Re: [RFC PATCH 3/4] mm/page_alloc: introduce __GFP_PTE_MAPPED flag to allocate pte-mapped pages

[Edgecombe, Rick P]

On Mon, 2021-08-23 at 16:25 +0300, Mike Rapoport wrote:
> From: Mike Rapoport <rppt@xxxxxxxxxxxxx>
> 
> When __GFP_PTE_MAPPED flag is passed to an allocation request of
> order 0,
> the allocated page will be mapped at PTE level in the direct map.
> 
> To reduce the direct map fragmentation, maintain a cache of 4K pages
> that
> are already mapped at PTE level in the direct map. Whenever the cache
> should be replenished, try to allocate 2M page and split it to 4K
> pages
> to localize shutter of the direct map. If the allocation of 2M page
> fails,
> fallback to a single page allocation at expense of the direct map
> fragmentation.
> 
> The cache registers a shrinker that releases free pages from the
> cache to
> the page allocator.
> 
> The __GFP_PTE_MAPPED and caching of 4K pages are enabled only if an
> architecture selects ARCH_WANTS_PTE_MAPPED_CACHE in its Kconfig.
> 
> [
> cache management are mostly copied from 
> 
https://lore.kernel.org/lkml/20210505003032.489164-4-rick.p.edgecombe@xxxxxxxxx/
> ]
> 
> Signed-off-by: Mike Rapoport <rppt@xxxxxxxxxxxxx>
> ---
>  arch/Kconfig                    |   8 +
>  arch/x86/Kconfig                |   1 +
>  include/linux/gfp.h             |  11 +-
>  include/linux/mm.h              |   2 +
>  include/linux/pageblock-flags.h |  26 ++++
>  init/main.c                     |   1 +
>  mm/internal.h                   |   3 +-
>  mm/page_alloc.c                 | 261
> +++++++++++++++++++++++++++++++-
>  8 files changed, 309 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/Kconfig b/arch/Kconfig
> index 129df498a8e1..2db95331201b 100644
> --- a/arch/Kconfig
> +++ b/arch/Kconfig
> @@ -243,6 +243,14 @@ config ARCH_HAS_SET_MEMORY
>  config ARCH_HAS_SET_DIRECT_MAP
>  	bool
>  
> +#
> +# Select if the architecture wants to minimize fragmentation of its
> +# direct/linear map cauesd by set_memory and set_direct_map
> operations
> +#
> +config ARCH_WANTS_PTE_MAPPED_CACHE
> +	bool
> +	depends on ARCH_HAS_SET_MEMORY || ARCH_HAS_SET_DIRECT_MAP
> +
>  #
>  # Select if the architecture provides the arch_dma_set_uncached
> symbol to
>  # either provide an uncached segment alias for a DMA allocation, or
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index 88fb922c23a0..9b4e6cf4a6aa 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -118,6 +118,7 @@ config X86
>  	select ARCH_WANTS_NO_INSTR
>  	select ARCH_WANT_HUGE_PMD_SHARE
>  	select ARCH_WANT_LD_ORPHAN_WARN
> +	select ARCH_WANTS_PTE_MAPPED_CACHE
>  	select ARCH_WANTS_THP_SWAP		if X86_64
>  	select BUILDTIME_TABLE_SORT
>  	select CLKEVT_I8253
> diff --git a/include/linux/gfp.h b/include/linux/gfp.h
> index 55b2ec1f965a..c9006e3c67ad 100644
> --- a/include/linux/gfp.h
> +++ b/include/linux/gfp.h
> @@ -55,8 +55,9 @@ struct vm_area_struct;
>  #define ___GFP_ACCOUNT		0x400000u
>  #define ___GFP_ZEROTAGS		0x800000u
>  #define ___GFP_SKIP_KASAN_POISON	0x1000000u
> +#define ___GFP_PTE_MAPPED	0x2000000u
>  #ifdef CONFIG_LOCKDEP
> -#define ___GFP_NOLOCKDEP	0x2000000u
> +#define ___GFP_NOLOCKDEP	0x4000000u
>  #else
>  #define ___GFP_NOLOCKDEP	0
>  #endif
> @@ -101,12 +102,18 @@ struct vm_area_struct;
>   * node with no fallbacks or placement policy enforcements.
>   *
>   * %__GFP_ACCOUNT causes the allocation to be accounted to kmemcg.
> + *
> + * %__GFP_PTE_MAPPED returns a page that is mapped with PTE in the
> + * direct map. On architectures that use higher page table levels to
> map
> + * physical memory, this flag will casue split of large pages in the
> direct
> + * mapping. Has effect only if CONFIG_ARCH_WANTS_PTE_MAPPED_CACHE is
> set.
