Linux Virtualization - Apr 2016 - [PATCH v3 02/16] mm/compaction: support non-lru movable page migration

Might have been better as a separate migration patch and then a compaction 
patch. It's prefixed mm/compaction, but most changed are in mm/migrate.c

On 03/30/2016 09:12 AM, Minchan Kim wrote:
> We have allowed migration for only LRU pages until now and it was
> enough to make high-order pages. But recently, embedded system(e.g.,
> webOS, android) uses lots of non-movable pages(e.g., zram, GPU memory)
> so we have seen several reports about troubles of small high-order
> allocation. For fixing the problem, there were several efforts
> (e,g,. enhance compaction algorithm, SLUB fallback to 0-order page,
> reserved memory, vmalloc and so on) but if there are lots of
> non-movable pages in system, their solutions are void in the long run.
>
> So, this patch is to support facility to change non-movable pages
> with movable. For the feature, this patch introduces functions related
> to migration to address_space_operations as well as some page flags.
>
> Basically, this patch supports two page-flags and two functions related
> to page migration. The flag and page->mapping stability are protected
> by PG_lock.
>
> 	PG_movable
> 	PG_isolated
>
> 	bool (*isolate_page) (struct page *, isolate_mode_t);
> 	void (*putback_page) (struct page *);
>
> Duty of subsystem want to make their pages as migratable are
> as follows:
>
> 1. It should register address_space to page->mapping then mark
> the page as PG_movable via __SetPageMovable.
>
> 2. It should mark the page as PG_isolated via SetPageIsolated
> if isolation is sucessful and return true.
Ah another thing to document (especially in the comments/Doc) is that the 
subsystem must not expect anything to survive in page.lru (or fields that union 
it) after having isolated successfully.
> 3. If migration is successful, it should clear PG_isolated and
> PG_movable of the page for free preparation then release the
> reference of the page to free.
>
> 4. If migration fails, putback function of subsystem should
> clear PG_isolated via ClearPageIsolated.
>
> 5. If a subsystem want to release isolated page, it should
> clear PG_isolated but not PG_movable. Instead, VM will do it.
Under lock? Or just with ClearPageIsolated?
> Cc: Vlastimil Babka <vbabka at suse.cz>
> Cc: Mel Gorman <mgorman at suse.de>
> Cc: Hugh Dickins <hughd at google.com>
> Cc: dri-devel at lists.freedesktop.org
> Cc: virtualization at lists.linux-foundation.org
> Signed-off-by: Gioh Kim <gurugio at hanmail.net>
> Signed-off-by: Minchan Kim <minchan at kernel.org>
> ---
>   Documentation/filesystems/Locking      |   4 +
>   Documentation/filesystems/vfs.txt      |   5 +
>   fs/proc/page.c                         |   3 +
>   include/linux/fs.h                     |   2 +
>   include/linux/migrate.h                |   2 +
>   include/linux/page-flags.h             |  31 ++++++
>   include/uapi/linux/kernel-page-flags.h |   1 +
>   mm/compaction.c                        |  14 ++-
>   mm/migrate.c                           | 174
+++++++++++++++++++++++++++++----
>   9 files changed, 217 insertions(+), 19 deletions(-)
>
> diff --git a/Documentation/filesystems/Locking
b/Documentation/filesystems/Locking
> index 619af9bfdcb3..0bb79560abb3 100644
> --- a/Documentation/filesystems/Locking
> +++ b/Documentation/filesystems/Locking
> @@ -195,7 +195,9 @@ unlocks and drops the reference.
>   	int (*releasepage) (struct page *, int);
>   	void (*freepage)(struct page *);
>   	int (*direct_IO)(struct kiocb *, struct iov_iter *iter, loff_t offset);
> +	bool (*isolate_page) (struct page *, isolate_mode_t);
>   	int (*migratepage)(struct address_space *, struct page *, struct page
*);
> +	void (*putback_page) (struct page *);
>   	int (*launder_page)(struct page *);
>   	int (*is_partially_uptodate)(struct page *, unsigned long, unsigned
long);
>   	int (*error_remove_page)(struct address_space *, struct page *);
> @@ -219,7 +221,9 @@ invalidatepage:		yes
>   releasepage:		yes
>   freepage:		yes
>   direct_IO:
> +isolate_page:		yes
>   migratepage:		yes (both)
> +putback_page:		yes
>   launder_page:		yes
>   is_partially_uptodate:	yes
>   error_remove_page:	yes
> diff --git a/Documentation/filesystems/vfs.txt
b/Documentation/filesystems/vfs.txt
> index b02a7d598258..4c1b6c3b4bc8 100644
> --- a/Documentation/filesystems/vfs.txt
> +++ b/Documentation/filesystems/vfs.txt
> @@ -592,9 +592,14 @@ struct address_space_operations {
>   	int (*releasepage) (struct page *, int);
>   	void (*freepage)(struct page *);
>   	ssize_t (*direct_IO)(struct kiocb *, struct iov_iter *iter, loff_t
offset);
> +	/* isolate a page for migration */
> +	bool (*isolate_page) (struct page *, isolate_mode_t);
>   	/* migrate the contents of a page to the specified target */
>   	int (*migratepage) (struct page *, struct page *);
> +	/* put the page back to right list */
... "after a failed migration" ?
> +	void (*putback_page) (struct page *);
>   	int (*launder_page) (struct page *);
> +
>   	int (*is_partially_uptodate) (struct page *, unsigned long,
>   					unsigned long);
>   	void (*is_dirty_writeback) (struct page *, bool *, bool *);
> diff --git a/fs/proc/page.c b/fs/proc/page.c
> index 3ecd445e830d..ce3d08a4ad8d 100644
> --- a/fs/proc/page.c
> +++ b/fs/proc/page.c
> @@ -157,6 +157,9 @@ u64 stable_page_flags(struct page *page)
>   	if (page_is_idle(page))
>   		u |= 1 << KPF_IDLE;
>
> +	if (PageMovable(page))
> +		u |= 1 << KPF_MOVABLE;
> +
>   	u |= kpf_copy_bit(k, KPF_LOCKED,	PG_locked);
>
>   	u |= kpf_copy_bit(k, KPF_SLAB,		PG_slab);
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index da9e67d937e5..36f2d610e7a8 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -401,6 +401,8 @@ struct address_space_operations {
>   	 */
>   	int (*migratepage) (struct address_space *,
>   			struct page *, struct page *, enum migrate_mode);
> +	bool (*isolate_page)(struct page *, isolate_mode_t);
> +	void (*putback_page)(struct page *);
>   	int (*launder_page) (struct page *);
>   	int (*is_partially_uptodate) (struct page *, unsigned long,
>   					unsigned long);
> diff --git a/include/linux/migrate.h b/include/linux/migrate.h
> index 9b50325e4ddf..404fbfefeb33 100644
> --- a/include/linux/migrate.h
> +++ b/include/linux/migrate.h
> @@ -37,6 +37,8 @@ extern int migrate_page(struct address_space *,
>   			struct page *, struct page *, enum migrate_mode);
>   extern int migrate_pages(struct list_head *l, new_page_t new, free_page_t
free,
>   		unsigned long private, enum migrate_mode mode, int reason);
> +extern bool isolate_movable_page(struct page *page, isolate_mode_t mode);
> +extern void putback_movable_page(struct page *page);
>
>   extern int migrate_prep(void);
>   extern int migrate_prep_local(void);
> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> index f4ed4f1b0c77..77ebf8fdbc6e 100644
> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h
> @@ -129,6 +129,10 @@ enum pageflags {
>
>   	/* Compound pages. Stored in first tail page's flags */
>   	PG_double_map = PG_private_2,
> +
> +	/* non-lru movable pages */
> +	PG_movable = PG_reclaim,
> +	PG_isolated = PG_owner_priv_1,
Documentation should probably state that these fields alias and subsystem 
supporting the movable pages shouldn't use them elsewhere.

