CVE-2024-42234: mm: fix crashes from deferred split racing folio migration

In the Linux kernel, the following vulnerability has been resolved:

mm: fix crashes from deferred split racing folio migration

Even on 6.10-rc6, I've been seeing elusive "Bad page state"s (often on flags when freeing, yet the flags shown are not bad: PGlocked had been set and cleared??), and VMBUGONPAGE(pagerefcount(page) == 0)s from deferredsplitscan()'s folioput(), and a variety of other BUG and WARN symptoms implying double free by deferred split and large folio migration.

6.7 commit 9bcef5973e31 ("mm: memcg: fix split queue list crash when large folio migration") was right to fix the memcg-dependent locking broken in 85ce2c517ade ("memcontrol: only transfer the memcg data for migration"), but missed a subtlety of deferredsplitscan(): it moves folios to its own local list to work on them without splitqueuelock, during which time folio->deferredlist is not empty, but even the "right" lock does nothing to secure the folio and the list it is on.

Fortunately, deferredsplitscan() is careful to use foliotryget(): so foliomigratemapping() can avoid the race by folioundolargermappable() while the old folio's reference count is temporarily frozen to 0 - adding such a freeze in the !mapping case too (originally, folio lock and unmapping and no swap cache left an anon folio unreachable, so no freezing was needed there: but the deferred split queue offers a way to reach it).