>   */
>  #define __GFP_RECLAIMABLE ((__force gfp_t)___GFP_RECLAIMABLE)
>  #define __GFP_WRITE	((__force gfp_t)___GFP_WRITE)
>  #define __GFP_HARDWALL   ((__force gfp_t)___GFP_HARDWALL)
>  #define __GFP_THISNODE	((__force gfp_t)___GFP_THISNODE)
>  #define __GFP_ACCOUNT	((__force gfp_t)___GFP_ACCOUNT)
> +#define __GFP_PTE_MAPPED ((__force gfp_t)___GFP_PTE_MAPPED)
>  
>  /**
>   * DOC: Watermark modifiers
> @@ -249,7 +256,7 @@ struct vm_area_struct;
>  #define __GFP_NOLOCKDEP ((__force gfp_t)___GFP_NOLOCKDEP)
>  
>  /* Room for N __GFP_FOO bits */
> -#define __GFP_BITS_SHIFT (25 + IS_ENABLED(CONFIG_LOCKDEP))
> +#define __GFP_BITS_SHIFT (26 + IS_ENABLED(CONFIG_LOCKDEP))
>  #define __GFP_BITS_MASK ((__force gfp_t)((1 << __GFP_BITS_SHIFT) -
> 1))
>  
>  /**
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 7ca22e6e694a..350ec98b82d2 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -3283,5 +3283,7 @@ static inline int seal_check_future_write(int
> seals, struct vm_area_struct *vma)
>  	return 0;
>  }
>  
> +void pte_mapped_cache_init(void);
> +
>  #endif /* __KERNEL__ */
>  #endif /* _LINUX_MM_H */
> diff --git a/include/linux/pageblock-flags.h
> b/include/linux/pageblock-flags.h
> index 973fd731a520..4faf8c542b00 100644
> --- a/include/linux/pageblock-flags.h
> +++ b/include/linux/pageblock-flags.h
> @@ -21,6 +21,8 @@ enum pageblock_bits {
>  			/* 3 bits required for migrate types */
>  	PB_migrate_skip,/* If set the block is skipped by compaction */
>  
> +	PB_pte_mapped, /* If set the block is mapped with PTEs in
> direct map */
> +
>  	/*
>  	 * Assume the bits will always align on a word. If this
> assumption
>  	 * changes then get/set pageblock needs updating.
> @@ -88,4 +90,28 @@ static inline void set_pageblock_skip(struct page
> *page)
>  }
>  #endif /* CONFIG_COMPACTION */
>  
> +#ifdef CONFIG_ARCH_WANTS_PTE_MAPPED_CACHE
> +#define get_pageblock_pte_mapped(page)				
> \
> +	get_pfnblock_flags_mask(page, page_to_pfn(page),	\
> +			(1 << (PB_pte_mapped)))
> +#define clear_pageblock_pte_mapped(page) \
> +	set_pfnblock_flags_mask(page, 0, page_to_pfn(page),	\
> +			(1 << PB_pte_mapped))
> +#define set_pageblock_pte_mapped(page) \
> +	set_pfnblock_flags_mask(page, (1 << PB_pte_mapped),	\
> +			page_to_pfn(page),			\
> +			(1 << PB_pte_mapped))
> +#else /* CONFIG_ARCH_WANTS_PTE_MAPPED_CACHE */
> +static inline bool get_pageblock_pte_mapped(struct page *page)
> +{
> +	return false;
> +}
> +static inline void clear_pageblock_pte_mapped(struct page *page)
> +{
> +}
> +static inline void set_pageblock_pte_mapped(struct page *page)
> +{
> +}
> +#endif /* CONFIG_ARCH_WANTS_PTE_MAPPED_CACHE */
> +
>  #endif	/* PAGEBLOCK_FLAGS_H */
> diff --git a/init/main.c b/init/main.c
> index f5b8246e8aa1..c0d59a183a39 100644
> --- a/init/main.c
> +++ b/init/main.c
> @@ -828,6 +828,7 @@ static void __init mm_init(void)
>  	page_ext_init_flatmem_late();
>  	kmem_cache_init();
>  	kmemleak_init();
> +	pte_mapped_cache_init();
>  	pgtable_init();
>  	debug_objects_mem_init();
>  	vmalloc_init();
> diff --git a/mm/internal.h b/mm/internal.h
> index 31ff935b2547..0557ece6ebf4 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -24,7 +24,8 @@
>  			__GFP_ATOMIC)
>  
>  /* The GFP flags allowed during early boot */
> -#define GFP_BOOT_MASK (__GFP_BITS_MASK &
> ~(__GFP_RECLAIM|__GFP_IO|__GFP_FS))
> +#define GFP_BOOT_MASK (__GFP_BITS_MASK &
> ~(__GFP_RECLAIM|__GFP_IO|__GFP_FS|\
> +					   __GFP_PTE_MAPPED))
>  
>  /* Control allocation cpuset and node placement constraints */
>  #define GFP_CONSTRAINT_MASK (__GFP_HARDWALL|__GFP_THISNODE)
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 856b175c15a4..7936d8dcb80b 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -72,6 +72,7 @@
>  #include <linux/padata.h>
>  #include <linux/khugepaged.h>
>  #include <linux/buffer_head.h>
> +#include <linux/set_memory.h>
>  #include <asm/sections.h>
>  #include <asm/tlbflush.h>
>  #include <asm/div64.h>
> @@ -107,6 +108,14 @@ typedef int __bitwise fpi_t;
>   */
>  #define FPI_TO_TAIL		((__force fpi_t)BIT(1))
>  
> +/*
> + * Free page directly to the page allocator rather than check if it
> should
> + * be placed into the cache of pte_mapped pages.
