Re: [PATCH] add generic callbacks into compaction

[Minchan Kim]

Hello Gioh,

I wanted to have such feature for zsmalloc.
Thanks for the work.

On Wed, Apr 01, 2015 at 08:11:30AM +0900, Gioh Kim wrote:
> I sent a patch about page allocation for less fragmentation.
> http://permalink.gmane.org/gmane.linux.kernel.mm/130599
> 
> It proposes a page allocator allocates pages in the same pageblock
> for the drivers to move their unmovable pages. Some drivers which comsumes many pages
> and increases system fragmentation use the allocator to move their pages to
> decrease fragmentation.
> 
> I think I can try another approach.
> There is a compaction code for balloon pages.
> But the compaction code cannot migrate pages of other drivers.
> If there is a generic migration framework applicable to any drivers,
> drivers can register their migration functions.
> And the compaction can migrate movable pages and also driver's pages.
> 
> I'm not familiar with virtualization so I couldn't test this patch yet.
> But if mm developers agree with this approach, I will complete this patch.
> 
> I would do appreciate any feedback.

Could you separate introducing migrate core patchset and balloon patchset for
using the feature?

> 
> Signed-off-by: Gioh Kim <gioh.kim@xxxxxxx>
> ---
>  drivers/virtio/virtio_balloon.c    |    2 ++
>  include/linux/balloon_compaction.h |   23 +++++++++---
>  include/linux/fs.h                 |    3 ++
>  include/linux/pagemap.h            |   26 ++++++++++++++
>  mm/balloon_compaction.c            |   68 ++++++++++++++++++++++++++++++++++--
>  mm/compaction.c                    |    7 ++--
>  mm/migrate.c                       |   24 ++++++-------
>  7 files changed, 129 insertions(+), 24 deletions(-)
> 
> diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
> index 0413157..cd9b8e4 100644
> --- a/drivers/virtio/virtio_balloon.c
> +++ b/drivers/virtio/virtio_balloon.c
> @@ -486,6 +486,8 @@ static int virtballoon_probe(struct virtio_device *vdev)
>  
>  	balloon_devinfo_init(&vb->vb_dev_info);
>  #ifdef CONFIG_BALLOON_COMPACTION
> +	vb->vb_dev_info.mapping = balloon_mapping_alloc(&vb->vb_dev_info,
> +							&balloon_aops);
>  	vb->vb_dev_info.migratepage = virtballoon_migratepage;
>  #endif
>  
> diff --git a/include/linux/balloon_compaction.h b/include/linux/balloon_compaction.h
> index 9b0a15d..0af32b3 100644
> --- a/include/linux/balloon_compaction.h
> +++ b/include/linux/balloon_compaction.h
> @@ -62,6 +62,7 @@ struct balloon_dev_info {
>  	struct list_head pages;		/* Pages enqueued & handled to Host */
>  	int (*migratepage)(struct balloon_dev_info *, struct page *newpage,
>  			struct page *page, enum migrate_mode mode);
> +	struct address_space *mapping;
>  };
>  
>  extern struct page *balloon_page_enqueue(struct balloon_dev_info *b_dev_info);
> @@ -76,10 +77,22 @@ static inline void balloon_devinfo_init(struct balloon_dev_info *balloon)
>  }
>  
>  #ifdef CONFIG_BALLOON_COMPACTION
> -extern bool balloon_page_isolate(struct page *page);
> -extern void balloon_page_putback(struct page *page);
> -extern int balloon_page_migrate(struct page *newpage,
> -				struct page *page, enum migrate_mode mode);
> +extern const struct address_space_operations balloon_aops;
> +extern int balloon_page_isolate(struct page *page);
> +extern int balloon_page_putback(struct page *page);
> +extern int balloon_page_migrate(struct address_space *mapping,
> +				struct page *newpage,
> +				struct page *page,
> +				enum migrate_mode mode);
> +
> +extern struct address_space
> +*balloon_mapping_alloc(struct balloon_dev_info *b_dev_info,
> +		       const struct address_space_operations *a_ops);
> +
> +static inline void balloon_mapping_free(struct address_space *balloon_mapping)
> +{
> +	kfree(balloon_mapping);
> +}
>  
>  /*
>   * __is_movable_balloon_page - helper to perform @page PageBalloon tests
> @@ -123,6 +136,7 @@ static inline bool isolated_balloon_page(struct page *page)
>  static inline void balloon_page_insert(struct balloon_dev_info *balloon,
>  				       struct page *page)
>  {
> +	page->mapping = balloon->mapping;
>  	__SetPageBalloon(page);
>  	SetPagePrivate(page);
>  	set_page_private(page, (unsigned long)balloon);
> @@ -139,6 +153,7 @@ static inline void balloon_page_insert(struct balloon_dev_info *balloon,
>   */
>  static inline void balloon_page_delete(struct page *page)
>  {
> +	page->mapping = NULL;
>  	__ClearPageBalloon(page);
>  	set_page_private(page, 0);
>  	if (PagePrivate(page)) {
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index b4d71b5..de463b9 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -368,6 +368,9 @@ struct address_space_operations {
>  	 */
>  	int (*migratepage) (struct address_space *,
>  			struct page *, struct page *, enum migrate_mode);
> +	int (*isolatepage)(struct page *);

