Re: [PATCH 01/12] mm/huge_memory: zap deposited page tables after an RCU grace period

[Kiryl Shutsemau]

On Tue, Sep 01, 2026 at 03:41:17PM +0100, Lorenzo Stoakes (ARM) wrote:
> On Tue, Sep 01, 2026 at 11:24:08AM -0300, Jason Gunthorpe wrote:
> > On Tue, Sep 01, 2026 at 03:12:45PM +0100, Lorenzo Stoakes (ARM) wrote:
> >
> > > It won't be costly at the time of the calls obviously as its deferred. Maybe
> > > increase some time spent in softirq but again is 512x that big of a deal?
> > >
> > > I'm not sure how you'd both defer the free and somehow utilise mmu_gather here
> > > either really, certainly not without it becoming extremely messy.
> >
> > The less costly version is to thread the page to be freed onto the
> > mmu_gather through a linked list in the struct page memory. This is
> > super cheap since it is just a singly linked list operation.
> >
> > Then when the mmu_gather is flushed it does a single call_rcu using
> > the rcu head of the struct page of the head of the list. The callback
> > clears the entire linked list of pages.
> >
> > Since you have to tlb flush anyhow, it makes sense to always use the
> > mmu_gather. For example the design I ended up with for iommupt
> > accumulates all the invalidations and all the free-able memory into a
> > gather then invalidates and frees.
> >
> > This allows maximizing the tlbi efficiency too. You can't do call_srcu
> > until you flush the tlb and if you call once per table then you are
> > also tlb flushing once per table too.
> >
> > So if the kernel really does want to clear out 512 leaf tables the
> > optimal implementation is one range tlbi for 512 entries followed by
> > one call_rcu to free the memory. Hence the gather..
> 
> I think there's some confusion here.
> 
> This isn't the path in which a page table is being freed, the _deposited_
> table is zapped, in zap_deposited_table().
> 
> That is, the page table kept in reserve for THP split, that is not
> currently mapped.
> 
> It amounts to a __free_pages() call.
> 
> The TLB operations are in e.g. zap_huge_pmd() etc. and nobody has
> complained about inefficiencies there.
> 
> So, unless I'm missing something here, TLB flushes play no role in this
> whatsoever.
> 
> The issue Kiryl raised was that instead of immediately freeing page tables,
> they are now batched up individually by call_rcu().
> 
> I personally find it difficult to imagine the numbers here would be
> problematic or certainly cause anything observable beyond what is
> observable now.
> 
> So I'm going to have to say, unless it can be clearly demonstrated this is
> problematic, I don't think there's any reason to add additional complexity
> here.

It would be nice to measure munmap() overhead here.

I am worried about hitting DEFAULT_MAX_RCU_BLIMIT and trigger
rcu_force_quiescent_state() which can be disruptive to the system.

DEFAULT_MAX_RCU_BLIMIT is 10K, so it is ~20G of THP unmapped on x86.

munmap() of 64G worth of THP should be enough to demonstrate the
problem.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov