FROMLIST: mm/migrate: set swap entry values of THP tail pages properly.

The tail pages in a THP can have swap entry information stored in their
private field. When migrating to a new page, all tail pages of the new
page need to update ->private to avoid future data corruption.

This fix is stable-only, since after commit 07e09c48 ("mm/huge_memory:
work on folio->swap instead of page->private when splitting folio"),
subpages of a swapcached THP no longer requires the maintenance.

Adding THPs to the swapcache was introduced in commit
38d8b4e6 ("mm, THP, swap: delay splitting THP during swap out"),
where each subpage of a THP added to the swapcache had its own swapcache
entry and required the ->private field to point to the correct swapcache
entry. Later, when THP migration functionality was implemented in commit
616b8371 ("mm: thp: enable thp migration in generic path"),
it initially did not handle the subpages of swapcached THPs, failing to
update their ->private fields or replace the subpage pointers in the
swapcache. Subsequently, commit e71769ae ("mm: enable thp migration
for shmem thp") addressed the swapcache update aspect. This patch fixes
the update of subpage ->private fields.

Bug: 324818390
Bug: 336202640
Fixes: 616b8371 ("mm: thp: enable thp migration in generic path")
Link: https://lore.kernel.org/linux-mm/20240306155217.118467-1-zi.yan@sent.com/


Reported-and-tested-by: Charan Teja Kalla <quic_charante@quicinc.com>
Change-Id: Ia4603cd58b76dc6ff46a2c53a735942a87221419
Signed-off-by: Zi Yan <ziy@nvidia.com>
Acked-by: David Hildenbrand <david@redhat.com>
Closes: https://lore.kernel.org/linux-mm/1707814102-22682-1-git-send-email-quic_charante@quicinc.com/


Signed-off-by: Charan Teja Kalla <quic_charante@quicinc.com>