Also I'm a bit uncomfortable how isolate_movable_page() blindly expects that
page->mapping->a_ops->isolate_page exists for PageMovable() pages. What
if it's
a false positive on a PG_reclaim page? Can we rely on PG_reclaim always (and 
without races) implying PageLRU() so that we don't even attempt 
isolate_movable_page()?
>   };
>
>   #ifndef __GENERATING_BOUNDS_H
> @@ -614,6 +618,33 @@ static inline void __ClearPageBalloon(struct page
*page)
>   	atomic_set(&page->_mapcount, -1);
>   }
>
> +#define PAGE_MOVABLE_MAPCOUNT_VALUE (-255)
IIRC this was what Gioh's previous attempts used instead of PG_movable? Is
it
still needed? Doesn't it prevent a driver providing movable *and* mapped
pages?
If it's to distinguish the PG_reclaim alias that I mention above, it seems
like
an overkill to me. Why would be need both special mapcount value and a flag? 
Checking that page->mapping->a_ops->isolate_page exists before calling
it should
be enough to resolve the ambiguity?
> +
> +static inline int PageMovable(struct page *page)
> +{
> +	return ((test_bit(PG_movable, &(page)->flags) &&
> +		atomic_read(&page->_mapcount) == PAGE_MOVABLE_MAPCOUNT_VALUE)
> +		|| PageBalloon(page));
> +}
> +
> +/* Caller should hold a PG_lock */
> +static inline void __SetPageMovable(struct page *page,
> +				struct address_space *mapping)
> +{
> +	page->mapping = mapping;
> +	__set_bit(PG_movable, &page->flags);
> +	atomic_set(&page->_mapcount, PAGE_MOVABLE_MAPCOUNT_VALUE);
> +}
> +
> +static inline void __ClearPageMovable(struct page *page)
> +{
> +	atomic_set(&page->_mapcount, -1);
> +	__clear_bit(PG_movable, &(page)->flags);
> +	page->mapping = NULL;
> +}
> +
> +PAGEFLAG(Isolated, isolated, PF_ANY);
> +
>   /*
>    * 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 5da5f8751ce7..a184fd2434fa 100644
> --- a/include/uapi/linux/kernel-page-flags.h
> +++ b/include/uapi/linux/kernel-page-flags.h
> @@ -34,6 +34,7 @@
>   #define KPF_BALLOON		23
>   #define KPF_ZERO_PAGE		24
>   #define KPF_IDLE		25
> +#define KPF_MOVABLE		26
>
>
>   #endif /* _UAPILINUX_KERNEL_PAGE_FLAGS_H */
> diff --git a/mm/compaction.c b/mm/compaction.c
> index ccf97b02b85f..7557aedddaee 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -703,7 +703,7 @@ isolate_migratepages_block(struct compact_control *cc,
unsigned long low_pfn,
>
>   		/*
>   		 * Check may be lockless but that's ok as we recheck later.
> -		 * It's possible to migrate LRU pages and balloon pages
> +		 * It's possible to migrate LRU and movable kernel pages.
>   		 * Skip any other type of page
>   		 */
>   		is_lru = PageLRU(page);
> @@ -714,6 +714,18 @@ isolate_migratepages_block(struct compact_control *cc,
unsigned long low_pfn,
>   					goto isolate_success;
>   				}
>   			}
> +
> +			if (unlikely(PageMovable(page)) &&
> +					!PageIsolated(page)) {
> +				if (locked) {
> +					spin_unlock_irqrestore(&zone->lru_lock,
> +									flags);
> +					locked = false;
> +				}
> +
> +				if (isolate_movable_page(page, isolate_mode))
> +					goto isolate_success;
> +			}
>   		}
>
>   		/*
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 53529c805752..b56bf2b3fe8c 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -73,6 +73,85 @@ int migrate_prep_local(void)
>   	return 0;
>   }
>
> +bool isolate_movable_page(struct page *page, isolate_mode_t mode)
> +{
> +	bool ret = false;
Maintaining "ret" seems useless here. All the "goto out*"
statements are
executed only when ret is false, and ret == true is returned by a different
return.
> +
> +	/*
> +	 * 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 movable 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.
> +	 */
> +	if (unlikely(!get_page_unless_zero(page)))
> +		goto out;
> +
> +	/*
> +	 * Check PG_movable before holding a PG_lock because page's owner
> +	 * assumes anybody doesn't touch PG_lock of newly allocated page.
> +	 */
> +	if (unlikely(!PageMovable(page)))
> +		goto out_putpage;
> +	/*
> +	 * As movable 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.
> +	 *
> +	 * In order to avoid having an already isolated movable 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 movable page isolation steps.
> +	 */
> +	if (unlikely(!trylock_page(page)))
> +		goto out_putpage;
> +
> +	if (!PageMovable(page) || PageIsolated(page))
> +		goto out_no_isolated;
> +
> +	ret = page->mapping->a_ops->isolate_page(page, mode);
> +	if (!ret)
> +		goto out_no_isolated;
> +
> +	WARN_ON_ONCE(!PageIsolated(page));
> +	unlock_page(page);
> +	return ret;
> +
> +out_no_isolated:
> +	unlock_page(page);
> +out_putpage:
> +	put_page(page);
> +out:
> +	return ret;
> +}
> +
> +/* It should be called on page which is PG_movable */
> +void putback_movable_page(struct page *page)
> +{
> +	/*
> +	 * 'lock_page()' stabilizes the page and prevents races against
> +	 * concurrent isolation threads attempting to re-isolate it.
> +	 */
> +	VM_BUG_ON_PAGE(!PageMovable(page), page);
> +
> +	lock_page(page);
> +	if (PageIsolated(page)) {
> +		struct address_space *mapping;
> +
> +		mapping = page_mapping(page);
> +		mapping->a_ops->putback_page(page);
> +		WARN_ON_ONCE(PageIsolated(page));
> +	} else {
> +		__ClearPageMovable(page);
> +	}
> +	unlock_page(page);
> +	/* drop the extra ref count taken for movable page isolation */
> +	put_page(page);
> +}
> +
>   /*
>    * Put previously isolated pages back onto the appropriate lists
>    * from where they were once taken off for compaction/migration.
> @@ -94,10 +173,18 @@ 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)))
> +		if (unlikely(isolated_balloon_page(page))) {
>   			balloon_page_putback(page);
> -		else
> +		} else if (unlikely(PageMovable(page))) {
> +			if (PageIsolated(page)) {
> +				putback_movable_page(page);
> +			} else {
> +				__ClearPageMovable(page);
We don't do lock_page() here, so what prevents parallel compaction isolating
the
same page?
> +				put_page(page);
> +			}
> +		} else {
>   			putback_lru_page(page);
> +		}
>   	}
>   }
>
> @@ -592,7 +679,7 @@ void migrate_page_copy(struct page *newpage, struct
page *page)
>    ***********************************************************/
>
>   /*
> - * Common logic to directly migrate a single page suitable for
> + * Common logic to directly migrate a single LRU page suitable for
>    * pages that do not use PagePrivate/PagePrivate2.
>    *
>    * Pages are locked upon entry and exit.
> @@ -755,24 +842,54 @@ static int move_to_new_page(struct page *newpage,
struct page *page,
>   				enum migrate_mode mode)
>   {
>   	struct address_space *mapping;
> -	int rc;
> +	int rc = -EAGAIN;
> +	bool lru_movable = true;
>
>   	VM_BUG_ON_PAGE(!PageLocked(page), page);
>   	VM_BUG_ON_PAGE(!PageLocked(newpage), newpage);
>
>   	mapping = page_mapping(page);
> -	if (!mapping)
> -		rc = migrate_page(mapping, newpage, page, mode);
> -	else if (mapping->a_ops->migratepage)
> -		/*
> -		 * Most pages have a mapping and most filesystems provide a
> -		 * migratepage callback. Anonymous pages are part of swap
> -		 * space which also has its own migratepage callback. This
> -		 * is the most common path for page migration.
> -		 */
> -		rc = mapping->a_ops->migratepage(mapping, newpage, page, mode);
> -	else
> -		rc = fallback_migrate_page(mapping, newpage, page, mode);
> +	/*
> +	 * In case of non-lru page, it could be released after
> +	 * isolation step. In that case, we shouldn't try
> +	 * fallback migration which was designed for LRU pages.
> +	 *
> +	 * The rule for such case is that subsystem should clear
> +	 * PG_isolated but remains PG_movable so VM should catch
> +	 * it and clear PG_movable for it.
> +	 */
> +	if (unlikely(PageMovable(page))) {
Can false positive from PG_reclaim occur here?
> +		lru_movable = false;
> +		VM_BUG_ON_PAGE(!mapping, page);
> +		if (!PageIsolated(page)) {
> +			rc = MIGRATEPAGE_SUCCESS;
> +			__ClearPageMovable(page);
> +			goto out;
> +		}
> +	}
> +
> +	if (likely(lru_movable)) {
> +		if (!mapping)
> +			rc = migrate_page(mapping, newpage, page, mode);
> +		else if (mapping->a_ops->migratepage)
> +			/*
> +			 * Most pages have a mapping and most filesystems
> +			 * provide a migratepage callback. Anonymous pages
> +			 * are part of swap space which also has its own
> +			 * migratepage callback. This is the most common path
> +			 * for page migration.
> +			 */
> +			rc = mapping->a_ops->migratepage(mapping, newpage,
> +							page, mode);
> +		else
> +			rc = fallback_migrate_page(mapping, newpage,
> +							page, mode);
> +	} else {
> +		rc = mapping->a_ops->migratepage(mapping, newpage,
> +						page, mode);
> +		WARN_ON_ONCE(rc == MIGRATEPAGE_SUCCESS &&
> +			PageIsolated(page));
> +	}
>
>   	/*
>   	 * When successful, old pagecache page->mapping must be cleared
before
> @@ -782,6 +899,7 @@ static int move_to_new_page(struct page *newpage,
struct page *page,
>   		if (!PageAnon(page))
>   			page->mapping = NULL;
>   	}
> +out:
>   	return rc;
>   }
>
> @@ -960,6 +1078,8 @@ static ICE_noinline int unmap_and_move(new_page_t
get_new_page,
>   			put_new_page(newpage, private);
>   		else
>   			put_page(newpage);
> +		if (PageMovable(page))
> +			__ClearPageMovable(page);
>   		goto out;
>   	}
>
> @@ -1000,8 +1120,26 @@ static ICE_noinline int unmap_and_move(new_page_t
get_new_page,
>   				num_poisoned_pages_inc();
>   		}
>   	} else {
> -		if (rc != -EAGAIN)
> -			putback_lru_page(page);
> +		if (rc != -EAGAIN) {
> +			/*
> +			 * subsystem couldn't remove PG_movable since page is
> +			 * isolated so PageMovable check is not racy in here.
> +			 * But PageIsolated check can be racy but it's okay
> +			 * because putback_movable_page checks it under PG_lock
> +			 * again.
> +			 */
> +			if (unlikely(PageMovable(page))) {
> +				if (PageIsolated(page))
> +					putback_movable_page(page);
> +				else {
> +					__ClearPageMovable(page);
Again, we don't do lock_page() here, so what prevents parallel compaction 
isolating the same page?

