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

[Peter Zijlstra]

On Mon, Feb 02, 2026 at 10:37:39PM +0800, Lance Yang wrote:
> 
> 
> 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?

Oh bah, reading is hard. I had missed they had more table_sync_one() calls,
rather than remove_table_one().

So you *can* replace table_sync_one() with rcu_sync(), that will provide
the same guarantees. Its just a 'little' bit slower on the update side,
but does not incur the read side cost.

I really think anything here needs to better explain the various
requirements. Because now everybody gets to pay the price for hugetlb
shared crud, while 'nobody' will actually use that.