Re: [PATCH RFC 3/9] x86/mm/cpa: Add grouped page allocations

[Mike Rapoport]

On Tue, May 04, 2021 at 05:30:26PM -0700, Rick Edgecombe wrote:
> For x86, setting memory permissions on the direct map results in fracturing
> large pages. Direct map fracturing can be reduced by locating pages that
> will have their permissions set close together.
> 
> Create a simple page cache that allocates pages from huge page size
> blocks. Don't guarantee that a page will come from a huge page grouping,
> instead fallback to non-grouped pages to fulfill the allocation if
> needed. Also, register a shrinker such that the system can ask for the
> pages back if needed. Since this is only needed when there is a direct
> map, compile it out on highmem systems.

I only had time to skim through the patches, I like the idea of having a
simple cache that allocates larger pages with a fallback to basic page
size.

I just think it should be more generic and closer to the page allocator.
I was thinking about adding a GFP flag that will tell that the allocated
pages should be removed from the direct map. Then alloc_pages() could use
such cache whenever this GFP flag is specified with a fallback for lower
order allocations.
 
> Free pages in the cache are kept track of in per-node list inside a
> list_lru. NUMA_NO_NODE requests are serviced by checking each per-node
> list in a round robin fashion. If pages are requested for a certain node
> but the cache is empty for that node, a whole additional huge page size
> page is allocated.
> 
> Signed-off-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
> ---
>  arch/x86/include/asm/set_memory.h |  14 +++
>  arch/x86/mm/pat/set_memory.c      | 151 ++++++++++++++++++++++++++++++
>  2 files changed, 165 insertions(+)
> 
> diff --git a/arch/x86/include/asm/set_memory.h b/arch/x86/include/asm/set_memory.h
> index 4352f08bfbb5..b63f09cc282a 100644
> --- a/arch/x86/include/asm/set_memory.h
> +++ b/arch/x86/include/asm/set_memory.h
> @@ -4,6 +4,9 @@
>  
>  #include <asm/page.h>
>  #include <asm-generic/set_memory.h>
> +#include <linux/gfp.h>
> +#include <linux/list_lru.h>
> +#include <linux/shrinker.h>
>  
>  /*
>   * The set_memory_* API can be used to change various attributes of a virtual
> @@ -135,4 +138,15 @@ static inline int clear_mce_nospec(unsigned long pfn)
>   */
>  #endif
>  
> +struct grouped_page_cache {
> +	struct shrinker shrinker;
> +	struct list_lru lru;
> +	gfp_t gfp;
> +	atomic_t nid_round_robin;
> +};
> +
> +int init_grouped_page_cache(struct grouped_page_cache *gpc, gfp_t gfp);
> +struct page *get_grouped_page(int node, struct grouped_page_cache *gpc);
> +void free_grouped_page(struct grouped_page_cache *gpc, struct page *page);
> +
>  #endif /* _ASM_X86_SET_MEMORY_H */
> diff --git a/arch/x86/mm/pat/set_memory.c b/arch/x86/mm/pat/set_memory.c
> index 16f878c26667..6877ef66793b 100644
> --- a/arch/x86/mm/pat/set_memory.c
> +++ b/arch/x86/mm/pat/set_memory.c
> @@ -2306,6 +2306,157 @@ int __init kernel_unmap_pages_in_pgd(pgd_t *pgd, unsigned long address,
>  	return retval;
>  }
>  
> +#ifndef HIGHMEM
> +static struct page *__alloc_page_order(int node, gfp_t gfp_mask, int order)
> +{
> +	if (node == NUMA_NO_NODE)
> +		return alloc_pages(gfp_mask, order);
> +
> +	return alloc_pages_node(node, gfp_mask, order);
> +}
> +
> +static struct grouped_page_cache *__get_gpc_from_sc(struct shrinker *shrinker)
> +{
> +	return container_of(shrinker, struct grouped_page_cache, shrinker);
> +}
> +
> +static unsigned long grouped_shrink_count(struct shrinker *shrinker,
> +					  struct shrink_control *sc)
> +{
> +	struct grouped_page_cache *gpc = __get_gpc_from_sc(shrinker);