Sorry for so many questions, hope they all have good answers and this series is 
a success :) Thanks for picking it up.
> +					put_page(page);
> +				}
> +			} else {
> +				putback_lru_page(page);
> +			}
> +		}
> +
>   		if (put_new_page)
>   			put_new_page(newpage, private);
>   		else
>

On Fri, Apr 01, 2016 at 11:29:14PM +0200, Vlastimil Babka
wrote:
> Might have been better as a separate migration patch and then a
> compaction patch. It's prefixed mm/compaction, but most changed are
> in mm/migrate.c
Indeed. The title is rather misleading but not sure it's a good idea
to separate compaction and migration part.
I will just resend to change the tile from "mm/compaction" to
"mm/migration".
> 
> On 03/30/2016 09:12 AM, Minchan Kim wrote:
> >We have allowed migration for only LRU pages until now and it was
> >enough to make high-order pages. But recently, embedded system(e.g.,
> >webOS, android) uses lots of non-movable pages(e.g., zram, GPU memory)
> >so we have seen several reports about troubles of small high-order
> >allocation. For fixing the problem, there were several efforts
> >(e,g,. enhance compaction algorithm, SLUB fallback to 0-order page,
> >reserved memory, vmalloc and so on) but if there are lots of
> >non-movable pages in system, their solutions are void in the long run.
> >
> >So, this patch is to support facility to change non-movable pages
> >with movable. For the feature, this patch introduces functions related
> >to migration to address_space_operations as well as some page flags.
> >
> >Basically, this patch supports two page-flags and two functions related
> >to page migration. The flag and page->mapping stability are
protected
> >by PG_lock.
> >
> >	PG_movable
> >	PG_isolated
> >
> >	bool (*isolate_page) (struct page *, isolate_mode_t);
> >	void (*putback_page) (struct page *);
> >
> >Duty of subsystem want to make their pages as migratable are
> >as follows:
> >
> >1. It should register address_space to page->mapping then mark
> >the page as PG_movable via __SetPageMovable.
> >
> >2. It should mark the page as PG_isolated via SetPageIsolated
> >if isolation is sucessful and return true.
> 
> Ah another thing to document (especially in the comments/Doc) is
> that the subsystem must not expect anything to survive in page.lru
> (or fields that union it) after having isolated successfully.
Indeed. I surprised I didn't miss because I wrote it down somewhere
but might miss it during rebase.
I will fix it.
> 
> >3. If migration is successful, it should clear PG_isolated and
> >PG_movable of the page for free preparation then release the
> >reference of the page to free.
> >
> >4. If migration fails, putback function of subsystem should
> >clear PG_isolated via ClearPageIsolated.
> >
> >5. If a subsystem want to release isolated page, it should
> >clear PG_isolated but not PG_movable. Instead, VM will do it.
> 
> Under lock? Or just with ClearPageIsolated?
Both:
ClearPageIsolated undert PG_lock.

