Message-ID: <20160324050012.GA18946@bbox>
Date:	Thu, 24 Mar 2016 14:00:12 +0900
From:	Minchan Kim <minchan@...nel.org>
To:	Joonsoo Kim <iamjoonsoo.kim@....com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	jlayton@...chiereds.net, bfields@...ldses.org,
	Vlastimil Babka <vbabka@...e.cz>, koct9i@...il.com,
	aquini@...hat.com, virtualization@...ts.linux-foundation.org,
	Mel Gorman <mgorman@...e.de>, Hugh Dickins <hughd@...gle.com>,
	Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
	Rik van Riel <riel@...hat.com>, rknize@...orola.com,
	Gioh Kim <gi-oh.kim@...fitbricks.com>,
	Sangseok Lee <sangseok.lee@....com>,
	Chan Gyun Jeong <chan.jeong@....com>,
	Al Viro <viro@...IV.linux.org.uk>,
	YiPing Xu <xuyiping@...ilicon.com>,
	dri-devel@...ts.freedesktop.org, Gioh Kim <gurugio@...mail.net>
Subject: Re: [PATCH v2 13/18] mm/compaction: support non-lru movable page
 migration

On Wed, Mar 23, 2016 at 02:05:11PM +0900, Joonsoo Kim wrote:
> On Tue, Mar 22, 2016 at 11:55:45PM +0900, Minchan Kim wrote:
> > On Tue, Mar 22, 2016 at 02:50:37PM +0900, Joonsoo Kim wrote:
> > > On Mon, Mar 21, 2016 at 03:31:02PM +0900, 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.
> > > > 
> > > > 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.
> > > 
> > > I think that this feature needs a separate document to describe
> > > requirement of each step in more detail. For example, #1 can be
> > > possible without holding a lock? I'm not sure because you lock
> > > the page when implementing zsmalloc page migration in 15th patch.
> > 
> > Yes, we needs PG_lock because install page->mapping and PG_movable
> > should be atomic and PG_lock protects it.
> > 
> > Better interface might be
> > 
> > void __SetPageMovable(struct page *page, sruct address_space *mapping);
> > 
> > > 
> > > #3 also need more explanation. Before release, we need to
> > > unregister address_space. I guess that it needs to be done
> > > in migratepage() but there is no explanation.
> > 
> > Okay, we can unregister address_space in __ClearPageMovable.
> > I will change it.
> > 
> > > 
> > > > 
> > > > Cc: Vlastimil Babka <vbabka@...e.cz>
> > > > Cc: Mel Gorman <mgorman@...e.de>
> > > > Cc: Hugh Dickins <hughd@...gle.com>
> > > > Cc: dri-devel@...ts.freedesktop.org
> > > > Cc: virtualization@...ts.linux-foundation.org
> > > > Signed-off-by: Gioh Kim <gurugio@...mail.net>
> > > > Signed-off-by: Minchan Kim <minchan@...nel.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             |  29 ++++++++
> > > >  include/uapi/linux/kernel-page-flags.h |   1 +
> > > >  mm/compaction.c                        |  14 +++-
> > > >  mm/migrate.c                           | 132 +++++++++++++++++++++++++++++----
> > > >  9 files changed, 177 insertions(+), 15 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 */
> > > > +	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 712f1b9992cc..e2066e73a9b8 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 14a97194b34b..b7ef2e41fa4a 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..3885064641c4 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,
> > > >  };
> > > >  
> > > >  #ifndef __GENERATING_BOUNDS_H
> > > > @@ -614,6 +618,31 @@ static inline void __ClearPageBalloon(struct page *page)
> > > >  	atomic_set(&page->_mapcount, -1);
> > > >  }
> > > >  
> > > > +#define PAGE_MOVABLE_MAPCOUNT_VALUE (-255)
> > > > +
> > > > +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)
> > > > +{
> > > > +	__set_bit(PG_movable, &page->flags);
> > > > +	atomic_set(&page->_mapcount, PAGE_MOVABLE_MAPCOUNT_VALUE);
> > > > +}
> > > 
> > > I think there is no big benefit to use non-atomic version here.
> > > PageMovable() is speculatively checked without holding a PG_lock
> > > so some cpu can miss this flag set if we use non-atomic version.
> > 
> > I wanted to show that double underscore is non-atomic so caller
> > should take care of the lock(i.e., PG_lock).
> > If we use atomic version, what kinds of benefit do we have?
> > Without holding PG_lock, atomic version could be raced, too.
> 
> My suggestion is holding PG_lock + atomic set. Compaction first
> checks PageMovable() without PG_lock so it can miss PageMovable() if
> non-atomic version is used.

