Re: [PATCH v4 1/4] mm: introduce debug_pagealloc_map_pages() helper

[Vlastimil Babka <vbabka@suse.cz>]

On 11/3/20 5:20 PM, Mike Rapoport wrote:
> From: Mike Rapoport <rppt@linux.ibm.com>
> 
> When CONFIG_DEBUG_PAGEALLOC is enabled, it unmaps pages from the kernel
> direct mapping after free_pages(). The pages than need to be mapped back
> before they could be used. Theese mapping operations use
> __kernel_map_pages() guarded with with debug_pagealloc_enabled().
> 
> The only place that calls __kernel_map_pages() without checking whether
> DEBUG_PAGEALLOC is enabled is the hibernation code that presumes
> availability of this function when ARCH_HAS_SET_DIRECT_MAP is set.
> Still, on arm64, __kernel_map_pages() will bail out when DEBUG_PAGEALLOC is
> not enabled but set_direct_map_invalid_noflush() may render some pages not
> present in the direct map and hibernation code won't be able to save such
> pages.
> 
> To make page allocation debugging and hibernation interaction more robust,
> the dependency on DEBUG_PAGEALLOC or ARCH_HAS_SET_DIRECT_MAP has to be made
> more explicit.
> 
> Start with combining the guard condition and the call to
> __kernel_map_pages() into a single debug_pagealloc_map_pages() function to
> emphasize that __kernel_map_pages() should not be called without
> DEBUG_PAGEALLOC and use this new function to map/unmap pages when page
> allocation debug is enabled.
> 
> Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
> Reviewed-by: David Hildenbrand <david@redhat.com>
> Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>

Acked-by: Vlastimil Babka <vbabka@suse.cz>

But, the "enable" param is hideous. I would rather have map and unmap variants 
(and just did the same split for page poisoning) and this seems to be a good 
opportunity. If David didn't propose it already, I'm surprised ;)

> ---
>   include/linux/mm.h  | 10 ++++++++++
>   mm/memory_hotplug.c |  3 +--
>   mm/page_alloc.c     |  6 ++----
>   mm/slab.c           |  8 +++-----
>   4 files changed, 16 insertions(+), 11 deletions(-)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index ef360fe70aaf..1fc0609056dc 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -2936,12 +2936,22 @@ kernel_map_pages(struct page *page, int numpages, int enable)
>   {
>   	__kernel_map_pages(page, numpages, enable);
>   }
> +
> +static inline void debug_pagealloc_map_pages(struct page *page,
> +					     int numpages, int enable)
> +{
> +	if (debug_pagealloc_enabled_static())
> +		__kernel_map_pages(page, numpages, enable);
> +}
> +
>   #ifdef CONFIG_HIBERNATION
>   extern bool kernel_page_present(struct page *page);
>   #endif	/* CONFIG_HIBERNATION */
>   #else	/* CONFIG_DEBUG_PAGEALLOC || CONFIG_ARCH_HAS_SET_DIRECT_MAP */
>   static inline void
>   kernel_map_pages(struct page *page, int numpages, int enable) {}
> +static inline void debug_pagealloc_map_pages(struct page *page,
> +					     int numpages, int enable) {}
>   #ifdef CONFIG_HIBERNATION
>   static inline bool kernel_page_present(struct page *page) { return true; }
>   #endif	/* CONFIG_HIBERNATION */
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index b44d4c7ba73b..e2b6043a4428 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -614,8 +614,7 @@ void generic_online_page(struct page *page, unsigned int order)
>   	 * so we should map it first. This is better than introducing a special
>   	 * case in page freeing fast path.
>   	 */
> -	if (debug_pagealloc_enabled_static())
> -		kernel_map_pages(page, 1 << order, 1);
> +	debug_pagealloc_map_pages(page, 1 << order, 1);
>   	__free_pages_core(page, order);
>   	totalram_pages_add(1UL << order);
>   #ifdef CONFIG_HIGHMEM
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 23f5066bd4a5..9a66a1ff9193 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1272,8 +1272,7 @@ static __always_inline bool free_pages_prepare(struct page *page,
>   	 */
>   	arch_free_page(page, order);
>   
> -	if (debug_pagealloc_enabled_static())
> -		kernel_map_pages(page, 1 << order, 0);
> +	debug_pagealloc_map_pages(page, 1 << order, 0);
>   
>   	kasan_free_nondeferred_pages(page, order);
>   
> @@ -2270,8 +2269,7 @@ inline void post_alloc_hook(struct page *page, unsigned int order,
>   	set_page_refcounted(page);
>   
>   	arch_alloc_page(page, order);
> -	if (debug_pagealloc_enabled_static())
> -		kernel_map_pages(page, 1 << order, 1);
> +	debug_pagealloc_map_pages(page, 1 << order, 1);
>   	kasan_alloc_pages(page, order);
>   	kernel_poison_pages(page, 1 << order, 1);
>   	set_page_owner(page, order, gfp_flags);
> diff --git a/mm/slab.c b/mm/slab.c
> index b1113561b98b..340db0ce74c4 100644
> --- a/mm/slab.c
> +++ b/mm/slab.c
> @@ -1431,10 +1431,8 @@ static bool is_debug_pagealloc_cache(struct kmem_cache *cachep)
>   #ifdef CONFIG_DEBUG_PAGEALLOC
>   static void slab_kernel_map(struct kmem_cache *cachep, void *objp, int map)
>   {
> -	if (!is_debug_pagealloc_cache(cachep))
> -		return;
> -
> -	kernel_map_pages(virt_to_page(objp), cachep->size / PAGE_SIZE, map);
> +	debug_pagealloc_map_pages(virt_to_page(objp),
> +				  cachep->size / PAGE_SIZE, map);
>   }
>   
>   #else
> @@ -2062,7 +2060,7 @@ int __kmem_cache_create(struct kmem_cache *cachep, slab_flags_t flags)
>   
>   #if DEBUG
>   	/*
> -	 * If we're going to use the generic kernel_map_pages()
> +	 * If we're going to use the generic debug_pagealloc_map_pages()
>   	 * poisoning, then it's going to smash the contents of
>   	 * the redzone and userword anyhow, so switch them off.
>   	 */
>