Yes, it's better to change ClearPageIsolated to __ClearPageIsolated.
> 
> >Cc: Vlastimil Babka <vbabka at suse.cz>
> >Cc: Mel Gorman <mgorman at suse.de>
> >Cc: Hugh Dickins <hughd at google.com>
> >Cc: dri-devel at lists.freedesktop.org
> >Cc: virtualization at lists.linux-foundation.org
> >Signed-off-by: Gioh Kim <gurugio at hanmail.net>
> >Signed-off-by: Minchan Kim <minchan at kernel.org>
> >---
> >  Documentation/filesystems/Locking      |   4 +
> >  Documentation/filesystems/vfs.txt      |   5 +
> >  fs/proc/page.c                         |   3 +
> >  include/linux/fs.h                     |   2 +
> >  include/linux/migrate.h                |   2 +
> >  include/linux/page-flags.h             |  31 ++++++
> >  include/uapi/linux/kernel-page-flags.h |   1 +
> >  mm/compaction.c                        |  14 ++-
> >  mm/migrate.c                           | 174
+++++++++++++++++++++++++++++----
> >  9 files changed, 217 insertions(+), 19 deletions(-)
> >
> >diff --git a/Documentation/filesystems/Locking
b/Documentation/filesystems/Locking
> >index 619af9bfdcb3..0bb79560abb3 100644
> >--- a/Documentation/filesystems/Locking
> >+++ b/Documentation/filesystems/Locking
> >@@ -195,7 +195,9 @@ unlocks and drops the reference.
> >  	int (*releasepage) (struct page *, int);
> >  	void (*freepage)(struct page *);
> >  	int (*direct_IO)(struct kiocb *, struct iov_iter *iter, loff_t
offset);
> >+	bool (*isolate_page) (struct page *, isolate_mode_t);
> >  	int (*migratepage)(struct address_space *, struct page *, struct
page *);
> >+	void (*putback_page) (struct page *);
> >  	int (*launder_page)(struct page *);
> >  	int (*is_partially_uptodate)(struct page *, unsigned long, unsigned
long);
> >  	int (*error_remove_page)(struct address_space *, struct page *);
> >@@ -219,7 +221,9 @@ invalidatepage:		yes
> >  releasepage:		yes
> >  freepage:		yes
> >  direct_IO:
> >+isolate_page:		yes
> >  migratepage:		yes (both)
> >+putback_page:		yes
> >  launder_page:		yes
> >  is_partially_uptodate:	yes
> >  error_remove_page:	yes
> >diff --git a/Documentation/filesystems/vfs.txt
b/Documentation/filesystems/vfs.txt
> >index b02a7d598258..4c1b6c3b4bc8 100644
> >--- a/Documentation/filesystems/vfs.txt
> >+++ b/Documentation/filesystems/vfs.txt
> >@@ -592,9 +592,14 @@ struct address_space_operations {
> >  	int (*releasepage) (struct page *, int);
> >  	void (*freepage)(struct page *);
> >  	ssize_t (*direct_IO)(struct kiocb *, struct iov_iter *iter, loff_t
offset);
> >+	/* isolate a page for migration */
> >+	bool (*isolate_page) (struct page *, isolate_mode_t);
> >  	/* migrate the contents of a page to the specified target */
> >  	int (*migratepage) (struct page *, struct page *);
> >+	/* put the page back to right list */
> 
> ... "after a failed migration" ?
Better.
> 
> >+	void (*putback_page) (struct page *);
> >  	int (*launder_page) (struct page *);
> >+
> >  	int (*is_partially_uptodate) (struct page *, unsigned long,
> >  					unsigned long);
> >  	void (*is_dirty_writeback) (struct page *, bool *, bool *);
> >diff --git a/fs/proc/page.c b/fs/proc/page.c
> >index 3ecd445e830d..ce3d08a4ad8d 100644
> >--- a/fs/proc/page.c
> >+++ b/fs/proc/page.c
> >@@ -157,6 +157,9 @@ u64 stable_page_flags(struct page *page)
> >  	if (page_is_idle(page))
> >  		u |= 1 << KPF_IDLE;
> >
> >+	if (PageMovable(page))
> >+		u |= 1 << KPF_MOVABLE;
> >+
> >  	u |= kpf_copy_bit(k, KPF_LOCKED,	PG_locked);
> >
> >  	u |= kpf_copy_bit(k, KPF_SLAB,		PG_slab);
> >diff --git a/include/linux/fs.h b/include/linux/fs.h
> >index da9e67d937e5..36f2d610e7a8 100644
> >--- a/include/linux/fs.h
> >+++ b/include/linux/fs.h
> >@@ -401,6 +401,8 @@ struct address_space_operations {
> >  	 */
> >  	int (*migratepage) (struct address_space *,
> >  			struct page *, struct page *, enum migrate_mode);
> >+	bool (*isolate_page)(struct page *, isolate_mode_t);
> >+	void (*putback_page)(struct page *);
> >  	int (*launder_page) (struct page *);
> >  	int (*is_partially_uptodate) (struct page *, unsigned long,
> >  					unsigned long);
> >diff --git a/include/linux/migrate.h b/include/linux/migrate.h
> >index 9b50325e4ddf..404fbfefeb33 100644
> >--- a/include/linux/migrate.h
> >+++ b/include/linux/migrate.h
> >@@ -37,6 +37,8 @@ extern int migrate_page(struct address_space *,
> >  			struct page *, struct page *, enum migrate_mode);
> >  extern int migrate_pages(struct list_head *l, new_page_t new,
free_page_t free,
> >  		unsigned long private, enum migrate_mode mode, int reason);
> >+extern bool isolate_movable_page(struct page *page, isolate_mode_t
mode);
> >+extern void putback_movable_page(struct page *page);
> >
> >  extern int migrate_prep(void);
> >  extern int migrate_prep_local(void);
> >diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> >index f4ed4f1b0c77..77ebf8fdbc6e 100644
> >--- a/include/linux/page-flags.h
> >+++ b/include/linux/page-flags.h
> >@@ -129,6 +129,10 @@ enum pageflags {
> >
> >  	/* Compound pages. Stored in first tail page's flags */
> >  	PG_double_map = PG_private_2,
> >+
> >+	/* non-lru movable pages */
> >+	PG_movable = PG_reclaim,
> >+	PG_isolated = PG_owner_priv_1,
> 
> Documentation should probably state that these fields alias and
> subsystem supporting the movable pages shouldn't use them elsewhere.
Yeb.
> 
> Also I'm a bit uncomfortable how isolate_movable_page() blindly expects
that
> page->mapping->a_ops->isolate_page exists for PageMovable() pages.
> What if it's a false positive on a PG_reclaim page? Can we rely on
> PG_reclaim always (and without races) implying PageLRU() so that we
> don't even attempt isolate_movable_page()?
For now, we shouldn't have such a false positive because PageMovable
checks page->_mapcount == PAGE_MOVABLE_MAPCOUNT_VALUE as well as PG_movable
under PG_lock.

