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Message-ID: <56FEE82A.30602@suse.cz>
Date: Fri, 1 Apr 2016 23:29:14 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: Minchan Kim <minchan@...nel.org>,
Andrew Morton <akpm@...ux-foundation.org>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
jlayton@...chiereds.net, bfields@...ldses.org,
Joonsoo Kim <iamjoonsoo.kim@....com>, 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 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@...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 | 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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