linux-kernel - Re: [PATCH v2 07/12] s390: add pte_free

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Date:   Thu, 29 Jun 2023 15:59:07 +0200
From:   Alexander Gordeev <agordeev@...ux.ibm.com>
To:     Gerald Schaefer <gerald.schaefer@...ux.ibm.com>
Cc:     Hugh Dickins <hughd@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Mike Rapoport <rppt@...nel.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Matthew Wilcox <willy@...radead.org>,
        David Hildenbrand <david@...hat.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Qi Zheng <zhengqi.arch@...edance.com>,
        Yang Shi <shy828301@...il.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Peter Xu <peterx@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will@...nel.org>, Yu Zhao <yuzhao@...gle.com>,
        Alistair Popple <apopple@...dia.com>,
        Ralph Campbell <rcampbell@...dia.com>,
        Ira Weiny <ira.weiny@...el.com>,
        Steven Price <steven.price@....com>,
        SeongJae Park <sj@...nel.org>,
        Lorenzo Stoakes <lstoakes@...il.com>,
        Huang Ying <ying.huang@...el.com>,
        Naoya Horiguchi <naoya.horiguchi@....com>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Zack Rusin <zackr@...are.com>, Jason Gunthorpe <jgg@...pe.ca>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Pasha Tatashin <pasha.tatashin@...een.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Minchan Kim <minchan@...nel.org>,
        Christoph Hellwig <hch@...radead.org>,
        Song Liu <song@...nel.org>,
        Thomas Hellstrom <thomas.hellstrom@...ux.intel.com>,
        Russell King <linux@...linux.org.uk>,
        "David S. Miller" <davem@...emloft.net>,
        Michael Ellerman <mpe@...erman.id.au>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        Jann Horn <jannh@...gle.com>,
        Vishal Moola <vishal.moola@...il.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        linux-arm-kernel@...ts.infradead.org, sparclinux@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, linux-s390@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v2 07/12] s390: add pte_free_defer() for pgtables sharing
 page

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@...gle.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!