Re: [PATCH] Fixes for prep_zero_page

[Andrew Morton <akpm@osdl.org>]

Zwane Mwaikambo <zwane@arm.linux.org.uk> wrote:
>
> On Sun, 9 Jan 2005, Andrew Morton wrote:
> 
> > Well it's doing clear_highpage() before __alloc_pages() has called
> > kernel_map_pages(), so CONFIG_DEBUG_PAGEALLOC is quite kaput.
> > 
> > So the current __GFP_ZERO buglist is:
> > 
> > 1: Breaks CONFIG_DEBUG_PAGEALLOC
> > 
> > 2: Breaks the cache aliasing protection for anonymous pages
> > 
> > 3: prep_zero_page() uses KM_USER0 so __GFP_ZERO from IRQ context will
> >    cause rare memory corruption.
> 
> The following should take care of 1 and 3. I opted to unmap the pages 
> again after the clear page so that it remains isolated and we don't have 
> to make additional checks to see if we should unmap the pages.
> 
> Signed-off-by: Zwane Mwaikambo <zwane@arm.linux.org.uk>
> 
> Index: linux-2.6.10-mm2/include/linux/highmem.h
> ===================================================================
> RCS file: /home/cvsroot/linux-2.6.10-mm2/include/linux/highmem.h,v
> retrieving revision 1.1.1.1
> diff -u -p -B -r1.1.1.1 highmem.h
> --- linux-2.6.10-mm2/include/linux/highmem.h	9 Jan 2005 04:51:52 -0000	1.1.1.1
> +++ linux-2.6.10-mm2/include/linux/highmem.h	9 Jan 2005 15:32:17 -0000
> @@ -50,6 +50,18 @@ static inline void clear_highpage(struct
>  	kunmap_atomic(kaddr, KM_USER0);
>  }
>  
> +static inline void clear_irq_highpage(struct page *page)
> +{
> +	char *kaddr;
> +	unsigned long flags;
> +
> +	local_irq_save(flags);
> +	kaddr = kmap_atomic(page, KM_IRQ0);
> +	clear_page(kaddr);
> +	kunmap_atomic(kaddr, KM_IRQ0);
> +	local_irq_restore(flags);
> +}
> +

This won't work right if someone tries to allocate memory while holding a
KM_IRQ0 atomic kmap.

It would be quite bizarre for anyone to be allocating highmem pages from
IRQ context anyway, but as a generic mechanism this really should work as
expected in all contexts.  That means a new kmap slot.

>  /*
>   * Same but also flushes aliased cache contents to RAM.
>   */
> Index: linux-2.6.10-mm2/mm/page_alloc.c
> ===================================================================
> RCS file: /home/cvsroot/linux-2.6.10-mm2/mm/page_alloc.c,v
> retrieving revision 1.1.1.1
> diff -u -p -B -r1.1.1.1 page_alloc.c
> --- linux-2.6.10-mm2/mm/page_alloc.c	9 Jan 2005 04:52:40 -0000	1.1.1.1
> +++ linux-2.6.10-mm2/mm/page_alloc.c	9 Jan 2005 15:46:00 -0000
> @@ -691,10 +691,17 @@ perthread_pages_alloc(void)
>   */
>  static inline void prep_zero_page(struct page *page, int order)
>  {
> -	int i;
> +	int i, nr_pages = (1 << order);
> +	int context = in_interrupt();
>  
> -	for(i = 0; i < (1 << order); i++)
> -		clear_highpage(page + i);
> +	kernel_map_pages(page, nr_pages, 1);
> +	for(i = 0; i < nr_pages; i++) {
> +		if (likely(!context))
> +			clear_highpage(page + i);
> +		else
> +			clear_irq_highpage(page + i);
> +	}
> +	kernel_map_pages(page, nr_pages, 0);
>  }

Can't we simply move the page zeroing to the very end of __alloc_pages()?