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SubjectRe: [PATCH v4 1/3] mm: introduce compaction and migration for virtio ballooned pages
On Wed, Jul 18, 2012 at 04:12:39PM -0700, Andrew Morton wrote:
> On Wed, 18 Jul 2012 20:07:07 -0300
> Rafael Aquini <aquini@redhat.com> wrote:
>
> > >
> > > > +}
> > > > +#else
> > > > +static inline bool is_balloon_page(struct page *page) { return false; }
> > > > +static inline bool isolate_balloon_page(struct page *page) { return false; }
> > > > +static inline bool putback_balloon_page(struct page *page) { return false; }
> > > > +#endif /* (VIRTIO_BALLOON || VIRTIO_BALLOON_MODULE) && COMPACTION */
> > >
> > > This means that if CONFIG_VIRTIO_BALLOON=y and CONFIG_COMPACTION=n,
> > > is_balloon_page() will always return NULL. IOW, no pages are balloon
> > > pages! This is wrong.
> > >
> > I believe it's right, actually, as we can see CONFIG_COMPACTION=n associated with
> > CONFIG_MIGRATION=y (and CONFIG_VIRTIO_BALLOON=y).
> > For such config case we cannot perform the is_balloon_page() test branches
> > placed on mm/migration.c
>
> No, it isn't right. Look at the name: "is_balloon_page". If a caller
> runs is_balloon_page() against a balloon page with CONFIG_COMPACTION=n
> then they will get "false", which is incorrect.
>
You're right, I got your point.

> So the function needs a better name - one which communicates that it is
> a balloon page *for the purposes of processing by the compaction code*.
> Making the function private to compaction.c would help with that, if
> feasible.
>

How about this (adjusted) approach:

diff --git a/include/linux/mm.h b/include/linux/mm.h
index b94f17a..02a8f80 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1629,8 +1629,7 @@ static inline unsigned int debug_guardpage_minorder(void)
{ return 0; }
static inline bool page_is_guard(struct page *page) { return false; }
#endif /* CONFIG_DEBUG_PAGEALLOC */

-#if (defined(CONFIG_VIRTIO_BALLOON) || \
- defined(CONFIG_VIRTIO_BALLOON_MODULE)) && defined(CONFIG_COMPACTION)
+#if (defined(CONFIG_VIRTIO_BALLOON) ||defined(CONFIG_VIRTIO_BALLOON_MODULE))
extern bool putback_balloon_page(struct page *);
extern struct address_space *balloon_mapping;

@@ -1638,11 +1637,13 @@ static inline bool is_balloon_page(struct page *page)
{
return (page->mapping && page->mapping == balloon_mapping);
}
+#if defined(CONFIG_COMPACTION)
+static inline bool balloon_compaction_enabled(void) { return true; }
#else
-static inline bool is_balloon_page(struct page *page) { return false; }
-static inline bool isolate_balloon_page(struct page *page) { return false; }
-static inline bool putback_balloon_page(struct page *page) { return false; }
-#endif /* (VIRTIO_BALLOON || VIRTIO_BALLOON_MODULE) && COMPACTION */
+static inline bool putback_balloon_page(struct page *page) { return false; }
+static inline bool balloon_compaction_enabled(void) { return false; }
+#endif /* CONFIG_COMPACTION */
+#endif /* (CONFIG_VIRTIO_BALLOON || CONFIG_VIRTIO_BALLOON_MODULE) */

#endif /* __KERNEL__ */
#endif /* _LINUX_MM_H */
diff --git a/mm/migrate.c b/mm/migrate.c
index 59c7bc5..f5f6a7d 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -78,7 +78,8 @@ void putback_lru_pages(struct list_head *l)
list_del(&page->lru);
dec_zone_page_state(page, NR_ISOLATED_ANON +
page_is_file_cache(page));
- if (unlikely(is_balloon_page(page)))
+ if (unlikely(is_balloon_page(page)) &&
+ balloon_compaction_enabled())
WARN_ON(!putback_balloon_page(page));
else
putback_lru_page(page);
@@ -786,7 +787,7 @@ static int __unmap_and_move(struct page *page, struct page
*newpage,
}
}

- if (is_balloon_page(page)) {
+ if (is_balloon_page(page) && balloon_compaction_enabled()) {
/*
* A ballooned page does not need any special attention from
* physical to virtual reverse mapping procedures.
@@ -867,7 +868,7 @@ static int unmap_and_move(new_page_t get_new_page, unsigned
long private,

rc = __unmap_and_move(page, newpage, force, offlining, mode);

- if (is_balloon_page(newpage)) {
+ if (is_balloon_page(newpage) && balloon_compaction_enabled()) {
/*
* A ballooned page has been migrated already. Now, it is the
* time to wrap-up counters, handle the old page back to Buddy


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