Re: [PATCH 2/4] mm/compaction: enable mobile-page migration

[Minchan Kim <minchan@kernel.org>]

Hi Gioh,

On Mon, Jul 13, 2015 at 05:35:17PM +0900, Gioh Kim wrote:
> From: Gioh Kim <gurugio@hanmail.net>
> 
> Add framework to register callback functions and check page mobility.
> There are some modes for page isolation so that isolate interface
> has arguments of page address and isolation mode while putback
> interface has only page address as argument.
> 
> Signed-off-by: Gioh Kim <gioh.kim@lge.com>
> Acked-by: Rafael Aquini <aquini@redhat.com>
> ---
>  fs/proc/page.c                         |  3 ++
>  include/linux/compaction.h             | 80 ++++++++++++++++++++++++++++++++++
>  include/linux/fs.h                     |  2 +
>  include/linux/page-flags.h             | 19 ++++++++
>  include/uapi/linux/kernel-page-flags.h |  1 +
>  5 files changed, 105 insertions(+)
> 
> diff --git a/fs/proc/page.c b/fs/proc/page.c
> index 7eee2d8..a4f5a00 100644
> --- a/fs/proc/page.c
> +++ b/fs/proc/page.c
> @@ -146,6 +146,9 @@ u64 stable_page_flags(struct page *page)
>  	if (PageBalloon(page))
>  		u |= 1 << KPF_BALLOON;
>  
> +	if (PageMobile(page))
> +		u |= 1 << KPF_MOBILE;
> +

Need a new page flag for non-LRU page migration?
I am worry that we don't have empty slot for page flag on 32-bit.

Aha, see SetPageMobile below. You use _mapcount.
It seems to be work for non-LRU pages but unfortunately, zsmalloc
already have used that field as own purpose so there is no room
for it.


>  	u |= kpf_copy_bit(k, KPF_LOCKED,	PG_locked);
>  
>  	u |= kpf_copy_bit(k, KPF_SLAB,		PG_slab);
> diff --git a/include/linux/compaction.h b/include/linux/compaction.h
> index aa8f61c..f693072 100644
> --- a/include/linux/compaction.h
> +++ b/include/linux/compaction.h
> @@ -1,6 +1,9 @@
>  #ifndef _LINUX_COMPACTION_H
>  #define _LINUX_COMPACTION_H
>  
> +#include <linux/page-flags.h>
> +#include <linux/pagemap.h>
> +
>  /* Return values for compact_zone() and try_to_compact_pages() */
>  /* compaction didn't start as it was deferred due to past failures */
>  #define COMPACT_DEFERRED	0
> @@ -51,6 +54,70 @@ extern void compaction_defer_reset(struct zone *zone, int order,
>  				bool alloc_success);
>  extern bool compaction_restarting(struct zone *zone, int order);
>  
> +static inline bool mobile_page(struct page *page)
> +{
> +	return page->mapping &&	(PageMobile(page) || PageBalloon(page));


What's the locking rule to test mobile_page?
Why we need such many checking?

Why we need PageBalloon check?
You are trying to make generic abstraction for non-LRU page to migrate
but PageBalloon check in here breaks your goal.

> +}
> +
> +static inline bool isolate_mobilepage(struct page *page, isolate_mode_t mode)
> +{
> +	bool ret = false;
> +
> +	/*
> +	 * Avoid burning cycles with pages that are yet under __free_pages(),
> +	 * or just got freed under us.
> +	 *
> +	 * In case we 'win' a race for a mobile page being freed under us and
> +	 * raise its refcount preventing __free_pages() from doing its job
> +	 * the put_page() at the end of this block will take care of
> +	 * release this page, thus avoiding a nasty leakage.
> +	 */

Good.

> +	if (unlikely(!get_page_unless_zero(page)))
> +		goto out;
> +
> +	/*
> +	 * As mobile pages are not isolated from LRU lists, concurrent
> +	 * compaction threads can race against page migration functions
> +	 * as well as race against the releasing a page.

How can it race with releasing a page?
We already get refcount above.

Do you mean page->mapping tearing race?
If so, zsmalloc should be chaned to hold a lock.


> +	 *
> +	 * In order to avoid having an already isolated mobile page
> +	 * being (wrongly) re-isolated while it is under migration,
> +	 * or to avoid attempting to isolate pages being released,
> +	 * lets be sure we have the page lock
> +	 * before proceeding with the mobile page isolation steps.
> +	 */
> +	if (unlikely(!trylock_page(page)))
> +		goto out_putpage;
> +
> +	if (!(mobile_page(page) && page->mapping->a_ops->isolatepage))
> +		goto out_not_isolated;

I cannot know how isolate_mobilepage is called by upper layer
because this patch doesn't include it.
Anyway, why we need such many checks to prove it's mobile page?

mobile_page() {
	page->mapping && (PageMobile(page) || PageBalloon(page));
}

Additionally, added page->mapping->a_ops->isolatepage check

Is it possible that a non-LRU page's address space provides
only page->maping->migratepage without isolate/putback?

