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

[Lance Yang]

On 2026/2/24 20:35, Peter Zijlstra wrote:
> On Tue, Feb 24, 2026 at 08:18:46PM +0800, Lance Yang wrote:
> > On 2026/2/24 19:41, Peter Zijlstra wrote:
> > > On Tue, Feb 24, 2026 at 11:07:00AM +0800, 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.
> > >
> > > So sending the IPIs also requires non-atomic context, so change there.
> >
> > Yeah, you're right!
> >
> > > What isn't explained, and very much not clear to me, is why
> > > tlb_remove_table_sync_one() is retained?
> >
> > Good point. tlb_remove_table_sync_one() is still needed in:
> >
> > 1) khugepaged (mm/khugepaged.c) - after pmdp_collapse_flush()
> > 2) tlb_finish_mmu() (tlb.h) - when tlb->fully_unshared_tables
> > 3) ...
> >
> > These are not slow paths like batch allocation failure. This patch only
> > converts this obvious slow path first.
> >
> > I'm working on converting the remaining callers as well, but not with
> > RCU, looking at other options (e.g. targeted IPI).
>
> OK, so with that addition to the Changelog,
>
> Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>

Thanks for taking time to review!