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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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