linux-kernel - Re: [PATCH 01/13] mm: Update ptep_get

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Date:   Mon, 31 Oct 2022 10:46:14 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Nadav Amit <nadav.amit@...il.com>, Jann Horn <jannh@...gle.com>,
        John Hubbard <jhubbard@...dia.com>, X86 ML <x86@...nel.org>,
        Matthew Wilcox <willy@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        kernel list <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        jroedel@...e.de, ubizjak@...il.com,
        Alistair Popple <apopple@...dia.com>
Subject: Re: [PATCH 01/13] mm: Update ptep_get_lockless()s comment

On Sun, Oct 30, 2022 at 06:47:23PM -0700, Linus Torvalds wrote:

> diff --git a/mm/memory.c b/mm/memory.c
> index ba1d08a908a4..c893f5ffc5a8 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -1451,9 +1451,8 @@ static unsigned long zap_pte_range(struct mmu_gather *tlb,
>  				if (pte_young(ptent) &&
>  				    likely(!(vma->vm_flags & VM_SEQ_READ)))
>  					mark_page_accessed(page);
> +			}
> +			page_zap_pte_rmap(page);
>  			munlock_vma_page(page, vma, false);
>  			rss[mm_counter(page)]--;
>  			if (unlikely(page_mapcount(page) < 0))
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 69de6c833d5c..28b51a31ebb0 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1413,47 +1413,26 @@ static void page_remove_anon_compound_rmap(struct page *page)
>  }
>  
>  /**
> + * page_zap_pte_rmap - take down a pte mapping from a page
>   * @page:	page to remove mapping from
>   *
> + * This is the simplified form of page_remove_rmap(), that only
> + * deals with last-level pages, so 'compound' is always false,
> + * and the caller does 'munlock_vma_page(page, vma, compound)'
> + * separately.
>   *
> + * This allows for a much simpler calling convention and code.
>   *
>   * The caller holds the pte lock.
>   */
> +void page_zap_pte_rmap(struct page *page)
>  {
>  	if (!atomic_add_negative(-1, &page->_mapcount))
>  		return;
>  
>  	lock_page_memcg(page);
> +	__dec_lruvec_page_state(page,
> +		PageAnon(page) ? NR_ANON_MAPPED : NR_FILE_MAPPED);
>  	unlock_page_memcg(page);
>  }

So we *could* use atomic_add_return() and include the print_bad_pte()
thing in this function -- however that turns the whole thing into a mess
again :/