Re: [PATCH v4 0/3] targeted TLB sync IPIs for lockless page table

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

On 2026/2/2 21:37, Peter Zijlstra wrote:
> On Mon, Feb 02, 2026 at 09:07:10PM +0800, Lance Yang wrote:
> 
>>>> Right, but if we can use full RCU for PT_RECLAIM, why can't we do so
>>>> unconditionally and not add overhead?
>>>
>>> The sync (IPI) is mainly needed for unshare (e.g. hugetlb) and collapse
>>> (khugepaged) paths, regardless of whether table free uses RCU, IIUC.
>>
>> In addition: We need the sync when we modify page tables (e.g. unshare,
>> collapse), not only when we free them. RCU can defer freeing but does
>> not prevent lockless walkers from seeing concurrent in-place
>> modifications, so we need the IPI to synchronize with those walkers
>> first.
> 
> Currently PT_RECLAIM=y has no IPI; are you saying that is broken? If
> not, then why do we need this at all?

PT_RECLAIM=y does have IPI for unshare/collapse — those paths call
tlb_flush_unshared_tables() (for hugetlb unshare) and collapse_huge_page()
(in khugepaged collapse), which already send IPIs today (broadcast to all
CPUs via tlb_remove_table_sync_one()).

What PT_RECLAIM=y doesn't need IPI for is table freeing (
__tlb_remove_table_one() uses call_rcu() instead). But table modification
(unshare, collapse) still needs IPI to synchronize with lockless walkers,
regardless of PT_RECLAIM.

So PT_RECLAIM=y is not broken; it already has IPI where needed. This series
just makes those IPIs targeted instead of broadcast. Does that clarify?


Thanks,
Lance