[PATCH 0/3] mm/page_alloc: pcp locking cleanup

[Vlastimil Babka <vbabka@kernel.org>]

This is a followup to the hotfix 038a102535eb ("mm/page_alloc: prevent
pcp corruption with SMP=n"), to simplify the code and deal with the
original issue properly. The previous RFC attempt [1] argued for
changing the UP spinlock implementation, which was discouraged, but
thanks to David's off-list suggestion, we can achieve the goal without
changing the spinlock implementation.

The main change in Patch 1 relies on the fact that on UP we don't need
the pcp lists for scalability, so just make them always bypassed during
alloc/free by making the pcp trylock an unconditional failure.

The various drain paths that use pcp_spin_lock_maybe_irqsave() continue
to exist but will never do any work in practice. In Patch 2 we can again
remove the irq saving from them that commit 038a102535eb added.

Besides simpler code with all the ugly UP_flags removed, we get less
bloat with CONFIG_SMP=n for mm/page_alloc.o as a result:

add/remove: 25/28 grow/shrink: 4/5 up/down: 2105/-6665 (-4560)
Function                                     old     new   delta
get_page_from_freelist                      5689    7248   +1559
free_unref_folios                           2006    2324    +318
make_alloc_exact                             270     286     +16
__zone_watermark_ok                          306     322     +16
drain_pages_zone.isra                        119     109     -10
decay_pcp_high                               181     149     -32
setup_pcp_cacheinfo                          193     147     -46
__free_frozen_pages                         1339    1089    -250
alloc_pages_bulk_noprof                     1054     419    -635
free_frozen_page_commit                      907       -    -907
try_to_claim_block                          1975       -   -1975
__rmqueue_pcplist                           2614       -   -2614
Total: Before=54624, After=50064, chg -8.35%

[1] https://lore.kernel.org/all/d762c46b-36f0-471a-b5b4-23c8cf5628ae@suse.cz/

Signed-off-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
---
Vlastimil Babka (3):
      mm/page_alloc: effectively disable pcp with CONFIG_SMP=n
      mm/page_alloc: remove IRQ saving/restoring from pcp locking
      mm/page_alloc: remove pcpu_spin_* wrappers

 mm/page_alloc.c | 146 ++++++++++++++++++++------------------------------------
 1 file changed, 51 insertions(+), 95 deletions(-)
---
base-commit: 8982358e1c87e3e1dc0aad37f4f93efe9c1cfe03
change-id: 20260227-b4-pcp-locking-cleanup-b7a2d5ff2ead

Best regards,
-- 
Vlastimil Babka (SUSE) <vbabka@kernel.org>