Re: [RFCv2 4/5] mm/compaction: compaction calls generic migration

[Konstantin Khlebnikov]

On Fri, Jun 26, 2015 at 12:58 PM, Gioh Kim <gioh.kim@xxxxxxx> wrote:
> Compaction calls interfaces of driver page migration
> instead of calling balloon migration directly.
>
> Signed-off-by: Gioh Kim <gioh.kim@xxxxxxx>
> ---
>  drivers/virtio/virtio_balloon.c |  1 +
>  mm/compaction.c                 |  9 +++++----
>  mm/migrate.c                    | 21 ++++++++++++---------
>  3 files changed, 18 insertions(+), 13 deletions(-)
>
> diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
> index c49b553..5e5cbea 100644
> --- a/drivers/virtio/virtio_balloon.c
> +++ b/drivers/virtio/virtio_balloon.c
> @@ -30,6 +30,7 @@
>  #include <linux/balloon_compaction.h>
>  #include <linux/oom.h>
>  #include <linux/wait.h>
> +#include <linux/anon_inodes.h>
>
>  /*
>   * Balloon device works in 4K page units.  So each page is pointed to by
> diff --git a/mm/compaction.c b/mm/compaction.c
> index 16e1b57..cc5ec81 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -14,7 +14,7 @@
>  #include <linux/backing-dev.h>
>  #include <linux/sysctl.h>
>  #include <linux/sysfs.h>
> -#include <linux/balloon_compaction.h>
> +#include <linux/compaction.h>
>  #include <linux/page-isolation.h>
>  #include <linux/kasan.h>
>  #include "internal.h"
> @@ -714,12 +714,13 @@ 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 pages and driver pages
>                  * Skip any other type of page
>                  */
>                 if (!PageLRU(page)) {
> -                       if (unlikely(balloon_page_movable(page))) {
> -                               if (balloon_page_isolate(page)) {
> +                       if (unlikely(driver_page_migratable(page))) {
> +                               if (page->mapping->a_ops->isolatepage(page,
> +                                                               isolate_mode)) {

Dereferencing page->mapping isn't safe here.
Page could be "truncated" from mapping at any time.

As you can see  balloon_page_isolate() calls get_page_unless_zero,
trylock_page and only after that checks balloon_page_movable again.

Existing code already does similar unsafe dereference in
__isolate_lru_page(): page->mapping->a_ops->migratepage

>                                         /* Successfully isolated */
>                                         goto isolate_success;
>                                 }
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 236ee25..a0bc1e4 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -35,7 +35,7 @@
>  #include <linux/hugetlb.h>
>  #include <linux/hugetlb_cgroup.h>
>  #include <linux/gfp.h>
> -#include <linux/balloon_compaction.h>
> +#include <linux/compaction.h>
>  #include <linux/mmu_notifier.h>
>
>  #include <asm/tlbflush.h>
> @@ -76,7 +76,7 @@ int migrate_prep_local(void)
>   * from where they were once taken off for compaction/migration.
>   *
>   * This function shall be used whenever the isolated pageset has been
> - * built from lru, balloon, hugetlbfs page. See isolate_migratepages_range()
> + * built from lru, driver, hugetlbfs page. See isolate_migratepages_range()
>   * and isolate_huge_page().
>   */
>  void putback_movable_pages(struct list_head *l)
> @@ -92,8 +92,8 @@ 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)))
> -                       balloon_page_putback(page);
> +               if (unlikely(driver_page_migratable(page)))
> +                       page->mapping->a_ops->putbackpage(page);
>                 else
>                         putback_lru_page(page);
>         }
> @@ -844,15 +844,18 @@ static int __unmap_and_move(struct page *page, struct page *newpage,
>                 }
>         }
>
> -       if (unlikely(isolated_balloon_page(page))) {
> +       if (unlikely(driver_page_migratable(page))) {
>                 /*
> -                * A ballooned page does not need any special attention from
> +                * A driver page does not need any special attention from
>                  * physical to virtual reverse mapping procedures.
>                  * Skip any attempt to unmap PTEs or to remap swap cache,
>                  * in order to avoid burning cycles at rmap level, and perform
>                  * the page migration right away (proteced by page lock).
>                  */
> -               rc = balloon_page_migrate(newpage, page, mode);
> +               rc = page->mapping->a_ops->migratepage(page->mapping,
> +                                                      newpage,
> +                                                      page,
> +                                                      mode);
>                 goto out_unlock;
>         }
>
> @@ -962,8 +965,8 @@ out:
>         if (rc != MIGRATEPAGE_SUCCESS && put_new_page) {
>                 ClearPageSwapBacked(newpage);
>                 put_new_page(newpage, private);
> -       } else if (unlikely(__is_movable_balloon_page(newpage))) {
> -               /* drop our reference, page already in the balloon */
> +       } else if (unlikely(driver_page_migratable(newpage))) {
> +               /* drop our reference */
>                 put_page(newpage);
>         } else
>                 putback_lru_page(newpage);
> --
> 1.9.1
>
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