mm/codetag: swap tags when migrate pages

mm/codetag: swap tags when migrate pages

Current solution to adjust codetag references during page migration is
done in 3 steps:

1. sets the codetag reference of the old page as empty (not pointing
   to any codetag);

2. subtracts counters of the new page to compensate for its own
   allocation;

3. sets codetag reference of the new page to point to the codetag of
   the old page.

This does not work if CONFIG_MEM_ALLOC_PROFILING_DEBUG=n because
set_codetag_empty() becomes NOOP.  Instead, let's simply swap codetag
references so that the new page is referencing the old codetag and the old
page is referencing the new codetag.  This way accounting stays valid and
the logic makes more sense.

Link: https://lkml.kernel.org/r/20241129025213.34836-1-00107082@163.com
Fixes: e0a955bf7f61 ("mm/codetag: add pgalloc_tag_copy()")
Signed-off-by: David Wang <00107082@163.com>
Closes: https://lore.kernel.org/lkml/20241124074318.399027-1-00107082@163.com/
Acked-by: Suren Baghdasaryan <surenb@google.com>
Suggested-by: Suren Baghdasaryan <surenb@google.com>
Acked-by: Yu Zhao <yuzhao@google.com>
Cc: Kent Overstreet <kent.overstreet@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>