem(64);
> free_bootmem(p, PAGE_SIZE);
> p = alloc_bootmem(PAGE_SIZE);
> r = alloc_bootmem(64);
> 
> --- after this sequence (assuming that the allocator was empty or 
> page-aligned before), pointer "q" will be equal to pointer "r".

Excellent detective work!

> What's hapenning inside the allocator:
> p = alloc_bootmem(PAGE_SIZE);
> in allocator: last_end_off == PAGE_SIZE, bitmap contains bits 10000...
> q = alloc_bootmem(64);
> in allocator: last_end_off == PAGE_SIZE + 64, bitmap contains 11000...
> free_bootmem(p, PAGE_SIZE);
> in allocator: last_end_off == PAGE_SIZE + 64, bitmap contains 01000...
> p = alloc_bootmem(PAGE_SIZE);
> in allocator: last_end_off == PAGE_SIZE, bitmap contains 11000...
> r = alloc_bootmem(64);
> and now:
> it finds bit "2", as a place where to allocate (sidx)
> it hits the condition
> if (bdata->last_end_off && PFN_DOWN(bdata->last_end_off) + 1 == sidx))
> start_off = ALIGN(bdata->last_end_off, align);
> --- you can see that the condition is true, so it assigns start_off = 
> ALIGN(bdata->last_end_off, align); --- that is PAGE_SIZE --- and allocates 
> over already allocated block.
> 
> This patch fixes it (kernels 2.6.27-rc2 and 2.6.27-rc3 boot ok after the 
> patch). Johannes, please review the patch and submit it to Linus.
> 
> With the patch it tries to continue at the end of previous allocation only 
> if the previous allocation ended in the middle of the page.
> 
> Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
>
> ---
>  mm/bootmem.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> Index: linux-2.6.27-rc2-orig/mm/bootmem.c
> ===================================================================
> --- linux-2.6.27-rc2-orig.orig/mm/bootmem.c	2008-08-15 00:10:38.000000000 +0200
> +++ linux-2.6.27-rc2-orig/mm/bootmem.c	2008-08-15 00:10:53.000000000 +0200
> @@ -473,7 +473,7 @@ find_block:
>  				goto find_block;
>  			}
>  
> -		if (bdata->last_end_off &&
> +		if (bdata->last_end_off & (PAGE_SIZE - 1) &&
>  				PFN_DOWN(bdata->last_end_off) + 1 == sidx)
>  			start_off = ALIGN(bdata->last_end_off, align);
>  		else
> 
>