It would be useful if isolatepage is aware of migrate_mode and address_space
For exmaple, if it is MIGRATE_SYNC, driver could wait  some of work to finish.
>From address_space, driver can get private data registered when isolate/putback
happens.


> +	int (*putbackpage)(struct page *);

Ditto.

> +
>  	int (*launder_page) (struct page *);
>  	int (*is_partially_uptodate) (struct page *, unsigned long,
>  					unsigned long);
> diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
> index 4b3736f..715b5b2 100644
> --- a/include/linux/pagemap.h
> +++ b/include/linux/pagemap.h
> @@ -25,6 +25,7 @@ enum mapping_flags {
>  	AS_MM_ALL_LOCKS	= __GFP_BITS_SHIFT + 2,	/* under mm_take_all_locks() */
>  	AS_UNEVICTABLE	= __GFP_BITS_SHIFT + 3,	/* e.g., ramdisk, SHM_LOCK */
>  	AS_EXITING	= __GFP_BITS_SHIFT + 4, /* final truncate in progress */
> +	AS_MIGRATABLE   = __GFP_BITS_SHIFT + 5,
>  };
>  
>  static inline void mapping_set_error(struct address_space *mapping, int error)
> @@ -54,6 +55,31 @@ static inline int mapping_unevictable(struct address_space *mapping)
>  	return !!mapping;
>  }
>  
> +static inline void mapping_set_migratable(struct address_space *mapping)
> +{
> +	set_bit(AS_MIGRATABLE, &mapping->flags);
> +}

It means every address_space which want to migrate pages mapped the address space
should call this function?

> +
> +static inline void mapping_clear_migratable(struct address_space *mapping)
> +{
> +	clear_bit(AS_MIGRATABLE, &mapping->flags);
> +}
> +
> +static inline int __mapping_ops(struct address_space *mapping)

The naming of __mapping_ops is awkward. How about "bool is_migratable(...)"?
Another question:
Why should they support all fucntions of migrate(ie, migratepage, isolatepage, putbackpage).
I mean currently inode of regular file supports only migratepage.
By your rule, they are not migratable?

> +{
> +	return mapping->a_ops &&
> +		mapping->a_ops->migratepage &&
> +		mapping->a_ops->isolatepage &&
> +		mapping->a_ops->putbackpage;
> +}
> +
> +static inline int mapping_migratable(struct address_space *mapping)
> +{
> +	if (mapping && __mapping_ops(mapping))

Why do we need mapping NULL check in here?
It's role of caller?

> +		return test_bit(AS_MIGRATABLE, &mapping->flags);

It means some of address_space has migratepage, isolatepage, putbackpage but
it doesn't set AS_MIGRATABLE? Why do we need it?

