Re: [PATCH 3/3] mm: Clear page->private when splitting or migrating a page

[Xiubo Li]

On 6/19/22 11:11 PM, Matthew Wilcox (Oracle) wrote:
> In our efforts to remove uses of PG_private, we have found folios with
> the private flag clear and folio->private not-NULL.  That is the root
> cause behind 642d51fb0775 ("ceph: check folio PG_private bit instead
> of folio->private").  It can also affect a few other filesystems that
> haven't yet reported a problem.
>
> compaction_alloc() can return a page with uninitialised page->private,
> and rather than checking all the callers of migrate_pages(), just zero
> page->private after calling get_new_page().  Similarly, the tail pages
> from split_huge_page() may also have an uninitialised page->private.
>
> Reported-by: Xiubo Li <xiubli@xxxxxxxxxx>
> Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
> ---
>   mm/huge_memory.c | 1 +
>   mm/migrate.c     | 1 +
>   2 files changed, 2 insertions(+)
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index f7248002dad9..9b31a50217b5 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -2377,6 +2377,7 @@ static void __split_huge_page_tail(struct page *head, int tail,
>   			page_tail);
>   	page_tail->mapping = head->mapping;
>   	page_tail->index = head->index + tail;
> +	page_tail->private = NULL;

There has a warning when compiling it:

mm/huge_memory.c: In function ‘__split_huge_page_tail’:
mm/huge_memory.c:2380:21: warning: assignment to ‘long unsigned int’ 
from ‘void *’ makes integer from pointer without a cast [-Wint-conversion]
  page_tail->private = NULL;
                     ^
  AR      mm/built-in.a


>   
>   	/* Page flags must be visible before we make the page non-compound. */
>   	smp_wmb();
> diff --git a/mm/migrate.c b/mm/migrate.c
> index e51588e95f57..6c1ea61f39d8 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -1106,6 +1106,7 @@ static int unmap_and_move(new_page_t get_new_page,
>   	if (!newpage)
>   		return -ENOMEM;
>   
> +	newpage->private = 0;
>   	rc = __unmap_and_move(page, newpage, force, mode);
>   	if (rc == MIGRATEPAGE_SUCCESS)
>   		set_page_owner_migrate_reason(newpage, reason);