mm/migrate: fix shmem xarray update during migration

JIRA: https://issues.redhat.com/browse/RHEL-84184
Conflicts:
  * notable context difference due to RHEL-9 missing support
    for mTHP and its follow-up commits

This patch is a backport of the following upstream commit:
commit 60cf233
Author: Zi Yan <ziy@nvidia.com>
Date:   Wed Mar 5 15:04:03 2025 -0500

    mm/migrate: fix shmem xarray update during migration

    A shmem folio can be either in page cache or in swap cache, but not at the
    same time.  Namely, once it is in swap cache, folio->mapping should be
    NULL, and the folio is no longer in a shmem mapping.

    In __folio_migrate_mapping(), to determine the number of xarray entries to
    update, folio_test_swapbacked() is used, but that conflates shmem in page
    cache case and shmem in swap cache case.  It leads to xarray multi-index
    entry corruption, since it turns a sibling entry to a normal entry during
    xas_store() (see [1] for a userspace reproduction).  Fix it by only using
    folio_test_swapcache() to determine whether xarray is storing swap cache
    entries or not to choose the right number of xarray entries to update.

    [1] https://lore.kernel.org/linux-mm/Z8idPCkaJW1IChjT@casper.infradead.org/

    Note:
    In __split_huge_page(), folio_test_anon() && folio_test_swapcache() is
    used to get swap_cache address space, but that ignores the shmem folio in
    swap cache case.  It could lead to NULL pointer dereferencing when a
    in-swap-cache shmem folio is split at __xa_store(), since
    !folio_test_anon() is true and folio->mapping is NULL.  But fortunately,
    its caller split_huge_page_to_list_to_order() bails out early with EBUSY
    when folio->mapping is NULL.  So no need to take care of it here.

    Link: https://lkml.kernel.org/r/20250305200403.2822855-1-ziy@nvidia.com
    Fixes: fc346d0 ("mm: migrate high-order folios in swap cache correctly")
    Signed-off-by: Zi Yan <ziy@nvidia.com>
    Reported-by: Liu Shixin <liushixin2@huawei.com>
    Closes: https://lore.kernel.org/all/28546fb4-5210-bf75-16d6-43e1f8646080@huawei.com/
    Suggested-by: Hugh Dickins <hughd@google.com>
    Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
    Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
    Cc: Barry Song <baohua@kernel.org>
    Cc: Charan Teja Kalla <quic_charante@quicinc.com>
    Cc: David Hildenbrand <david@redhat.com>
    Cc: Hugh Dickins <hughd@google.com>
    Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
    Cc: Lance Yang <ioworker0@gmail.com>
    Cc: Ryan Roberts <ryan.roberts@arm.com>
    Cc: <stable@vger.kernel.org>
    Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Signed-off-by: Rafael Aquini <raquini@redhat.com>

Author: Rafael Aquini

mm/migrate.c

@@ -437,15 +437,13 @@ int folio_migrate_mapping(struct address_space *mapping,
 	newfolio->index = folio->index;
 	newfolio->mapping = folio->mapping;
 	folio_ref_add(newfolio, nr); /* add cache reference */
-	if (folio_test_swapbacked(folio)) {
+	if (folio_test_swapbacked(folio))
 		__folio_set_swapbacked(newfolio);
-		if (folio_test_swapcache(folio)) {
-			folio_set_swapcache(newfolio);
-			newfolio->private = folio_get_private(folio);
-		}
+	if (folio_test_swapcache(folio)) {
+		folio_set_swapcache(newfolio);
+		newfolio->private = folio_get_private(folio);
 		entries = nr;
 	} else {
-		VM_BUG_ON_FOLIO(folio_test_swapcache(folio), folio);
 		entries = 1;
 	}