Re: [PATCH 08/12] mm/pgtable: add pte_free_defer() for pgtable as page

[Hugh Dickins]

On Wed, 31 May 2023, Jason Gunthorpe wrote:
> On Sun, May 28, 2023 at 11:23:47PM -0700, Hugh Dickins wrote:
> > Add the generic pte_free_defer(), to call pte_free() via call_rcu().
> > pte_free_defer() will be called inside khugepaged's retract_page_tables()
> > loop, where allocating extra memory cannot be relied upon.  This version
> > suits all those architectures which use an unfragmented page for one page
> > table (none of whose pte_free()s use the mm arg which was passed to it).
> > 
> > Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
> > ---
> > +	page = pgtable;
> > +	call_rcu(&page->rcu_head, pte_free_now);
> 
> People have told me that we can't use the rcu_head on the struct page
> backing page table blocks. I understood it was because PPC was using
> that memory for something else.

In the 05/12 thread, Matthew pointed out that powerpc (and a few others)
use the one struct page for multiple page tables, and the lack of
multiple rcu_heads means I've got that patch and 06/12 sparc and
07/12 s390 embarrassingly wrong (whereas this generic 08/12 is okay).

I believe I know the extra grossness needed for powerpc and sparc: I had
it already for powerpc, but fooled myself into thinking not yet needed.

But (I haven't quite got there yet) it looks like Gerald is pointing
out that s390 is using lru which coincides with rcu_head: I already knew
s390 the most difficult, but that will be another layer of difficulty.

I expect it was s390 which people warned you of.

> 
> I was hoping Mathew's folio conversion would help clarify this..

I doubt that: what we have for use today is pages, however they are
dressed up.

> 
> On the flip side, if we are able to use rcu_head here then we should
> use it everywhere and also use it mmu_gather.c instead of allocating
> memory and having the smp_call_function() fallback. This would fix it
> to be actual RCU.
> 
> There have been a few talks that it sure would be nice if the page
> tables were always freed via RCU and every arch just turns on
> CONFIG_MMU_GATHER_RCU_TABLE_FREE. It seems to me that patch 10 is kind
> of half doing that by making this one path always use RCU on all
> arches.
> 
> AFAIK the main reason it hasn't been done was the lack of a rcu_head..

I haven't paid attention to that part of the history, and won't be
competent to propagate this further, into MMU-Gather-World; but agree
that would be a satisfying conclusion.

Hugh