But I read your question about user-mapped drvier pages so we cannot
use _mapcount anymore so I will find another thing. A option is this.

static inline int PageMovable(struct page *page)
{
        int ret = 0;
        struct address_space *mapping;
        struct address_space_operations *a_op;

        if (!test_bit(PG_movable, &(page->flags))
                goto out;

        mapping = page->mapping;
        if (!mapping)
                goto out;

        a_op = mapping->a_op;
        if (!aop)
                goto out;
        if (a_op->isolate_page)
                ret = 1;
out:
        return ret;

}

It works under PG_lock but with this, we need trylock_page to peek
whether it's movable non-lru or not for scanning pfn.
For avoiding that, we need another function to peek which just checks
PG_movable bit instead of all things.


/*
 * If @page_locked is false, we cannot guarantee page->mapping's
stability
 * so just the function checks with PG_movable which could be false positive
 * so caller should check it again under PG_lock to check
a_ops->isolate_page.
 */
static inline int PageMovable(struct page *page, bool page_locked)
{
        int ret = 0;
        struct address_space *mapping;
        struct address_space_operations *a_op;

        if (!test_bit(PG_movable, &(page->flags))
                goto out;

        if (!page_locked) {
                ret = 1;
                goto out;
        }

        mapping = page->mapping;
        if (!mapping)
                goto out;