> + * Used by the pte_mapped cache shrinker.
> + * Has effect only when pte-mapped cache is enabled
> + */
> +#define FPI_NO_PTE_MAP		((__force fpi_t)BIT(2))
> +
>  /*
>   * Don't poison memory with KASAN (only for the tag-based modes).
>   * During boot, all non-reserved memblock memory is exposed to
> page_alloc.
> @@ -225,6 +234,19 @@ static inline void
> set_pcppage_migratetype(struct page *page, int migratetype)
>  	page->index = migratetype;
>  }
>  
> +#ifdef CONFIG_ARCH_WANTS_PTE_MAPPED_CACHE
> +static struct page *alloc_page_pte_mapped(gfp_t gfp);
> +static void free_page_pte_mapped(struct page *page);
> +#else
> +static inline struct page *alloc_page_pte_mapped(gfp_t gfp)
> +{
> +	return NULL;
> +}
> +static void free_page_pte_mapped(struct page *page)
> +{
> +}
> +#endif
> +
>  #ifdef CONFIG_PM_SLEEP
>  /*
>   * The following functions are used by the suspend/hibernate code to
> temporarily
> @@ -536,7 +558,7 @@ void set_pfnblock_flags_mask(struct page *page,
> unsigned long flags,
>  	unsigned long bitidx, word_bitidx;
>  	unsigned long old_word, word;
>  
> -	BUILD_BUG_ON(NR_PAGEBLOCK_BITS != 4);
> +	BUILD_BUG_ON(NR_PAGEBLOCK_BITS != 5);
>  	BUILD_BUG_ON(MIGRATE_TYPES > (1 << PB_migratetype_bits));
>  
>  	bitmap = get_pageblock_bitmap(page, pfn);
> @@ -1352,6 +1374,16 @@ static __always_inline bool
> free_pages_prepare(struct page *page,
>  					   PAGE_SIZE << order);
>  	}
>  
> +	/*
> +	 * Unless are we shrinking pte_mapped cache return a page from
> +	 * a pageblock mapped with PTEs to that cache.
> +	 */
> +	if (!order && !(fpi_flags & FPI_NO_PTE_MAP) &&
> +	    get_pageblock_pte_mapped(page)) {
> +		free_page_pte_mapped(page);
> +		return false;
> +	}
> +
>  	kernel_poison_pages(page, 1 << order);
>  
>  	/*
> @@ -3445,6 +3477,13 @@ void free_unref_page_list(struct list_head
> *list)
>  	/* Prepare pages for freeing */
>  	list_for_each_entry_safe(page, next, list, lru) {
>  		pfn = page_to_pfn(page);
> +
> +		if (get_pageblock_pte_mapped(page)) {
> +			list_del(&page->lru);
> +			free_page_pte_mapped(page);
> +			continue;
> +		}
> +
>  		if (!free_unref_page_prepare(page, pfn, 0))
>  			list_del(&page->lru);
>  
> @@ -5381,6 +5420,12 @@ struct page *__alloc_pages(gfp_t gfp, unsigned
> int order, int preferred_nid,
>  			&alloc_gfp, &alloc_flags))
>  		return NULL;
>  
> +	if ((alloc_gfp & __GFP_PTE_MAPPED) && order == 0) {
> +		page = alloc_page_pte_mapped(alloc_gfp);
> +		if (page)
> +			goto out;
> +	}
> +
>  	/*
>  	 * Forbid the first pass from falling back to types that
> fragment
>  	 * memory until all local zones are considered.
