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    Subject[PATCH AUTOSEL for 4.9 276/293] xen/grant-table: Use put_page instead of free_page
    Date
    From: Ross Lagerwall <ross.lagerwall@citrix.com>

    [ Upstream commit 3ac7292a25db1c607a50752055a18aba32ac2176 ]

    The page given to gnttab_end_foreign_access() to free could be a
    compound page so use put_page() instead of free_page() since it can
    handle both compound and single pages correctly.

    This bug was discovered when migrating a Xen VM with several VIFs and
    CONFIG_DEBUG_VM enabled. It hits a BUG usually after fewer than 10
    iterations. All netfront devices disconnect from the backend during a
    suspend/resume and this will call gnttab_end_foreign_access() if a
    netfront queue has an outstanding skb. The mismatch between calling
    get_page() and free_page() on a compound page causes a reference
    counting error which is detected when DEBUG_VM is enabled.

    Signed-off-by: Ross Lagerwall <ross.lagerwall@citrix.com>
    Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
    Signed-off-by: Juergen Gross <jgross@suse.com>
    Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
    ---
    drivers/xen/grant-table.c | 4 ++--
    1 file changed, 2 insertions(+), 2 deletions(-)

    diff --git a/drivers/xen/grant-table.c b/drivers/xen/grant-table.c
    index bb36b1e1dbcc..775d4195966c 100644
    --- a/drivers/xen/grant-table.c
    +++ b/drivers/xen/grant-table.c
    @@ -327,7 +327,7 @@ static void gnttab_handle_deferred(unsigned long unused)
    if (entry->page) {
    pr_debug("freeing g.e. %#x (pfn %#lx)\n",
    entry->ref, page_to_pfn(entry->page));
    - __free_page(entry->page);
    + put_page(entry->page);
    } else
    pr_info("freeing g.e. %#x\n", entry->ref);
    kfree(entry);
    @@ -383,7 +383,7 @@ void gnttab_end_foreign_access(grant_ref_t ref, int readonly,
    if (gnttab_end_foreign_access_ref(ref, readonly)) {
    put_free_entry(ref);
    if (page != 0)
    - free_page(page);
    + put_page(virt_to_page(page));
    } else
    gnttab_add_deferred(ref, readonly,
    page ? virt_to_page(page) : NULL);
    --
    2.15.1
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