[RFC PATCH 0/2] mm/migrate: wait for folio refcount during longterm pin migration

[John Hubbard <jhubbard@nvidia.com>]

Hi,

This adds a bounded sleep to migration so that FOLL_LONGTERM pinning can
wait for transient folio references to drain, instead of failing after a
fixed number of retries. The wait uses a one-second timeout. An
alternative approach would be to call wait_var_event_killable() with no
timeout, but that doesn't match as well with migration's "this will
probably work" API. In other words, a short sleeping wait is more
appropriate here.

When migrating pages for FOLL_LONGTERM pinning, migration can fail with
-EAGAIN if a folio has unexpected references. These references are often
transient, but the current retry loop gives up too quickly. This series
adds wait_var_event_timeout() at the retry points, paired with
wake_up_var() in folio_put() to wake the sleeper as soon as the refcount
drops.

The wake_up_var() calls in folio_put() are gated behind a static key,
disabled by default, so non-migration workloads pay zero cost.
migrate_pages() enables the key on entry when the reason is
MR_LONGTERM_PIN, and disables it on exit.

Toggling the key is not free. folio_put() is static inline, so every
compilation unit that calls it gets its own patch site (roughly 500 in
vmlinux, plus modules). On x86, jump label patching is batched (256
sites per batch, 3 IPI rounds per batch), so enabling the key costs
6-9 IPI broadcasts, a few hundred microseconds on a large machine.
That cost is paid twice per migrate_pages() call. Migration itself
spends several milliseconds per batch on LRU isolation, TLB flushes,
and page copies. Concurrent longterm-pin migrations after the first
just do an atomic_inc (no patching).

Matthew Brost offered to performance-test this series [1], as Intel has
tests that stress migration and good metrics to catch regressions.

[1] https://lore.kernel.org/all/aX+oUorOWPt1xbgw@lstrano-desk.jf.intel.com/

John Hubbard (2):
  mm: wake up folio refcount waiters on folio_put()
  mm/migrate: wait for folio refcount during longterm pin migration

 include/linux/mm.h |  8 ++++++++
 mm/migrate.c       | 30 ++++++++++++++++++++++++++++++
 mm/swap.c          | 10 +++++++++-
 3 files changed, 47 insertions(+), 1 deletion(-)


base-commit: 9a9c8ce300cd3859cc87b408ef552cd697cc2ab7
-- 
2.53.0