Re: [RFC 3/4] lockless vma caching

[Minchan Kim]

KAMEZAWA Hiroyuki wrote:
> For accessing vma in lockless style, some modification for vma lookup is
> required. Now, rb-tree is used and it doesn't allow read while modification.
> 
> This is a trial to caching vma rather than diving into rb-tree. The last
> fault vma is cached to pgd's page->cached_vma field. And, add reference count
> and waitqueue to vma.
> 
> The accessor will have to do
> 
> 	vma = lookup_vma_cache(mm, address);
> 	if (vma) {
> 		if (mm_check_version(mm) && /* no write lock at this point ? */
> 		    (vma->vm_start <= address) && (vma->vm_end > address))
> 			goto found_vma; /* start speculative job */
> 		else
> 			vma_release_cache(vma);
> 		vma = NULL;
> 	}
> 	vma = find_vma();
> found_vma:
> 	....do some jobs....
> 	vma_release_cache(vma);
> 
> Maybe some more consideration for invalidation point is necessary.
> 
> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
> ---
>  include/linux/mm.h       |   20 +++++++++
>  include/linux/mm_types.h |    5 ++
>  mm/memory.c              |   14 ++++++
>  mm/mmap.c                |  102 +++++++++++++++++++++++++++++++++++++++++++++--
>  mm/page_alloc.c          |    1 
>  5 files changed, 138 insertions(+), 4 deletions(-)
> 
> Index: mmotm-mm-accessor/include/linux/mm.h
> ===================================================================
> --- mmotm-mm-accessor.orig/include/linux/mm.h
> +++ mmotm-mm-accessor/include/linux/mm.h
> @@ -763,6 +763,26 @@ unsigned long unmap_vmas(struct mmu_gath
>  		unsigned long end_addr, unsigned long *nr_accounted,
>  		struct zap_details *);
>  
> +struct vm_area_struct *lookup_vma_cache(struct mm_struct *mm,
> +		unsigned long address);
> +void invalidate_vma_cache(struct mm_struct *mm,
> +		struct vm_area_struct *vma);
> +void wait_vmas_cache_range(struct vm_area_struct *vma, unsigned long end);
> +
> +static inline void vma_hold(struct vm_area_struct *vma)
Nitpick:
How about static inline void vma_cache_[get/put] naming?

> +{
> +	atomic_inc(&vma->cache_access);
> +}
> +
> +void __vma_release(struct vm_area_struct *vma);
> +static inline void vma_release(struct vm_area_struct *vma)
> +{
> +	if (atomic_dec_and_test(&vma->cache_access)) {
> +		if (waitqueue_active(&vma->cache_wait))
> +			__vma_release(vma);
> +	}
> +}
> +
>  /**
>   * mm_walk - callbacks for walk_page_range
>   * @pgd_entry: if set, called for each non-empty PGD (top-level) entry
> Index: mmotm-mm-accessor/include/linux/mm_types.h
> ===================================================================
> --- mmotm-mm-accessor.orig/include/linux/mm_types.h
> +++ mmotm-mm-accessor/include/linux/mm_types.h
> @@ -12,6 +12,7 @@
>  #include <linux/completion.h>
>  #include <linux/cpumask.h>
>  #include <linux/page-debug-flags.h>
> +#include <linux/wait.h>
>  #include <asm/page.h>
>  #include <asm/mmu.h>
>  
> @@ -77,6 +78,7 @@ struct page {
>  	union {
>  		pgoff_t index;		/* Our offset within mapping. */
>  		void *freelist;		/* SLUB: freelist req. slab lock */
> +		void *cache;

Let's add annotation "/* vm_area_struct cache when the page is used as page table */".


>  	};
>  	struct list_head lru;		/* Pageout list, eg. active_list
>  					 * protected by zone->lru_lock !
> @@ -180,6 +182,9 @@ struct vm_area_struct {
>  	void * vm_private_data;		/* was vm_pte (shared mem) */
>  	unsigned long vm_truncate_count;/* truncate_count or restart_addr */
>  
> +	atomic_t cache_access;
> +	wait_queue_head_t cache_wait;
> +
>  #ifndef CONFIG_MMU
>  	struct vm_region *vm_region;	/* NOMMU mapping region */
>  #endif
> Index: mmotm-mm-accessor/mm/memory.c
> ===================================================================
> --- mmotm-mm-accessor.orig/mm/memory.c
> +++ mmotm-mm-accessor/mm/memory.c
> @@ -145,6 +145,14 @@ void pmd_clear_bad(pmd_t *pmd)
>  	pmd_clear(pmd);
>  }
>  

Let's put the note here. "The caller needs to hold the pte lock"

> +static void update_vma_cache(pmd_t *pmd, struct vm_area_struct *vma)
> +{
> +	struct page *page;
> +	/* ptelock is held */
> +	page = pmd_page(*pmd);
> +	page->cache = vma;
> +}
> +
>  /*
>   * Note: this doesn't free the actual pages themselves. That
>   * has been handled earlier when unmapping all the memory regions.
> @@ -2118,6 +2126,7 @@ reuse:
>  		if (ptep_set_access_flags(vma, address, page_table, entry,1))
>  			update_mmu_cache(vma, address, entry);
>  		ret |= VM_FAULT_WRITE;
> +		update_vma_cache(pmd, vma);
>  		goto unlock;
>  	}
>  
..
<snip>
..

> Index: mmotm-mm-accessor/mm/page_alloc.c
> ===================================================================
> --- mmotm-mm-accessor.orig/mm/page_alloc.c
> +++ mmotm-mm-accessor/mm/page_alloc.c
> @@ -698,6 +698,7 @@ static int prep_new_page(struct page *pa
>  
>  	set_page_private(page, 0);
>  	set_page_refcounted(page);
> +	page->cache = NULL;

Is here is proper place to initialize page->cache?
It cause unnecessary overhead about not pmd page.

How about pmd_alloc?

>  
>  	arch_alloc_page(page, order);
>  	kernel_map_pages(page, 1 << order, 1);
> 
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