[PATCH RESEND v3 0/2] skip redundant TLB sync IPIs

[Lance Yang <lance.yang@linux.dev>]

Hi all,

When unsharing hugetlb PMD page tables or collapsing pages in khugepaged,
we send two IPIs: one for TLB invalidation, and another to synchronize
with concurrent GUP-fast walkers. However, if the TLB flush already
reaches all CPUs, the second IPI is redundant. GUP-fast runs with IRQs
disabled, so when the TLB flush IPI completes, any concurrent GUP-fast
must have finished.

We now track whether IPIs were actually sent during TLB flush. We pass
the mmu_gather context through the flush path, and native_flush_tlb_multi()
sets a flag when sending IPIs. Works with PV and INVLPGB since only
native_flush_tlb_multi() sets the flag - no matter what replaces
pv_ops.mmu.flush_tlb_multi or whether INVLPGB is available.

David Hildenbrand did the initial implementation. I built on his work and
relied on off-list discussions to push it further - thanks a lot David!

v2 -> v3:
- Complete rewrite: use dynamic IPI tracking instead of static checks
  (per Dave Hansen, thanks!)
- Track IPIs via mmu_gather: native_flush_tlb_multi() sets flag when
  actually sending IPIs
- Motivation for skipping redundant IPIs explained by David:
  https://lore.kernel.org/linux-mm/1b27a3fa-359a-43d0-bdeb-c31341749367@kernel.org/
- https://lore.kernel.org/linux-mm/20251229145245.85452-1-lance.yang@linux.dev/

v1 -> v2:
- Fix cover letter encoding to resolve send-email issues. Apologies for
  any email flood caused by the failed send attempts :(

RFC -> v1:
- Use a callback function in pv_mmu_ops instead of comparing function
  pointers (per David)
- Embed the check directly in tlb_remove_table_sync_one() instead of
  requiring every caller to check explicitly (per David)
- Move tlb_table_flush_implies_ipi_broadcast() outside of
  CONFIG_MMU_GATHER_RCU_TABLE_FREE to fix build error on architectures
  that don't enable this config.
  https://lore.kernel.org/oe-kbuild-all/202512142156.cShiu6PU-lkp@intel.com/
- https://lore.kernel.org/linux-mm/20251213080038.10917-1-lance.yang@linux.dev/

Lance Yang (2):
  mm/tlb: skip redundant IPI when TLB flush already synchronized
  mm: introduce pmdp_collapse_flush_sync() to skip redundant IPI

 arch/x86/include/asm/tlb.h      |  3 ++-
 arch/x86/include/asm/tlbflush.h |  9 +++++----
 arch/x86/kernel/alternative.c   |  2 +-
 arch/x86/kernel/ldt.c           |  2 +-
 arch/x86/mm/tlb.c               | 22 +++++++++++++++------
 include/asm-generic/tlb.h       | 14 +++++++++-----
 include/linux/pgtable.h         | 13 +++++++++----
 mm/khugepaged.c                 |  9 +++------
 mm/mmu_gather.c                 | 26 ++++++++++++++++++-------
 mm/pgtable-generic.c            | 34 +++++++++++++++++++++++++++++++++
 10 files changed, 99 insertions(+), 35 deletions(-)

-- 
2.49.0