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

From: Mike Rapoport <[email-masked]>
Date: Thu, 5 Nov 2020 13:31:54 +0200

On Wed, Nov 04, 2020 at 06:35:50PM +0100, Vlastimil Babka wrote:
> On 11/3/20 5:20 PM, Mike Rapoport wrote:
> > From: Mike Rapoport <[email-masked]>
> > 
> > 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 <[email-masked]>
> > Reviewed-by: David Hildenbrand <[email-masked]>
> > Acked-by: Kirill A. Shutemov <[email-masked]>
> 
> Acked-by: Vlastimil Babka <[email-masked]>
> 
> 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 ;)

I'm ok with map and unmap, and no, David didn't propose it already :)

> > ---
> >   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.
> >   	 */
> > 
> 

-- 
Sincerely yours,
Mike.