> +	return !!mapping;
> +}
> +
>  static inline void mapping_set_exiting(struct address_space *mapping)
>  {
>  	set_bit(AS_EXITING, &mapping->flags);
> diff --git a/mm/balloon_compaction.c b/mm/balloon_compaction.c
> index fcad832..2e9d635 100644
> --- a/mm/balloon_compaction.c
> +++ b/mm/balloon_compaction.c
> @@ -131,8 +131,11 @@ static inline void __putback_balloon_page(struct page *page)
>  }
>  
>  /* __isolate_lru_page() counterpart for a ballooned page */
> -bool balloon_page_isolate(struct page *page)
> +int balloon_page_isolate(struct page *page)
>  {
> +	if (!balloon_page_movable(page))
> +		return false;
> +
>  	/*
>  	 * Avoid burning cycles with pages that are yet under __free_pages(),
>  	 * or just got freed under us.
> @@ -173,8 +176,11 @@ bool balloon_page_isolate(struct page *page)
>  }
>  
>  /* putback_lru_page() counterpart for a ballooned page */
> -void balloon_page_putback(struct page *page)
> +int balloon_page_putback(struct page *page)
>  {
> +	if (!isolated_balloon_page(page))
> +		return 0;
> +
>  	/*
>  	 * 'lock_page()' stabilizes the page and prevents races against
>  	 * concurrent isolation threads attempting to re-isolate it.
> @@ -190,15 +196,20 @@ void balloon_page_putback(struct page *page)
>  		dump_page(page, "not movable balloon page");
>  	}
>  	unlock_page(page);
> +	return 0;
>  }
>  
>  /* move_to_new_page() counterpart for a ballooned page */
> -int balloon_page_migrate(struct page *newpage,
> +int balloon_page_migrate(struct address_space *mapping,
> +			 struct page *newpage,
>  			 struct page *page, enum migrate_mode mode)
>  {
>  	struct balloon_dev_info *balloon = balloon_page_device(page);
>  	int rc = -EAGAIN;
>  
> +	if (!isolated_balloon_page(page))
> +		return 0;
> +
>  	/*
>  	 * Block others from accessing the 'newpage' when we get around to
>  	 * establishing additional references. We should be the only one
> @@ -218,4 +229,55 @@ int balloon_page_migrate(struct page *newpage,
>  	unlock_page(newpage);
>  	return rc;
>  }
> +
> +/* define the balloon_mapping->a_ops callback to allow balloon page migration */
> +const struct address_space_operations balloon_aops = {
> +	.migratepage = balloon_page_migrate,
> +	.isolatepage = balloon_page_isolate,
> +	.putbackpage = balloon_page_putback,
> +};
> +EXPORT_SYMBOL_GPL(balloon_aops);
> +
> +struct address_space *balloon_mapping_alloc(struct balloon_dev_info *b_dev_info,
> +				const struct address_space_operations *a_ops)
> +{
> +	struct address_space *mapping;
> +
> +	mapping = kmalloc(sizeof(*mapping), GFP_KERNEL);
> +	if (!mapping)
> +		return ERR_PTR(-ENOMEM);
> +
> +	/*
> +	 * Give a clean 'zeroed' status to all elements of this special
> +	 * balloon page->mapping struct address_space instance.
> +	 */
> +	address_space_init_once(mapping);
> +
> +	/*
> +	 * Set mapping->flags appropriately, to allow balloon pages
> +	 * ->mapping identification.
> +	 */
> +	mapping_set_migratable(mapping);
> +	mapping_set_gfp_mask(mapping, balloon_mapping_gfp_mask());
> +
> +	/* balloon's page->mapping->a_ops callback descriptor */
> +	mapping->a_ops = a_ops;
> +
> +	/*
> +	 * Establish a pointer reference back to the balloon device descriptor
> +	 * this particular page->mapping will be servicing.
> +	 * This is used by compaction / migration procedures to identify and
> +	 * access the balloon device pageset while isolating / migrating pages.
> +	 *
> +	 * As some balloon drivers can register multiple balloon devices
> +	 * for a single guest, this also helps compaction / migration to
> +	 * properly deal with multiple balloon pagesets, when required.
> +	 */
> +	mapping->private_data = b_dev_info;
> +	b_dev_info->mapping = mapping;
> +
> +	return mapping;
> +}
> +EXPORT_SYMBOL_GPL(balloon_mapping_alloc);
> +
>  #endif /* CONFIG_BALLOON_COMPACTION */
> diff --git a/mm/compaction.c b/mm/compaction.c
> index 8c0d945..9fbcb7b 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -740,12 +740,9 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
>  		 * Skip any other type of page
>  		 */
>  		if (!PageLRU(page)) {
> -			if (unlikely(balloon_page_movable(page))) {
> -				if (balloon_page_isolate(page)) {
> -					/* Successfully isolated */
> +			if (mapping_migratable(page->mapping))

