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

From: Minchan Kim
Date: Mon Aug 10 2015 - 03:19:33 EST

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On Fri, Jul 31, 2015 at 07:43:09PM +0900, Minchan Kim wrote:
> Hi Gioh,
>
> On Mon, Jul 13, 2015 at 05:35:17PM +0900, Gioh Kim wrote:
> > From: Gioh Kim <gurugio@xxxxxxxxxxx>
> >
> > Add framework to register callback functions and check page mobility.
> > There are some modes for page isolation so that isolate interface
> > has arguments of page address and isolation mode while putback
> > interface has only page address as argument.
> >
> > Signed-off-by: Gioh Kim <gioh.kim@xxxxxxx>
> > Acked-by: Rafael Aquini <aquini@xxxxxxxxxx>
> > ---
> > fs/proc/page.c | 3 ++
> > include/linux/compaction.h | 80 ++++++++++++++++++++++++++++++++++
> > include/linux/fs.h | 2 +
> > include/linux/page-flags.h | 19 ++++++++
> > include/uapi/linux/kernel-page-flags.h | 1 +
> > 5 files changed, 105 insertions(+)
> >
> > diff --git a/fs/proc/page.c b/fs/proc/page.c
> > index 7eee2d8..a4f5a00 100644
> > --- a/fs/proc/page.c
> > +++ b/fs/proc/page.c
> > @@ -146,6 +146,9 @@ u64 stable_page_flags(struct page *page)
> > if (PageBalloon(page))
> > u |= 1 << KPF_BALLOON;
> >
> > + if (PageMobile(page))
> > + u |= 1 << KPF_MOBILE;
> > +
>
> Need a new page flag for non-LRU page migration?
> I am worry that we don't have empty slot for page flag on 32-bit.
>
> Aha, see SetPageMobile below. You use _mapcount.
> It seems to be work for non-LRU pages but unfortunately, zsmalloc
> already have used that field as own purpose so there is no room
> for it.

Gioh, I just sent a patch which zsmalloc doesn't use page->mapping
and _mapcount so I think zsmalloc could support compaction with your
patchset. Although zsmalloc can support compaction with it, I still
don't think it's a good that driver have to mark pages mobile via
_mapcount.

I hope you can find another way.

Thanks.