> @@ -5681,6 +5726,220 @@ void free_pages_exact(void *virt, size_t
> size)
>  }
>  EXPORT_SYMBOL(free_pages_exact);
>  
> +#ifdef CONFIG_ARCH_WANTS_PTE_MAPPED_CACHE
> +
> +struct pte_mapped_cache {
> +	struct shrinker shrinker;
> +	struct list_lru lru;
> +	atomic_t nid_round_robin;
> +	unsigned long free_cnt;
> +};
> +
> +static struct pte_mapped_cache pte_mapped_cache;
> +
> +static struct page *pte_mapped_cache_get(struct pte_mapped_cache
> *cache)
> +{
> +	unsigned long start_nid, i;
> +	struct list_head *head;
> +	struct page *page = NULL;
> +
> +	start_nid = atomic_fetch_inc(&cache->nid_round_robin) %
> nr_node_ids;
> +	for (i = 0; i < nr_node_ids; i++) {
> +		int cur_nid = (start_nid + i) % nr_node_ids;
> +
> +		head = list_lru_get_mru(&cache->lru, cur_nid);
> +		if (head) {
> +			page = list_entry(head, struct page, lru);
> +			break;
> +		}
> +	}
> +
> +	return page;
> +}
> +
> +static void pte_mapped_cache_add(struct pte_mapped_cache *cache,
> +				 struct page *page)
> +{
> +	INIT_LIST_HEAD(&page->lru);
> +	list_lru_add_node(&cache->lru, &page->lru, page_to_nid(page));
> +	set_page_count(page, 0);
> +}
> +
> +static void pte_mapped_cache_add_neighbour_pages(struct page *page)
> +{
> +#if 0
> +	/*
> +	 * TODO: if pte_mapped_cache_replenish() had to fallback to
> order-0
> +	 * allocation, the large page in the direct map will be split
> +	 * anyway and if there are free pages in the same pageblock
> they
> +	 * can be added to pte_mapped cache.
> +	 */
> +	unsigned int order = (1 << HUGETLB_PAGE_ORDER);
> +	unsigned int nr_pages = (1 << order);
> +	unsigned long pfn = page_to_pfn(page);
> +	struct page *page_head = page - (pfn & (order - 1));
> +
> +	for (i = 0; i < nr_pages; i++) {
> +		page = page_head + i;
> +		if (is_free_buddy_page(page)) {
> +			take_page_off_buddy(page);
> +			pte_mapped_cache_add(&pte_mapped_cache, page);
> +		}
> +	}
> +#endif
> +}
> 
This seems a nice benefit of doing this sort of stuff in the page
allocator if it can work.

> +static struct page *pte_mapped_cache_replenish(struct
> pte_mapped_cache *cache,
> +					       gfp_t gfp)
> +{
> +	unsigned int order = HUGETLB_PAGE_ORDER;
> +	unsigned int nr_pages;
> +	struct page *page;
> +	int i, err;
> +
> +	gfp &= ~__GFP_PTE_MAPPED;
> +
> +	page = alloc_pages(gfp, order);
> +	if (!page) {
> +		order = 0;
> +		page = alloc_pages(gfp, order);
> +		if (!page)
> +			return NULL;
> +	}
> +
> +	nr_pages = (1 << order);
> +	err = set_memory_4k((unsigned long)page_address(page),
> nr_pages);
> +	if (err)
> +		goto err_free_pages;
> +
> +	if (order)
> +		split_page(page, order);
> +	else
> +		pte_mapped_cache_add_neighbour_pages(page);
> +
> +	for (i = 1; i < nr_pages; i++)
> +		pte_mapped_cache_add(cache, page + i);
> +
> +	set_pageblock_pte_mapped(page);
> +
> +	return page;
> +
> +err_free_pages:
> +	__free_pages(page, order);
> +	return NULL;
> +}
> +
> +static struct page *alloc_page_pte_mapped(gfp_t gfp)
I'm a little disappointed building into the page allocator didn't
automatically make higher order allocations easy. It seems this mostly
bolts the grouped pages code on to the page allocator and splits out of
the allocation/free paths to call into it?

I was thinking the main benefit of handling direct map permissions in
the page allocator would be re-using the buddy part to support high
order pages, etc. Did you try to build it in like that? If we can't get
that, what is the benefit to doing permission stuff in the pageallocator? 