I see. Do you want to support this feature for only "non-LRU pages"?
Hmm, your term for functions was too generic so I got confused.
At least, it needs prefix like "special_xxx" so that we could classify
regular inode migrate and special driver migrate.

Otherwise, we could make logic more general so every address_space
want to migrate their page has all of migrate functions(ie, migrate,isolate,putback)
and we support default functions so addresss_space of regular inode will
have default functions while special driver overrides it by own functions.
With it, compact/migrate just calls registered function of each subsystem and
pass many role to them.


> +				if (page->mapping->a_ops->isolatepage(page))
>  					goto isolate_success;
> -				}
> -			}
>  			continue;
>  		}
>  
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 85e0426..37cb366 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -71,6 +71,12 @@ int migrate_prep_local(void)
>  	return 0;
>  }
>  
> +static bool isolated_migratable_page(struct page *page)
> +{
> +	return page_mapped(page) == 0 &&
> +		mapping_migratable(page->mapping);
> +}

Why does we need to check like this?
Whether isolate was successful or not depends on the driver.
We cannot expect it with only page_mapped.

> +
>  /*
>   * Put previously isolated pages back onto the appropriate lists
>   * from where they were once taken off for compaction/migration.
> @@ -91,9 +97,9 @@ void putback_movable_pages(struct list_head *l)
>  		}
>  		list_del(&page->lru);
>  		dec_zone_page_state(page, NR_ISOLATED_ANON +
> -				page_is_file_cache(page));
> -		if (unlikely(isolated_balloon_page(page)))
> -			balloon_page_putback(page);
> +				    page_is_file_cache(page));
> +		if (unlikely(isolated_migratable_page(page)))
> +			page->mapping->a_ops->putbackpage(page);
>  		else
>  			putback_lru_page(page);

As I said, how about having putback_lru_page for page->mapping->a_ops->putbackpage
for regular inode? With it, we may remove some of branch in migrate/compact.c to
separate special driver and regular address_space.
Hope to enhance readability and make more flexible.

>  	}
> @@ -843,15 +849,9 @@ static int __unmap_and_move(struct page *page, struct page *newpage,
>  		}
>  	}
>  
> -	if (unlikely(isolated_balloon_page(page))) {
> -		/*
> -		 * A ballooned page does not need any special attention from
> -		 * physical to virtual reverse mapping procedures.
> -		 * Skip any attempt to unmap PTEs or to remap swap cache,
> -		 * in order to avoid burning cycles at rmap level, and perform
> -		 * the page migration right away (proteced by page lock).
> -		 */
> -		rc = balloon_page_migrate(newpage, page, mode);
> +	if (unlikely(isolated_migratable_page(page))) {
> +		rc = page->mapping->a_ops->migratepage(page->mapping,
> +						       newpage, page, mode);
>  		goto out_unlock;
>  	}
>  
> -- 
> 1.7.9.5
> 

-- 
Kind regards,
Minchan Kim
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