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

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

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@linux.dev>
>>>>
>>>> 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@infradead.org>

Thanks for taking time to review!