> +{
> +	struct pte_mapped_cache *cache = &pte_mapped_cache;
> +	struct page *page;
> +
> +	page = pte_mapped_cache_get(cache);
> +	if (page) {
> +		prep_new_page(page, 0, gfp, 0);
> +		goto out;
> +	}
> +
> +	page = pte_mapped_cache_replenish(cache, gfp);
> +
> +out:
> +	return page;
> +}
> +
We probably want to exclude GFP_ATOMIC before calling into CPA unless
debug page alloc is on, because it may need to split and sleep for the
allocation. There is a page table allocation with GFP_ATOMIC passed
actually.

In my next series of this I added support for GFP_ATOMIC to this code,
but that solution should only work for permission changing grouped page
allocators in the protected page tables case where the direct map
tables are handled differently. As a general solution though (that's
the long term intention right?), GFP_ATOMIC might deserve some
consideration.

The other thing is we probably don't want to clean out the atomic
reserves and add them to a cache just for one page. I opted to just
convert one page in the GFP_ATOMIC case.

> +static void free_page_pte_mapped(struct page *page)
> +{
> +	pte_mapped_cache_add(&pte_mapped_cache, page);
> +}
> +
> +static struct pte_mapped_cache *pte_mapped_cache_from_sc(struct
> shrinker *sh)
> +{
> +	return container_of(sh, struct pte_mapped_cache, shrinker);
> +}
> +
> +static unsigned long pte_mapped_cache_shrink_count(struct shrinker
> *shrinker,
> +						   struct
> shrink_control *sc)
> +{
> +	struct pte_mapped_cache *cache =
> pte_mapped_cache_from_sc(shrinker);
> +	unsigned long count = list_lru_shrink_count(&cache->lru, sc);
> +
> +	return count ? count : SHRINK_EMPTY;
> +}
> +
> +static enum lru_status pte_mapped_cache_shrink_isolate(struct
> list_head *item,
> +						       struct
> list_lru_one *lst,
> +						       spinlock_t
> *lock,
> +						       void *cb_arg)
> +{
> +	struct list_head *freeable = cb_arg;
> +
> +	list_lru_isolate_move(lst, item, freeable);
> +
> +	return LRU_REMOVED;
> +}
> +
> +static unsigned long pte_mapped_cache_shrink_scan(struct shrinker
> *shrinker,
> +						  struct shrink_control
> *sc)
> +{
> +	struct pte_mapped_cache *cache =
> pte_mapped_cache_from_sc(shrinker);
> +	struct list_head *cur, *next;
> +	unsigned long isolated;
> +	LIST_HEAD(freeable);
> +
> +	isolated = list_lru_shrink_walk(&cache->lru, sc,
> +					pte_mapped_cache_shrink_isolate
> ,
> +					&freeable);
> +
> +	list_for_each_safe(cur, next, &freeable) {
> +		struct page *page = list_entry(cur, struct page, lru);
> +
> +		list_del(cur);
> +		__free_pages_ok(page, 0, FPI_NO_PTE_MAP);
> +	}
> +
> +	/* Every item walked gets isolated */
> +	sc->nr_scanned += isolated;
> +
> +	return isolated;
> +}
> +
> +static int __pte_mapped_cache_init(struct pte_mapped_cache *cache)
> +{
> +	int err;
> +
> +	err = list_lru_init(&cache->lru);
> +	if (err)
> +		return err;
> +
> +	cache->shrinker.count_objects = pte_mapped_cache_shrink_count;
> +	cache->shrinker.scan_objects = pte_mapped_cache_shrink_scan;
> +	cache->shrinker.seeks = DEFAULT_SEEKS;
> +	cache->shrinker.flags = SHRINKER_NUMA_AWARE;
> +
> +	err = register_shrinker(&cache->shrinker);
> +	if (err)
> +		goto err_list_lru_destroy;
> +
> +	return 0;
> +
> +err_list_lru_destroy:
> +	list_lru_destroy(&cache->lru);
> +	return err;
> +}
> +
> +void __init pte_mapped_cache_init(void)
> +{
> +	if (gfp_allowed_mask & __GFP_PTE_MAPPED)
> +		return;
> +
> +	if (!__pte_mapped_cache_init(&pte_mapped_cache))
> +		gfp_allowed_mask |= __GFP_PTE_MAPPED;
> +}
> +#else
> +void __init pte_mapped_cache_init(void)
> +{
> +}
> +#endif /* CONFIG_ARCH_WANTS_PTE_MAPPED_CACHE */
> +
>  /**
>   * nr_free_zone_pages - count number of pages beyond high watermark
>   * @offset: The zone index of the highest zone