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    Subject[PATCH 3/3] mm: Clear page->private when splitting or migrating a page
    Date
    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@redhat.com>
    Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
    ---
    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;

    /* 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);
    --
    2.35.1
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