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Date:   Thu, 29 Jun 2023 15:59:07 +0200
From:   Alexander Gordeev <agordeev@linux.ibm.com>
To:     Gerald Schaefer <gerald.schaefer@linux.ibm.com>
Cc:     Hugh Dickins <hughd@google.com>,
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Subject: Re: [PATCH v2 07/12] s390: add pte_free_defer() for pgtables sharing
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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()?

...
> +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.

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.

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) {
		...
	}

By contrast, with "one bit at a time only" policy every
of three bits effectevely indicates which state a page
half is currently in.

Thanks!