Re: [PATCH v3 02/16] mm/compaction: support non-lru movable page migration

From: Vlastimil Babka
Date: Fri Apr 01 2016 - 17:29:26 EST

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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@xxxxxxx>
Cc: Mel Gorman <mgorman@xxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: dri-devel@xxxxxxxxxxxxxxxxxxxxx
Cc: virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Gioh Kim <gurugio@xxxxxxxxxxx>
Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx>
---
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

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