I think it's really unlikely to race so the chance it can miss PageMovable
is really small so we don't need to add unncessary overhead caused by
atomic op in PG_movable setting side. As well, I want to use double
understcore to show the intention it's not atomic so user should take
care of the function.

> 
> > 
> > > 
> > > > +
> > > > +static inline void __ClearPageMovable(struct page *page)
> > > > +{
> > > > +	atomic_set(&page->_mapcount, -1);
> > > > +	__clear_bit(PG_movable, &(page)->flags);
> > > > +}
> > > > +
> > > > +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 b65c84267ce0..fc2842a15807 100644
> > > > --- a/mm/migrate.c
> > > > +++ b/mm/migrate.c
> > > > @@ -73,6 +73,75 @@ int migrate_prep_local(void)
> > > >  	return 0;
> > > >  }
> > > >  
> > > > +bool isolate_movable_page(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 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;
> > > 
> > > After getting the ref counter, we need to re-check PageMovable()
> > > to ensure that we indeed handle PageMovable() type page. Without it,
> > > the page we handle can be freed and re-allocated to someone else
> > > that isn't related to PageMovable() before grabbing the page. Trying
> > > trylock_page() in this case could cause a problem.
> > 
> > I don't get it. Why do you think trylock_page could cause a problem?
> > Could you elaborate it more?
> 
> Okay. Consider following sequence.
> 
> CPU-A                               CPU-B
> check PageMovable() in compacton
> ...                            free the page
> ...                            allocate the page for other usecase
> ...                            (maybe for file cache or slub)
> get unless 0 in isolate_movable_page()
> trylock (success)
>                                (try) lock! failed!
> 
> In this case, someone can see failure even if they are owner of the
> page. IIUC, this also can happen in zsmalloc. See init_zspage() in
> 15th patch. It assume that allocated page can be locked
> unconditionally.

Oops, As you pointed out, other places already assumed page owner's
trylock will always succeed. :(
Thanks for the notice.
I will add PageMovable check right before trylock.

> 
> > > 
> > > > +	/*
> > > > +	 * 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;
> > > > +}
> > > > +
> > > > +void putback_movable_page(struct page *page)
> > > > +{
> > > > +	struct address_space *mapping;
> > > > +
> > > > +	/*
> > > > +	 * 'lock_page()' stabilizes the page and prevents races against
> > > > +	 * concurrent isolation threads attempting to re-isolate it.
> > > > +	 */
> > > > +	lock_page(page);
> > > > +	mapping = page_mapping(page);
> > > > +	if (mapping) {
> > > > +		mapping->a_ops->putback_page(page);
> > > > +		WARN_ON_ONCE(PageIsolated(page));
> > > > +	}
> > > > +	unlock_page(page);
> > > > +	/* drop the extra ref count taken for movable page isolation */
> > > > +	put_page(page);
> > > > +}
> > > 
> > > This is complicated part for me. mapping can disappear? In this case,
> > > who clear PageIsolated()?
> > 
> > Page's owner, for exmaple, zsmalloc, virtio-balloon.
> > They can free page whenever they want once it holds a PG_lock.
> > They should clear mapping and PG_movable with PG_lock.
> > 
> > > 
> > > > +
> > > > +
> > > >  /*
> > > >   * Put previously isolated pages back onto the appropriate lists
> > > >   * from where they were once taken off for compaction/migration.
> > > > @@ -96,6 +165,8 @@ void putback_movable_pages(struct list_head *l)
> > > >  				page_is_file_cache(page));
> > > >  		if (unlikely(isolated_balloon_page(page)))
> > > >  			balloon_page_putback(page);
> > > > +		else if (unlikely(PageIsolated(page)))
> > > > +			putback_movable_page(page);
> > > >  		else
> > > >  			putback_lru_page(page);
> > > >  	}
> > > 
> > > I think that this will not work. You uses PG_owner_priv_1 as
> > > PG_isolated and it is possible that some lru pages has this flag.
> > > I guess you need to add PageMovable() check but it seems that mapping
> > > and this flag can be cleared by others.
> > 
> > Hmm, PageMovable check may work because If PageMovable check fails,
> > it means page's owner free the page so we can simple put the page to
> > release refcount in here.
> > I will check it.
> 
> Hmmm... But, in failure case, is it safe to call putback_lru_page() for them?

At the moment, it's safe to work. It is just added to lrupvec to release
although it was not LRU page. Yes, it's error-pronce so someday it hurts
someone. I will think more about it.


> And, PageIsolated() would be left. Is it okay? It's not symmetric that

Page owner should clear PageIsolated once he freed the page.

> isolated page can be freed by decreasing ref count without calling
> putback function. This should be clarified and documented.

Yes, I will add it on document.

Thanks!

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