>
>
> > u |= kpf_copy_bit(k, KPF_LOCKED, PG_locked);
> >
> > u |= kpf_copy_bit(k, KPF_SLAB, PG_slab);
> > diff --git a/include/linux/compaction.h b/include/linux/compaction.h
> > index aa8f61c..f693072 100644
> > --- a/include/linux/compaction.h
> > +++ b/include/linux/compaction.h
> > @@ -1,6 +1,9 @@
> > #ifndef _LINUX_COMPACTION_H
> > #define _LINUX_COMPACTION_H
> >
> > +#include <linux/page-flags.h>
> > +#include <linux/pagemap.h>
> > +
> > /* Return values for compact_zone() and try_to_compact_pages() */
> > /* compaction didn't start as it was deferred due to past failures */
> > #define COMPACT_DEFERRED 0
> > @@ -51,6 +54,70 @@ extern void compaction_defer_reset(struct zone *zone, int order,
> > bool alloc_success);
> > extern bool compaction_restarting(struct zone *zone, int order);
> >
> > +static inline bool mobile_page(struct page *page)
> > +{
> > + return page->mapping && (PageMobile(page) || PageBalloon(page));
>
>
> What's the locking rule to test mobile_page?
> Why we need such many checking?
>
> Why we need PageBalloon check?
> You are trying to make generic abstraction for non-LRU page to migrate
> but PageBalloon check in here breaks your goal.
>
> > +}
> > +
> > +static inline bool isolate_mobilepage(struct page *page, isolate_mode_t mode)
> > +{
> > + bool ret = false;
> > +
> > + /*
> > + * Avoid burning cycles with pages that are yet under __free_pages(),
> > + * or just got freed under us.
> > + *
> > + * In case we 'win' a race for a mobile page being freed under us and
> > + * raise its refcount preventing __free_pages() from doing its job
> > + * the put_page() at the end of this block will take care of
> > + * release this page, thus avoiding a nasty leakage.
> > + */
>
> Good.
>
> > + if (unlikely(!get_page_unless_zero(page)))
> > + goto out;
> > +
> > + /*
> > + * As mobile pages are not isolated from LRU lists, concurrent
> > + * compaction threads can race against page migration functions
> > + * as well as race against the releasing a page.
>
> How can it race with releasing a page?
> We already get refcount above.
>
> Do you mean page->mapping tearing race?
> If so, zsmalloc should be chaned to hold a lock.
>
>
> > + *
> > + * In order to avoid having an already isolated mobile page
> > + * being (wrongly) re-isolated while it is under migration,
> > + * or to avoid attempting to isolate pages being released,
> > + * lets be sure we have the page lock
> > + * before proceeding with the mobile page isolation steps.
> > + */
> > + if (unlikely(!trylock_page(page)))
> > + goto out_putpage;
> > +
> > + if (!(mobile_page(page) && page->mapping->a_ops->isolatepage))
> > + goto out_not_isolated;
>
> I cannot know how isolate_mobilepage is called by upper layer
> because this patch doesn't include it.
> Anyway, why we need such many checks to prove it's mobile page?
>
> mobile_page() {
> page->mapping && (PageMobile(page) || PageBalloon(page));
> }
>
> Additionally, added page->mapping->a_ops->isolatepage check
>
> Is it possible that a non-LRU page's address space provides
> only page->maping->migratepage without isolate/putback?
>
> Couldn't we make it simple like this?
>
> page->mapping && page->mapping->migratepage
>
> So, couldn't we make mobile_page like this
>
> PageMobile(struct page *page)
> {
> VM_BUG_ON_PAGE(!PageLocked(page), page);
> VM_BUG_ON_PAGE(PageLRU(page), page);
>
> return page->mapping && page->mapping->migratepage;
> }
>
> It will save _mapcount of struct page.
>
> > + ret = page->mapping->a_ops->isolatepage(page, mode);
> > + if (!ret)
> > + goto out_not_isolated;
> > + unlock_page(page);
> > + return ret;
> > +
> > +out_not_isolated:
> > + unlock_page(page);
> > +out_putpage:
> > + put_page(page);
> > +out:
> > + return ret;
> > +}
> > +
> > +static inline void putback_mobilepage(struct page *page)
> > +{
> > + /*
> > + * 'lock_page()' stabilizes the page and prevents races against
>
> What does it mean 'stabilize'?
> Clear comment is always welcome rather than vague word.
>
> > + * concurrent isolation threads attempting to re-isolate it.
> > + */
> > + lock_page(page);
> > + if (page->mapping && page->mapping->a_ops->putbackpage)
> > + page->mapping->a_ops->putbackpage(page);
> > + unlock_page(page);
> > + /* drop the extra ref count taken for mobile page isolation */
> > + put_page(page);
> > +}
> > #else
> > static inline unsigned long try_to_compact_pages(gfp_t gfp_mask,
> > unsigned int order, int alloc_flags,
> > @@ -83,6 +150,19 @@ static inline bool compaction_deferred(struct zone *zone, int order)
> > return true;
> > }
> >
> > +static inline bool mobile_page(struct page *page)
> > +{
> > + return false;
> > +}
> > +
> > +static inline bool isolate_mobilepage(struct page *page, isolate_mode_t mode)
> > +{
> > + return false;
> > +}
> > +
> > +static inline void putback_mobilepage(struct page *page)
> > +{
> > +}
> > #endif /* CONFIG_COMPACTION */
> >
> > #if defined(CONFIG_COMPACTION) && defined(CONFIG_SYSFS) && defined(CONFIG_NUMA)
> > diff --git a/include/linux/fs.h b/include/linux/fs.h
> > index a0653e5..2cc4b24 100644
> > --- a/include/linux/fs.h
> > +++ b/include/linux/fs.h
> > @@ -396,6 +396,8 @@ struct address_space_operations {
> > */
> > int (*migratepage) (struct address_space *,
> > struct page *, struct page *, enum migrate_mode);
> > + bool (*isolatepage) (struct page *, isolate_mode_t);
> > + void (*putbackpage) (struct page *);
> > int (*launder_page) (struct page *);
> > int (*is_partially_uptodate) (struct page *, unsigned long,
> > unsigned long);
> > diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> > index f34e040..abef145 100644
> > --- a/include/linux/page-flags.h
> > +++ b/include/linux/page-flags.h
> > @@ -582,6 +582,25 @@ static inline void __ClearPageBalloon(struct page *page)
> > atomic_set(&page->_mapcount, -1);
> > }
> >
> > +#define PAGE_MOBILE_MAPCOUNT_VALUE (-255)
> > +
> > +static inline int PageMobile(struct page *page)
> > +{
> > + return atomic_read(&page->_mapcount) == PAGE_MOBILE_MAPCOUNT_VALUE;
> > +}
> > +
> > +static inline void __SetPageMobile(struct page *page)
> > +{
> > + VM_BUG_ON_PAGE(atomic_read(&page->_mapcount) != -1, page);
> > + atomic_set(&page->_mapcount, PAGE_MOBILE_MAPCOUNT_VALUE);
> > +}
> > +
> > +static inline void __ClearPageMobile(struct page *page)
> > +{
> > + VM_BUG_ON_PAGE(!PageMobile(page), page);
> > + atomic_set(&page->_mapcount, -1);
> > +}
> > +
> > /*
> > * If network-based swap is enabled, sl*b must keep track of whether pages
> > * were allocated from pfmemalloc reserves.
> > diff --git a/include/uapi/linux/kernel-page-flags.h b/include/uapi/linux/kernel-page-flags.h
> > index a6c4962..d50d9e8 100644
> > --- a/include/uapi/linux/kernel-page-flags.h
> > +++ b/include/uapi/linux/kernel-page-flags.h
> > @@ -33,6 +33,7 @@
> > #define KPF_THP 22
> > #define KPF_BALLOON 23
> > #define KPF_ZERO_PAGE 24
> > +#define KPF_MOBILE 25
> >
> >
> > #endif /* _UAPILINUX_KERNEL_PAGE_FLAGS_H */
> > --
> > 2.1.4
> >
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