Re: [PATCH v2 1/1] mm/mmu_gather: replace IPI with synchronize_rcu() when batch allocation fails

[Lance Yang]

On 2026/2/24 19:04, David Hildenbrand (Arm) wrote:
> On 2/24/26 04:07, Lance Yang wrote:
> > From: Lance Yang <lance.yang@xxxxxxxxx>
> >
> > When freeing page tables, we try to batch them. If batch allocation fails
> > (GFP_NOWAIT), __tlb_remove_table_one() immediately frees the one without
> > batching.
> >
> > On !CONFIG_PT_RECLAIM, the fallback sends an IPI to all CPUs via
> > tlb_remove_table_sync_one(). It disrupts all CPUs even when only a single
> > process is unmapping memory. IPI broadcast was reported to hurt RT
> > workloads[1].
> >
> > tlb_remove_table_sync_one() synchronizes with lockless page-table walkers
> > (e.g. GUP-fast) that rely on IRQ disabling. These walkers use
> > local_irq_disable(), which is also an RCU read-side critical section.
> >
> > This patch introduces tlb_remove_table_sync_rcu() which uses RCU grace
> > period (synchronize_rcu()) instead of IPI broadcast. This provides the
> > same guarantee as IPI but without disrupting all CPUs. Since batch
> > allocation already failed, we are in a way slow path where sleeping is
> > acceptable - we are in process context (unmap_region, exit_mmap) with only
> > mmap_lock held. might_sleep() will catch any invalid context.
> >
> > [1] https://lore.kernel.org/linux-mm/1b27a3fa-359a-43d0-bdeb-c31341749367@xxxxxxxxxx/
> >
> > Link: https://lore.kernel.org/linux-mm/20260202150957.GD1282955@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/
> > Link: https://lore.kernel.org/linux-mm/dfdfeac9-5cd5-46fc-a5c1-9ccf9bd3502a@xxxxxxxxx/
> > Link: https://lore.kernel.org/linux-mm/bc489455-bb18-44dc-8518-ae75abda6bec@xxxxxxxxxx/
> > Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > Suggested-by: Dave Hansen <dave.hansen@xxxxxxxxx>
> > Suggested-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>
> > Signed-off-by: Lance Yang <lance.yang@xxxxxxxxx>
> > ---
> > v1 -> v2:
> > - Wrap synchronize_rcu() in tlb_remove_table_sync_rcu() with proper
> >    kerneldoc (per David)
> > - Add might_sleep() to make sleeping constraint explicit (per Dave)
> > - Clarify this is for synchronization, not memory freeing (per Dave)
> > - https://lore.kernel.org/linux-mm/20260223033604.10198-1-lance.yang@xxxxxxxxx/
> >
> >   include/asm-generic/tlb.h |  4 ++++
> >   mm/mmu_gather.c           | 22 +++++++++++++++++++++-
> >   2 files changed, 25 insertions(+), 1 deletion(-)
> >
> > diff --git a/include/asm-generic/tlb.h b/include/asm-generic/tlb.h
> > index 4aeac0c3d3f0..bdcc2778ac64 100644
> > --- a/include/asm-generic/tlb.h
> > +++ b/include/asm-generic/tlb.h
> > @@ -251,6 +251,8 @@ static inline void tlb_remove_table(struct mmu_gather *tlb, void *table)
> >   
> >   void tlb_remove_table_sync_one(void);
> >   
> > +void tlb_remove_table_sync_rcu(void);
> > +
> >   #else
> >   
> >   #ifdef tlb_needs_table_invalidate
> > @@ -259,6 +261,8 @@ void tlb_remove_table_sync_one(void);
> >   
> >   static inline void tlb_remove_table_sync_one(void) { }
> >   
> > +static inline void tlb_remove_table_sync_rcu(void) { }
> > +
> >   #endif /* CONFIG_MMU_GATHER_RCU_TABLE_FREE */
> >   
> >   
> > diff --git a/mm/mmu_gather.c b/mm/mmu_gather.c
> > index fe5b6a031717..2c6fa8db55df 100644
> > --- a/mm/mmu_gather.c
> > +++ b/mm/mmu_gather.c
> > @@ -296,6 +296,26 @@ static void tlb_remove_table_free(struct mmu_table_batch *batch)
> >   	call_rcu(&batch->rcu, tlb_remove_table_rcu);
> >   }
> >   
> > +/**
> > + * tlb_remove_table_sync_rcu() - synchronize with software page-table walkers
>
> Nit: no need for the "()"

Oops, @Andrew could you drop the "()" here when applying?

> Thanks!
>
> Acked-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>

Thanks for taking time to review!