Re: [PATCH v2 07/12] s390: add pte_free_defer() for pgtables sharing page

[Gerald Schaefer <gerald.schaefer@linux.ibm.com>]

On Thu, 29 Jun 2023 15:59:07 +0200
Alexander Gordeev <agordeev@linux.ibm.com> wrote:

> On Wed, Jun 28, 2023 at 09:16:24PM +0200, Gerald Schaefer wrote:
> > On Tue, 20 Jun 2023 00:51:19 -0700 (PDT)
> > Hugh Dickins <hughd@google.com> wrote:  
> 
> Hi Gerald, Hugh!
> 
> ...
> > @@ -407,6 +445,88 @@ void __tlb_remove_table(void *_table)
> >  	__free_page(page);
> >  }
> >  
> > +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> > +static void pte_free_now0(struct rcu_head *head);
> > +static void pte_free_now1(struct rcu_head *head);  
> 
> What about pte_free_lower() / pte_free_upper()?

I actually like the 0/1 better, I always get confused what exactly we
mean with "lower / upper" in our code and comments. Is it the first
or second half? With 0/1 it is immediately clear to me.

> 
> ...
> > +void pte_free_defer(struct mm_struct *mm, pgtable_t pgtable)
> > +{
> > +	unsigned int bit, mask;
> > +	struct page *page;
> > +
> > +	page = virt_to_page(pgtable);
> > +	if (mm_alloc_pgste(mm)) {
> > +		/*
> > +		 * TODO: Do we need gmap_unlink(mm, pgtable, addr), like in
> > +		 * page_table_free_rcu()?
> > +		 * If yes -> need addr parameter here, like in pte_free_tlb().
> > +		 */
> > +		call_rcu(&page->rcu_head, pte_free_pgste);
> > +		return;
> > +}
> > +	bit = ((unsigned long)pgtable & ~PAGE_MASK) / (PTRS_PER_PTE * sizeof(pte_t));
> > +
> > +	spin_lock_bh(&mm->context.lock);
> > +	mask = atomic_xor_bits(&page->_refcount, 0x15U << (bit + 24));  
> 
> This  makes the bit logic increasingly complicated to me.

I think it is well in line with existing code in page_table_free[_rcu].
Only instead of doing xor with 0x11U, it does xor with 0x15U to also
switch on the H bit while at it.

> 
> What if instead we set the rule "one bit at a time only"?
> That means an atomic group bit flip is only allowed between
> pairs of bits, namely:
> 
> bit flip	initiated from
> -----------	----------------------------------------
> P      <- A	page_table_free(), page_table_free_rcu()
>      H <- A	pte_free_defer()
> P <- H		pte_free_half()
> 
> In the current model P bit could be on together with H
> bit simultaneously. That actually brings in equation
> nothing.

P bit has to be set at the latest when __tlb_remove_table() gets
called, because there it is checked / cleared. It might be possible
to not set it in pte_free_defer() already, but only later in
pte_free_half() RCU callback, before calling __tlb_remove_table().
But that would not be in line any more with existing code, where it
is already set before scheduling the RCU callback.

Therefore, I would rather stick to the current approach, unless
you see some bug in it.

> 
> Besides, this check in page_table_alloc() (while still
> correct) makes one (well, me) wonder "what about HH bits?":
> 
> 	mask = (mask | (mask >> 4)) & 0x03U;
> 	if (mask != 0x03U) {
> 		...
> 	}

Without adding fragments back to the list, it is not necessary
to check any H bits page_table_alloc(), or so I hope. Actually,
I like that aspect most, i.e. we have as little impact on current
code as possible.

And H bits are only relevant for preventing double use of rcu_head,
which is what they were designed for, and only the new code has
to care about them.