On Tue, Jan 12, 2021 at 1:54 AM Oscar Salvador [off-list ref] wrote:

On Tue, Jan 12, 2021 at 01:34:58AM -0800, Dan Williams wrote:

quoted

The conversion to move pfn_to_online_page() internal to
soft_offline_page() missed that the get_user_pages() reference needs to
be dropped when pfn_to_online_page() fails.

I would be more specific here wrt. get_user_pages (madvise).
soft_offline_page gets called from more places besides madvise_*.

Sure.

quoted

When soft_offline_page() is handed a pfn_valid() &&
!pfn_to_online_page() pfn the kernel hangs at dax-device shutdown due to
a leaked reference.

Fixes: feec24a6139d ("mm, soft-offline: convert parameter to pfn")
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Naoya Horiguchi <nao.horiguchi@gmail.com>
Cc: David Hildenbrand <redacted>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: <redacted>
Signed-off-by: Dan Williams <redacted>

LGTM, thanks for catching this:

Reviewed-by: Oscar Salvador <osalvador@suse.de>

A nit below.

quoted

---
 mm/memory-failure.c |   20 ++++++++++++++++----
 1 file changed, 16 insertions(+), 4 deletions(-)

diff --git a/mm/memory-failure.c b/mm/memory-failure.c
index 5a38e9eade94..78b173c7190c 100644
--- a/mm/memory-failure.c
+++ b/mm/memory-failure.c

@@ -1885,6 +1885,12 @@ static int soft_offline_free_page(struct page *page)
      return rc;
 }

+static void put_ref_page(struct page *page)
+{
+     if (page)
+             put_page(page);
+}

I am not sure this warrants a function.
I would probably go with "if (ref_page).." in the two corresponding places,
but not feeling strong here.

I'll take another look, it felt cluttered...

quoted

+
 /**
  * soft_offline_page - Soft offline a page.
  * @pfn: pfn to soft-offline

@@ -1910,20 +1916,26 @@ static int soft_offline_free_page(struct page *page)
 int soft_offline_page(unsigned long pfn, int flags)
 {
      int ret;
-     struct page *page;
      bool try_again = true;
+     struct page *page, *ref_page = NULL;
+
+     WARN_ON_ONCE(!pfn_valid(pfn) && (flags & MF_COUNT_INCREASED));

Did you see any scenario where this could happen? I understand that you are
adding this because we will leak a reference in case pfn is not valid anymore.

I did not, more future proofing / documenting against refactoring that
fails to consider that case.