linux-kernel - Re: Bootmem allocator broken

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Message-ID: <8763q3xj0p.fsf@skyscraper.fehenstaub.lan>
Date:	Fri, 15 Aug 2008 01:40:38 +0200
From:	Johannes Weiner <hannes@...urebad.de>
To:	Mikulas Patocka <mpatocka@...hat.com>
Cc:	David Miller <davem@...emloft.net>, sparclinux@...r.kernel.org,
	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	torvalds@...ux-foundation.org
Subject: Re: Bootmem allocator broken

Hi Mikulas,

Mikulas Patocka <mpatocka@...hat.com> writes:

> Examining the problem further, it turned out that Johannes Weiner 
> committed new bootmem allocator to 2.6.27-rc1 and the allocator is broken.
>
> 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".
>
> 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.

Yes, taking last_end_off into account when it's page-aligned is bogus as
the whole merging thing is about partial pages.

Cool spot and nice fix!

> Signed-off-by: Mikulas Patocka <mpatocka@...hat.com>

Acked-by: Johannes Weiner <hannes@...urebad.de>

	Hannes

> ---
>  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
--
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