[PATCH v2 0/2] mm: don't migrate folios containing hwpoisoned pages

[Yuanhe Shu]

v2 of this series.  No changes to the approach; the two deltas are a
wording correction promised in the v1 discussion and a third check
suggested by Kaitao Cheng.

memory_failure() can end up abandoning a folio while PG_hwpoison is
already set on it.  Two ways to get there:

  1. Race: any isolator (compaction, NUMA balancing, move_pages(),
     mbind(), CMA, long-term GUP) clears PG_lru from under
     memory_failure(), which then fails the HWPoisonHandlable()
     check, exhausts the get_any_page() retries and bails out with
     MF_IGNORED - without holding any reference, but with the flag
     set.

  2. No race at all: the THP-split-failure path
     (kill_procs_now() + MF_MSG_UNSPLIT_THP) leaves a large folio
     on the LRU with a PG_hwpoisoned subpage by design.

In both cases the migrator still holds the folio on its migration
list and migrates it like any other: the corrupted data is copied
into a fresh folio with no poison marker, and no hwpoison PTE entry
is installed on the way back, so the userspace mapping is silently
redirected to the corrupted copy - no SIGBUS is ever delivered.

This was reproduced on v7.3-rc4-75-g62f4c998b297 (the base of
this series): an order-0 stress workload (madvise(MADV_HWPOISON)
racing move_pages()) left the poison flag set on 8426 abandoned
folios in a 180-second run; 698 of them were subsequently
migrated, and in every case the address then served the copied
corrupted data with no SIGBUS.  With patch 2, 0 of 9711 were
propagated.

The compaction-side guard is needed regardless of how the race in
memory_failure() itself might be addressed: the steady state
(split failure leaves the folio on the LRU with the flag already
set) is not a race at all.  Deterministically validated 16/16
poisoned mTHP folios migrated unpatched vs 0/16 with patch 1,
with clean controls migrating 16/16 in both runs.

The checks compose with, not replace, copy_mc: folio_mc_copy()
still catches real-DRAM corruption during the copy on archs that
implement it, and the flag checks refuse software-injected poison
on all archs before and after the copy.  Extending the
!ARCH_HAS_COPY_MC fallbacks of copy_mc_user_highpage() and
copy_mc_highpage() with the same flag check - which would also
cover their non-migration callers - was proposed by Huang, Ying
and welcomed by David in the v1 discussion; that is posted as a
separate patch right after this series.

Changes since v1:

- Patch 1: the isolation check's claim is narrowed to folios that
  are already flagged when compaction sees them ("avoids the
  migration of a folio that is already flagged"); a poison landing
  after isolation is patch 2's business, as David pointed out.

- Patch 2: a third check after the copy, before the mapping is
  transferred, suggested by Kaitao Cheng.  It closes the copy
  window itself, which the debugfs corrupt-pfn injector can hit
  since it has no GUP or PTE ordering constraint.  In the v1
  racing workload the pre-copy checks already propagated 0 of
  9711, so the new check is measured-neutral there.

- Both patches carry Assisted-by: LLM, as promised in the v1
  discussion.

Yuanhe Shu (2):
  mm/compaction: skip folios containing hwpoisoned pages
  mm/migrate: refuse to migrate folios containing hwpoisoned pages

 mm/compaction.c |  8 ++++++++
 mm/migrate.c    | 49 ++++++++++++++++++++++++++++++++++++++++++++++--
 2 files changed, 55 insertions(+), 2 deletions(-)

base-commit: 62f4c998b297cf233997a2b4cd6fc2d2df0319c9
-- 
2.43.7