Subject: Re: [PATCH 2/4] powerpc: Prepare to support kernel handling of IOMMU map/unmap

From: Alexey Kardashevskiy <[email-masked]>
Date: Mon, 17 Jun 2013 19:17:35 +1000

On 06/16/2013 02:26 PM, Benjamin Herrenschmidt wrote:
>> +#if defined(CONFIG_SPARSEMEM_VMEMMAP) || defined(CONFIG_FLATMEM)
>> +int realmode_get_page(struct page *page)
>> +{
>> +	if (PageCompound(page))
>> +		return -EAGAIN;
>> +
>> +	get_page(page);
>> +
>> +	return 0;
>> +}
>> +EXPORT_SYMBOL_GPL(realmode_get_page);
>> +
>> +int realmode_put_page(struct page *page)
>> +{
>> +	if (PageCompound(page))
>> +		return -EAGAIN;
>> +
>> +	if (!atomic_add_unless(&page->_count, -1, 1))
>> +		return -EAGAIN;
>> +
>> +	return 0;
>> +}
>> +EXPORT_SYMBOL_GPL(realmode_put_page);
>> +#endif
> 
> Several worries here, mostly that if the generic code ever changes
> (something gets added to get_page() that makes it no-longer safe for use
> in real mode for example, or some other condition gets added to
> put_page()), we go out of sync and potentially end up with very hard and
> very subtle bugs.
> 
> It might be worth making sure that:
> 
>  - This is reviewed by some generic VM people (and make sure they
> understand why we need to do that)
> 
>  - A comment is added to get_page() and put_page() to make sure that if
> they are changed in any way, dbl check the impact on our
> realmode_get_page() (or "ping" us to make sure things are still ok).

After changing get_page() to get_page_unless_zero(), the get_page API I use is:
get_page_unless_zero() - basically atomic_inc_not_zero()
atomic_add_unless() - just operated with the counter
PageCompound() - check if it is a huge page.

No usage of get_page or put_page.

If any of those changes, I would expect it to hit us immediately, no?

So it may only make sense to add a comment to PageCompound(). But the
comment says "PageCompound is generally not used in hot code paths", and
our path is hot. Heh.

diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index 6d53675..c70a654 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -329,7 +329,8 @@ static inline void set_page_writeback(struct page *page)
  * System with lots of page flags available. This allows separate
  * flags for PageHead() and PageTail() checks of compound pages so that bit
  * tests can be used in performance sensitive paths. PageCompound is
- * generally not used in hot code paths.
+ * generally not used in hot code paths except arch/powerpc/mm/init_64.c
+ * which uses it to detect huge pages and avoid handling those in real mode.
  */
 __PAGEFLAG(Head, head) CLEARPAGEFLAG(Head, head)
 __PAGEFLAG(Tail, tail)

So?

-- 
Alexey