        a_op = mapping->a_op;
        if (!aop)
                goto out;
        if (a_op->isolate_page)
                ret = 1;
out:
        return ret;
}
> 
> >  };
> >
> >  #ifndef __GENERATING_BOUNDS_H
> >@@ -614,6 +618,33 @@ static inline void __ClearPageBalloon(struct page
*page)
> >  	atomic_set(&page->_mapcount, -1);
> >  }
> >
> >+#define PAGE_MOVABLE_MAPCOUNT_VALUE (-255)
> 
> IIRC this was what Gioh's previous attempts used instead of
> PG_movable? Is it still needed? Doesn't it prevent a driver
It needs to avoid false positive as I said.
> providing movable *and* mapped pages?
Absolutely true. I will rethink about it.
> If it's to distinguish the PG_reclaim alias that I mention above, it
> seems like an overkill to me. Why would be need both special
> mapcount value and a flag? Checking that
> page->mapping->a_ops->isolate_page exists before calling it should
> be enough to resolve the ambiguity?
As I mentioned, using a_ops->isolate_page needs to be done under PG_lock.
And the idea I suggested above will work, I guess.
I will try it.
> 
> >+
> >+static inline int PageMovable(struct page *page)
> >+{
> >+	return ((test_bit(PG_movable, &(page)->flags) &&
> >+		atomic_read(&page->_mapcount) == PAGE_MOVABLE_MAPCOUNT_VALUE)
> >+		|| PageBalloon(page));
> >+}
> >+
> >+/* Caller should hold a PG_lock */
> >+static inline void __SetPageMovable(struct page *page,
> >+				struct address_space *mapping)
> >+{
> >+	page->mapping = mapping;
> >+	__set_bit(PG_movable, &page->flags);
> >+	atomic_set(&page->_mapcount, PAGE_MOVABLE_MAPCOUNT_VALUE);
> >+}
> >+
> >+static inline void __ClearPageMovable(struct page *page)
> >+{
> >+	atomic_set(&page->_mapcount, -1);
> >+	__clear_bit(PG_movable, &(page)->flags);
> >+	page->mapping = NULL;
> >+}
> >+
> >+PAGEFLAG(Isolated, isolated, PF_ANY);
> >+
> >  /*
> >   * 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 5da5f8751ce7..a184fd2434fa 100644
> >--- a/include/uapi/linux/kernel-page-flags.h
> >+++ b/include/uapi/linux/kernel-page-flags.h
> >@@ -34,6 +34,7 @@
> >  #define KPF_BALLOON		23
> >  #define KPF_ZERO_PAGE		24
> >  #define KPF_IDLE		25
> >+#define KPF_MOVABLE		26
> >
> >
> >  #endif /* _UAPILINUX_KERNEL_PAGE_FLAGS_H */
> >diff --git a/mm/compaction.c b/mm/compaction.c
> >index ccf97b02b85f..7557aedddaee 100644
> >--- a/mm/compaction.c
> >+++ b/mm/compaction.c
> >@@ -703,7 +703,7 @@ isolate_migratepages_block(struct compact_control
*cc, unsigned long low_pfn,
> >
> >  		/*
> >  		 * Check may be lockless but that's ok as we recheck later.
> >-		 * It's possible to migrate LRU pages and balloon pages
> >+		 * It's possible to migrate LRU and movable kernel pages.
> >  		 * Skip any other type of page
> >  		 */
> >  		is_lru = PageLRU(page);
> >@@ -714,6 +714,18 @@ isolate_migratepages_block(struct compact_control
*cc, unsigned long low_pfn,
> >  					goto isolate_success;
> >  				}
> >  			}
> >+
> >+			if (unlikely(PageMovable(page)) &&
> >+					!PageIsolated(page)) {
> >+				if (locked) {
> >+					spin_unlock_irqrestore(&zone->lru_lock,
> >+									flags);
> >+					locked = false;
> >+				}
> >+
> >+				if (isolate_movable_page(page, isolate_mode))
> >+					goto isolate_success;
> >+			}
> >  		}
> >
> >  		/*
> >diff --git a/mm/migrate.c b/mm/migrate.c
> >index 53529c805752..b56bf2b3fe8c 100644
> >--- a/mm/migrate.c
> >+++ b/mm/migrate.c
> >@@ -73,6 +73,85 @@ int migrate_prep_local(void)
> >  	return 0;
> >  }
> >
> >+bool isolate_movable_page(struct page *page, isolate_mode_t mode)
> >+{
> >+	bool ret = false;
> 
> Maintaining "ret" seems useless here. All the "goto
out*" statements
> are executed only when ret is false, and ret == true is returned by
> a different return.
Yeb. Will change.
> 
> >+
> >+	/*
> >+	 * 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 movable 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.
> >+	 */
> >+	if (unlikely(!get_page_unless_zero(page)))
> >+		goto out;
> >+
> >+	/*
> >+	 * Check PG_movable before holding a PG_lock because page's owner
> >+	 * assumes anybody doesn't touch PG_lock of newly allocated page.
> >+	 */
> >+	if (unlikely(!PageMovable(page)))
> >+		goto out_putpage;
> >+	/*
> >+	 * As movable 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.
> >+	 *
> >+	 * In order to avoid having an already isolated movable 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 movable page isolation steps.
> >+	 */
> >+	if (unlikely(!trylock_page(page)))
> >+		goto out_putpage;
> >+
> >+	if (!PageMovable(page) || PageIsolated(page))
> >+		goto out_no_isolated;
> >+
> >+	ret = page->mapping->a_ops->isolate_page(page, mode);
> >+	if (!ret)
> >+		goto out_no_isolated;
> >+
> >+	WARN_ON_ONCE(!PageIsolated(page));
> >+	unlock_page(page);
> >+	return ret;
> >+
> >+out_no_isolated:
> >+	unlock_page(page);
> >+out_putpage:
> >+	put_page(page);
> >+out:
> >+	return ret;
> >+}
> >+
> >+/* It should be called on page which is PG_movable */
> >+void putback_movable_page(struct page *page)
> >+{
> >+	/*
> >+	 * 'lock_page()' stabilizes the page and prevents races
against
> >+	 * concurrent isolation threads attempting to re-isolate it.
> >+	 */
> >+	VM_BUG_ON_PAGE(!PageMovable(page), page);
> >+
> >+	lock_page(page);
> >+	if (PageIsolated(page)) {
> >+		struct address_space *mapping;
> >+
> >+		mapping = page_mapping(page);
> >+		mapping->a_ops->putback_page(page);