Couldn't we make it simple like this?

        page->mapping && page->mapping->migratepage

So, couldn't we make mobile_page like this

PageMobile(struct page *page)
{
        VM_BUG_ON_PAGE(!PageLocked(page), page);
        VM_BUG_ON_PAGE(PageLRU(page), page);

        return page->mapping && page->mapping->migratepage;
}

It will save _mapcount of struct page.

> +	ret = page->mapping->a_ops->isolatepage(page, mode);
> +	if (!ret)
> +		goto out_not_isolated;
> +	unlock_page(page);
> +	return ret;
> +
> +out_not_isolated:
> +	unlock_page(page);
> +out_putpage:
> +	put_page(page);
> +out:
> +	return ret;
> +}
> +
> +static inline void putback_mobilepage(struct page *page)
> +{
> +	/*
> +	 * 'lock_page()' stabilizes the page and prevents races against

What does it mean 'stabilize'?
Clear comment is always welcome rather than vague word.

> +	 * concurrent isolation threads attempting to re-isolate it.
> +	 */
> +	lock_page(page);
> +	if (page->mapping && page->mapping->a_ops->putbackpage)
> +		page->mapping->a_ops->putbackpage(page);
> +	unlock_page(page);
> +	/* drop the extra ref count taken for mobile page isolation */
> +	put_page(page);
> +}
>  #else
>  static inline unsigned long try_to_compact_pages(gfp_t gfp_mask,
>  			unsigned int order, int alloc_flags,
> @@ -83,6 +150,19 @@ static inline bool compaction_deferred(struct zone *zone, int order)
>  	return true;
>  }
>  
> +static inline bool mobile_page(struct page *page)
> +{
> +	return false;
> +}
> +
> +static inline bool isolate_mobilepage(struct page *page, isolate_mode_t mode)
> +{
> +	return false;
> +}
> +
> +static inline void putback_mobilepage(struct page *page)
> +{
> +}
>  #endif /* CONFIG_COMPACTION */
>  
>  #if defined(CONFIG_COMPACTION) && defined(CONFIG_SYSFS) && defined(CONFIG_NUMA)
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index a0653e5..2cc4b24 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -396,6 +396,8 @@ struct address_space_operations {
>  	 */
>  	int (*migratepage) (struct address_space *,
>  			struct page *, struct page *, enum migrate_mode);
> +	bool (*isolatepage) (struct page *, isolate_mode_t);
> +	void (*putbackpage) (struct page *);
>  	int (*launder_page) (struct page *);
>  	int (*is_partially_uptodate) (struct page *, unsigned long,
>  					unsigned long);
> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> index f34e040..abef145 100644
> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h
> @@ -582,6 +582,25 @@ static inline void __ClearPageBalloon(struct page *page)
>  	atomic_set(&page->_mapcount, -1);
>  }
>  
> +#define PAGE_MOBILE_MAPCOUNT_VALUE (-255)
> +
> +static inline int PageMobile(struct page *page)
> +{
> +	return atomic_read(&page->_mapcount) == PAGE_MOBILE_MAPCOUNT_VALUE;
> +}
> +
> +static inline void __SetPageMobile(struct page *page)
> +{
> +	VM_BUG_ON_PAGE(atomic_read(&page->_mapcount) != -1, page);
> +	atomic_set(&page->_mapcount, PAGE_MOBILE_MAPCOUNT_VALUE);
> +}
> +
> +static inline void __ClearPageMobile(struct page *page)
> +{
> +	VM_BUG_ON_PAGE(!PageMobile(page), page);
> +	atomic_set(&page->_mapcount, -1);
> +}
> +
>  /*
>   * If network-based swap is enabled, sl*b must keep track of whether pages
>   * were allocated from pfmemalloc reserves.
> diff --git a/include/uapi/linux/kernel-page-flags.h b/include/uapi/linux/kernel-page-flags.h
> index a6c4962..d50d9e8 100644
> --- a/include/uapi/linux/kernel-page-flags.h
> +++ b/include/uapi/linux/kernel-page-flags.h
> @@ -33,6 +33,7 @@
>  #define KPF_THP			22
>  #define KPF_BALLOON		23
>  #define KPF_ZERO_PAGE		24
> +#define KPF_MOBILE		25
>  
>  
>  #endif /* _UAPILINUX_KERNEL_PAGE_FLAGS_H */
> -- 
> 2.1.4
> 

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