linux-kernel - Re: [PATCH] generic debug pagealloc

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Date:	Tue, 3 Mar 2009 13:36:10 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Akinobu Mita <akinobu.mita@...il.com>
Cc:	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [PATCH] generic debug pagealloc

On Wed, 4 Mar 2009 01:01:04 +0900
Akinobu Mita <akinobu.mita@...il.com> wrote:

> +static void unpoison_page(struct page *page)
> +{
> +	unsigned char *mem;
> +	int i;
> +
> +	if (!page->poison)
> +		return;
> +
> +	mem = kmap_atomic(page, KM_USER0);
> +	for (i = 0; i < PAGE_SIZE; i++) {
> +		if (mem[i] != PAGE_POISON) {
> +			dump_broken_mem(mem);
> +			break;
> +		}
> +	}
> +	kunmap_atomic(mem, KM_USER0);
> +	page->poison = false;
> +}
> +
> +static void unpoison_pages(struct page *page, int n)
> +{
> +	int i;
> +
> +	for (i = 0; i < n; i++)
> +		unpoison_page(page + i);
> +}
> +
> +void kernel_map_pages(struct page *page, int numpages, int enable)
> +{
> +	if (!debug_pagealloc_enabled)
> +		return;
> +
> +	if (enable)
> +		unpoison_pages(page, numpages);
> +	else
> +		poison_pages(page, numpages);
> +}

kernel_map_pages() is called from the memory-allocation and
memory-freeing paths.  Hence it can be called from interrupt contexts.

KM_USER0 must not be used from interrupt context - it will corrupt the
non-interrupt context's pte, causing unpleasing very hard to track down
memory corruption.  Often memory which is getting written to the user's
disk.  This makes users unhappy.

We could use KM_IRQ0 here.  The code should disable local interrupts
when holding a KM_IRQ0 kmap.

If this code were to switch to using KM_IRQ0 then we still have a
problem - if any other place in the kernel does a memory allcoation or
free while holding a KM_IRQ0 kmap, then this new code will corrupt the
caller's pte.

So I guess we'll need to create a new kmap_atomic slot for this
application.  It will need interrupt protection - the page allocator can
be called from interrupt context while it is already running in
non-interrupt context.

Alternatively, we could just not do the kmap_atomic() at all.  i386
won't be using this code and IIRC the only other highmem architecture
is powerpc32, and ppc32 appears to also have its own DEBUG_PAGEALLOC
implementation.  So you could remove the kmap_atomic() stuff and put

#ifdef CONFIG_HIGHMEM
#error i goofed
#endif

in there.
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