> >+		WARN_ON_ONCE(PageIsolated(page));
> >+	} else {
> >+		__ClearPageMovable(page);
> >+	}
> >+	unlock_page(page);
> >+	/* drop the extra ref count taken for movable page isolation */
> >+	put_page(page);
> >+}
> >+
> >  /*
> >   * Put previously isolated pages back onto the appropriate lists
> >   * from where they were once taken off for compaction/migration.
> >@@ -94,10 +173,18 @@ 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)))
> >+		if (unlikely(isolated_balloon_page(page))) {
> >  			balloon_page_putback(page);
> >-		else
> >+		} else if (unlikely(PageMovable(page))) {
> >+			if (PageIsolated(page)) {
> >+				putback_movable_page(page);
> >+			} else {
> >+				__ClearPageMovable(page);
> 
> We don't do lock_page() here, so what prevents parallel compaction
> isolating the same page?
Need PG_lock.
> 
> >+				put_page(page);
> >+			}
> >+		} else {
> >  			putback_lru_page(page);
> >+		}
> >  	}
> >  }
> >
> >@@ -592,7 +679,7 @@ void migrate_page_copy(struct page *newpage, struct
page *page)
> >   ***********************************************************/
> >
> >  /*
> >- * Common logic to directly migrate a single page suitable for
> >+ * Common logic to directly migrate a single LRU page suitable for
> >   * pages that do not use PagePrivate/PagePrivate2.
> >   *
> >   * Pages are locked upon entry and exit.
> >@@ -755,24 +842,54 @@ static int move_to_new_page(struct page *newpage,
struct page *page,
> >  				enum migrate_mode mode)
> >  {
> >  	struct address_space *mapping;
> >-	int rc;
> >+	int rc = -EAGAIN;
> >+	bool lru_movable = true;
> >
> >  	VM_BUG_ON_PAGE(!PageLocked(page), page);
> >  	VM_BUG_ON_PAGE(!PageLocked(newpage), newpage);
> >
> >  	mapping = page_mapping(page);
> >-	if (!mapping)
> >-		rc = migrate_page(mapping, newpage, page, mode);
> >-	else if (mapping->a_ops->migratepage)
> >-		/*
> >-		 * Most pages have a mapping and most filesystems provide a
> >-		 * migratepage callback. Anonymous pages are part of swap
> >-		 * space which also has its own migratepage callback. This
> >-		 * is the most common path for page migration.
> >-		 */
> >-		rc = mapping->a_ops->migratepage(mapping, newpage, page,
mode);
> >-	else
> >-		rc = fallback_migrate_page(mapping, newpage, page, mode);
> >+	/*
> >+	 * In case of non-lru page, it could be released after
> >+	 * isolation step. In that case, we shouldn't try
> >+	 * fallback migration which was designed for LRU pages.
> >+	 *
> >+	 * The rule for such case is that subsystem should clear
> >+	 * PG_isolated but remains PG_movable so VM should catch
> >+	 * it and clear PG_movable for it.
> >+	 */
> >+	if (unlikely(PageMovable(page))) {
> 
> Can false positive from PG_reclaim occur here?
PageMovable has _mapcount == PAGE_MOVALBE_MAPCOUNT_VALUE check.
> 
> >+		lru_movable = false;
> >+		VM_BUG_ON_PAGE(!mapping, page);
> >+		if (!PageIsolated(page)) {
> >+			rc = MIGRATEPAGE_SUCCESS;
> >+			__ClearPageMovable(page);
> >+			goto out;
> >+		}
> >+	}
> >+
> >+	if (likely(lru_movable)) {
> >+		if (!mapping)
> >+			rc = migrate_page(mapping, newpage, page, mode);
> >+		else if (mapping->a_ops->migratepage)
> >+			/*
> >+			 * Most pages have a mapping and most filesystems
> >+			 * provide a migratepage callback. Anonymous pages
> >+			 * are part of swap space which also has its own
> >+			 * migratepage callback. This is the most common path
> >+			 * for page migration.
> >+			 */
> >+			rc = mapping->a_ops->migratepage(mapping, newpage,
> >+							page, mode);
> >+		else
> >+			rc = fallback_migrate_page(mapping, newpage,
> >+							page, mode);
> >+	} else {
> >+		rc = mapping->a_ops->migratepage(mapping, newpage,
> >+						page, mode);
> >+		WARN_ON_ONCE(rc == MIGRATEPAGE_SUCCESS &&
> >+			PageIsolated(page));
> >+	}
> >
> >  	/*
> >  	 * When successful, old pagecache page->mapping must be cleared
before
> >@@ -782,6 +899,7 @@ static int move_to_new_page(struct page *newpage,
struct page *page,
> >  		if (!PageAnon(page))
> >  			page->mapping = NULL;
> >  	}
> >+out:
> >  	return rc;
> >  }
> >
> >@@ -960,6 +1078,8 @@ static ICE_noinline int unmap_and_move(new_page_t
get_new_page,
> >  			put_new_page(newpage, private);
> >  		else
> >  			put_page(newpage);
> >+		if (PageMovable(page))
> >+			__ClearPageMovable(page);
> >  		goto out;
> >  	}
> >
> >@@ -1000,8 +1120,26 @@ static ICE_noinline int
unmap_and_move(new_page_t get_new_page,
> >  				num_poisoned_pages_inc();
> >  		}
> >  	} else {
> >-		if (rc != -EAGAIN)
> >-			putback_lru_page(page);
> >+		if (rc != -EAGAIN) {
> >+			/*
> >+			 * subsystem couldn't remove PG_movable since page is
> >+			 * isolated so PageMovable check is not racy in here.
> >+			 * But PageIsolated check can be racy but it's okay
> >+			 * because putback_movable_page checks it under PG_lock
> >+			 * again.
> >+			 */
> >+			if (unlikely(PageMovable(page))) {
> >+				if (PageIsolated(page))
> >+					putback_movable_page(page);
> >+				else {
> >+					__ClearPageMovable(page);
> 
> Again, we don't do lock_page() here, so what prevents parallel
> compaction isolating the same page?
It seems to need PG_lock in there, too.
Thanks for catching it up.
> 
> Sorry for so many questions, hope they all have good answers and
> this series is a success :) Thanks for picking it up.
No problem at all. Many question means the code or/and doc is not
clear and still need be improved.

