Re: [uml-devel] [UML] fix crash in block layer

[Blaisorblade <blaisorblade@yahoo.it>]

On Friday 16 February 2007 18:02, Jason Lunz wrote:
> On Thu, Feb 15, 2007 at 08:29:28PM +0100, Blaisorblade wrote:
> > The following could be a suggestion, if max_low_pfn is not used between
> > the old and the new moment of assignment (and it seems it is not). This
> > is just an idea however:

> blk_queue_bounce_limit() calls init_emergency_isa_pool() to get dma-zone
> pages to use as bounce buffers when its caller passes a dma_addr limit
> that is less than max_low_pfn. The BLK_BOUNCE_ANY macro is supposed to
> mean "never bounce", and it's defined as:

> #define BLK_BOUNCE_ANY          ((u64)blk_max_pfn << PAGE_SHIFT)

> So it presumes that max_pfn >= max_low_pfn. uml's mem_init() is
> violating this assumption - when uml_reserved is subtracted from
> max_pfn, we end up with max_pfn < max_low_pfn, so BLK_BOUNCE_ANY has the
> opposite of the intended effect. blk_queue_bounce_limit therefore tries
> to create a mempool with zone-dma pages on a no-dma-zone arch and the
> kernel goes BUG().

Thanks for the explaination; but I meant that the meaning of those variables 
and the code to set them was convoluted.

> So I think your idea is correct. It passes my testing - I can still use
> lvm within uml. I have not tested CONFIG_HIGHMEM, but here's an 
> implementation against 2.6.20.

Do not worry for HIGHMEM - I doubt it builds currently; uml_reserved is also 
used for other stuff.

I've also verified that moving the assignment to max_low_pfn to later cannot 
possibly introduce bugs, as max_{low_,}pfn are not used by core kernel code.

> Jeff, please drop my other patch and use this one.

> Signed-off-by: Jason Lunz <lunz@falooley.org>
>
> ---
>  arch/um/kernel/mem.c |    3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
>
> Index: linux-2.6.20-uml/arch/um/kernel/mem.c
> ===================================================================
> --- linux-2.6.20-uml.orig/arch/um/kernel/mem.c
> +++ linux-2.6.20-uml/arch/um/kernel/mem.c
> @@ -63,8 +63,6 @@
>
>  void mem_init(void)
>  {
> -	max_low_pfn = (high_physmem - uml_physmem) >> PAGE_SHIFT;
> -
>          /* clear the zero-page */
>          memset((void *) empty_zero_page, 0, PAGE_SIZE);
>
> @@ -79,6 +77,7 @@
>
>  	/* this will put all low memory onto the freelists */
>  	totalram_pages = free_all_bootmem();
> +	max_low_pfn = totalram_pages;
>  #ifdef CONFIG_HIGHMEM
>  	totalhigh_pages = highmem >> PAGE_SHIFT;
>  	totalram_pages += totalhigh_pages;

-- 
Inform me of my mistakes, so I can add them to my list!
Paolo Giarrusso, aka Blaisorblade
http://www.user-mode-linux.org/~blaisorblade
Chiacchiera con i tuoi amici in tempo reale! 
 http://it.yahoo.com/mail_it/foot/*http://it.messenger.yahoo.com 


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