> +	unsigned long page_cnt = list_lru_shrink_count(&gpc->lru, sc);
> +
> +	return page_cnt ? page_cnt : SHRINK_EMPTY;
> +}
> +
> +static enum lru_status grouped_isolate(struct list_head *item,
> +				       struct list_lru_one *list,
> +				       spinlock_t *lock, void *cb_arg)
> +{
> +	struct list_head *dispose = cb_arg;
> +
> +	list_lru_isolate_move(list, item, dispose);
> +
> +	return LRU_REMOVED;
> +}
> +
> +static void __dispose_pages(struct grouped_page_cache *gpc, struct list_head *head)
> +{
> +	struct list_head *cur, *next;
> +
> +	list_for_each_safe(cur, next, head) {
> +		struct page *page = list_entry(head, struct page, lru);
> +
> +		list_del(cur);
> +
> +		__free_pages(page, 0);
> +	}
> +}
> +
> +static unsigned long grouped_shrink_scan(struct shrinker *shrinker,
> +					 struct shrink_control *sc)
> +{
> +	struct grouped_page_cache *gpc = __get_gpc_from_sc(shrinker);
> +	unsigned long isolated;
> +	LIST_HEAD(freeable);
> +
> +	if (!(sc->gfp_mask & gpc->gfp))
> +		return SHRINK_STOP;
> +
> +	isolated = list_lru_shrink_walk(&gpc->lru, sc, grouped_isolate,
> +					&freeable);
> +	__dispose_pages(gpc, &freeable);
> +
> +	/* Every item walked gets isolated */
> +	sc->nr_scanned += isolated;
> +
> +	return isolated;
> +}
> +
> +static struct page *__remove_first_page(struct grouped_page_cache *gpc, int node)
> +{
> +	unsigned int start_nid, i;
> +	struct list_head *head;
> +
> +	if (node != NUMA_NO_NODE) {
> +		head = list_lru_get_mru(&gpc->lru, node);
> +		if (head)
> +			return list_entry(head, struct page, lru);
> +		return NULL;
> +	}
> +
> +	/* If NUMA_NO_NODE, search the nodes in round robin for a page */
> +	start_nid = (unsigned int)atomic_fetch_inc(&gpc->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(&gpc->lru, cur_nid);
> +		if (head)
> +			return list_entry(head, struct page, lru);
> +	}
> +
> +	return NULL;
> +}
> +
> +/* Get and add some new pages to the cache to be used by VM_GROUP_PAGES */
> +static struct page *__replenish_grouped_pages(struct grouped_page_cache *gpc, int node)
> +{
> +	const unsigned int hpage_cnt = HPAGE_SIZE >> PAGE_SHIFT;
> +	struct page *page;
> +	int i;
> +
> +	page = __alloc_page_order(node, gpc->gfp, HUGETLB_PAGE_ORDER);
> +	if (!page)
> +		return __alloc_page_order(node, gpc->gfp, 0);
> +
> +	split_page(page, HUGETLB_PAGE_ORDER);
> +
> +	for (i = 1; i < hpage_cnt; i++)
> +		free_grouped_page(gpc, &page[i]);
> +
> +	return &page[0];
> +}
> +
> +int init_grouped_page_cache(struct grouped_page_cache *gpc, gfp_t gfp)
> +{
> +	int err = 0;
> +
> +	memset(gpc, 0, sizeof(struct grouped_page_cache));
> +
> +	if (list_lru_init(&gpc->lru))
> +		goto out;
> +
> +	gpc->shrinker.count_objects = grouped_shrink_count;
> +	gpc->shrinker.scan_objects = grouped_shrink_scan;
> +	gpc->shrinker.seeks = DEFAULT_SEEKS;
> +	gpc->shrinker.flags = SHRINKER_NUMA_AWARE;
> +
> +	err = register_shrinker(&gpc->shrinker);
> +	if (err)
> +		list_lru_destroy(&gpc->lru);
> +
> +out:
> +	return err;
> +}
> +
> +struct page *get_grouped_page(int node, struct grouped_page_cache *gpc)
> +{
> +	struct page *page;
> +
> +	page = __remove_first_page(gpc, node);
> +
> +	if (page)
> +		return page;
> +
> +	return __replenish_grouped_pages(gpc, node);
> +}
> +
> +void free_grouped_page(struct grouped_page_cache *gpc, struct page *page)
> +{
> +	INIT_LIST_HEAD(&page->lru);
> +	list_lru_add_node(&gpc->lru, &page->lru, page_to_nid(page));
> +}
> +#endif /* !HIGHMEM */
>  /*
>   * The testcases use internal knowledge of the implementation that shouldn't
>   * be exposed to the rest of the kernel. Include these directly here.
> -- 
> 2.30.2
> 

-- 
Sincerely yours,
Mike.