Thanks for detail review, Vlastimil!
I will resend new versions after vacation in this week.

On 04/04/2016 07:12 AM, Minchan Kim wrote:
> On Fri, Apr 01, 2016 at 11:29:14PM +0200, Vlastimil Babka wrote:
>> Might have been better as a separate migration patch and then a
>> compaction patch. It's prefixed mm/compaction, but most changed are
>> in mm/migrate.c
>
> Indeed. The title is rather misleading but not sure it's a good idea
> to separate compaction and migration part.
Guess it's better to see the new functions together with its user after 
all, OK.
> I will just resend to change the tile from "mm/compaction" to
> "mm/migration".
OK!
>> Also I'm a bit uncomfortable how isolate_movable_page() blindly
expects that
>> page->mapping->a_ops->isolate_page exists for PageMovable()
pages.
>> What if it's a false positive on a PG_reclaim page? Can we rely on
>> PG_reclaim always (and without races) implying PageLRU() so that we
>> don't even attempt isolate_movable_page()?
>
> For now, we shouldn't have such a false positive because PageMovable
> checks page->_mapcount == PAGE_MOVABLE_MAPCOUNT_VALUE as well as
PG_movable
> under PG_lock.
>
> But I read your question about user-mapped drvier pages so we cannot
> use _mapcount anymore so I will find another thing. A option is this.
>
> static inline int PageMovable(struct page *page)
> {
>          int ret = 0;
>          struct address_space *mapping;
>          struct address_space_operations *a_op;
>
>          if (!test_bit(PG_movable, &(page->flags))
>                  goto out;
>
>          mapping = page->mapping;
>          if (!mapping)
>                  goto out;
>
>          a_op = mapping->a_op;
>          if (!aop)
>                  goto out;
>          if (a_op->isolate_page)
>                  ret = 1;
> out:
>          return ret;
>
> }
>
> It works under PG_lock but with this, we need trylock_page to peek
> whether it's movable non-lru or not for scanning pfn.
Hm I hoped that with READ_ONCE() we could do the peek safely without 
trylock_page, if we use it only as a heuristic. But I guess it would 
require at least RCU-level protection of the 
page->mapping->a_op->isolate_page chain.
> For avoiding that, we need another function to peek which just checks
> PG_movable bit instead of all things.
>
>
> /*
>   * If @page_locked is false, we cannot guarantee page->mapping's
stability
>   * so just the function checks with PG_movable which could be false
positive
>   * so caller should check it again under PG_lock to check
a_ops->isolate_page.
>   */
> static inline int PageMovable(struct page *page, bool page_locked)
> {
>          int ret = 0;
>          struct address_space *mapping;
>          struct address_space_operations *a_op;
>
>          if (!test_bit(PG_movable, &(page->flags))
>                  goto out;
>
>          if (!page_locked) {
>                  ret = 1;
>                  goto out;
>          }
>
>          mapping = page->mapping;
>          if (!mapping)
>                  goto out;
>
>          a_op = mapping->a_op;
>          if (!aop)
>                  goto out;
>          if (a_op->isolate_page)
>                  ret = 1;
> out:
>          return ret;
> }
I wouldn't put everything into single function, but create something 
like __PageMovable() just for the unlocked peek. Unlike the 
zone->lru_lock, we don't keep page_lock() across iterations in 
isolate_migratepages_block(), as obviously each page has different lock.
So the page_locked parameter would be always passed as constant, and at 
that point it's better to have separate functions.

So I guess the question is how many false positives from overlap with 
PG_reclaim the scanner will hit if we give up on 
PAGE_MOVABLE_MAPCOUNT_VALUE, as that will increase number of page locks 
just to realize that it's not actual PageMovable() page...
> Thanks for detail review, Vlastimil!
> I will resend new versions after vacation in this week.
You're welcome, great!