linux-kernel - Re: [PATCH v3 4/5] mm: Add zap_skip_check

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Date:   Thu, 9 Sep 2021 11:16:14 +1000
From:   Alistair Popple <apopple@...dia.com>
To:     <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Hugh Dickins <hughd@...gle.com>, <linux-mm@...ck.org>,
        Peter Xu <peterx@...hat.com>
CC:     Miaohe Lin <linmiaohe@...wei.com>,
        David Hildenbrand <david@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Yang Shi <shy828301@...il.com>,
        Matthew Wilcox <willy@...radead.org>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Jerome Glisse <jglisse@...hat.com>, <peterx@...hat.com>,
        Liam Howlett <liam.howlett@...cle.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>
Subject: Re: [PATCH v3 4/5] mm: Add zap_skip_check_mapping() helper

Reviewed-by: Alistair Popple <apopple@...dia.com>

On Thursday, 9 September 2021 2:36:25 AM AEST Peter Xu wrote:
> Use the helper for the checks.  Rename "check_mapping" into "zap_mapping"
> because "check_mapping" looks like a bool but in fact it stores the mapping
> itself.  When it's set, we check the mapping (it must be non-NULL).  When it's
> cleared we skip the check, which works like the old way.
> 
> Move the duplicated comments to the helper too.
> 
> Signed-off-by: Peter Xu <peterx@...hat.com>
> ---
>  include/linux/mm.h | 16 +++++++++++++++-
>  mm/memory.c        | 29 ++++++-----------------------
>  2 files changed, 21 insertions(+), 24 deletions(-)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index d1126f731221..ed44f31615d9 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -1721,10 +1721,24 @@ extern void user_shm_unlock(size_t, struct ucounts *);
>   * Parameter block passed down to zap_pte_range in exceptional cases.
>   */
>  struct zap_details {
> -	struct address_space *check_mapping;	/* Check page->mapping if set */
> +	struct address_space *zap_mapping;	/* Check page->mapping if set */
>  	struct page *single_page;		/* Locked page to be unmapped */
>  };
>  
> +/*
> + * We set details->zap_mappings when we want to unmap shared but keep private
> + * pages. Return true if skip zapping this page, false otherwise.
> + */
> +static inline bool
> +zap_skip_check_mapping(struct zap_details *details, struct page *page)
> +{
> +	if (!details || !page)
> +		return false;
> +
> +	return details->zap_mapping &&
> +	    (details->zap_mapping != page_rmapping(page));
> +}
> +
>  struct page *vm_normal_page(struct vm_area_struct *vma, unsigned long addr,
>  			     pte_t pte);
>  struct page *vm_normal_page_pmd(struct vm_area_struct *vma, unsigned long addr,
> diff --git a/mm/memory.c b/mm/memory.c
> index 6bba3b9fef7c..e5ee8399d270 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -1333,16 +1333,8 @@ static unsigned long zap_pte_range(struct mmu_gather *tlb,
>  			struct page *page;
>  
>  			page = vm_normal_page(vma, addr, ptent);
> -			if (unlikely(details) && page) {
> -				/*
> -				 * unmap_shared_mapping_pages() wants to
> -				 * invalidate cache without truncating:
> -				 * unmap shared but keep private pages.
> -				 */
> -				if (details->check_mapping &&
> -				    details->check_mapping != page_rmapping(page))
> -					continue;
> -			}
> +			if (unlikely(zap_skip_check_mapping(details, page)))
> +				continue;
>  			ptent = ptep_get_and_clear_full(mm, addr, pte,
>  							tlb->fullmm);
>  			tlb_remove_tlb_entry(tlb, pte, addr);
> @@ -1375,17 +1367,8 @@ static unsigned long zap_pte_range(struct mmu_gather *tlb,
>  		    is_device_exclusive_entry(entry)) {
>  			struct page *page = pfn_swap_entry_to_page(entry);
>  
> -			if (unlikely(details && details->check_mapping)) {
> -				/*
> -				 * unmap_shared_mapping_pages() wants to
> -				 * invalidate cache without truncating:
> -				 * unmap shared but keep private pages.
> -				 */
> -				if (details->check_mapping !=
> -				    page_rmapping(page))
> -					continue;
> -			}
> -
> +			if (unlikely(zap_skip_check_mapping(details, page)))
> +				continue;
>  			pte_clear_not_present_full(mm, addr, pte, tlb->fullmm);
>  			rss[mm_counter(page)]--;
>  
> @@ -3368,7 +3351,7 @@ void unmap_mapping_page(struct page *page)
>  	first_index = page->index;
>  	last_index = page->index + thp_nr_pages(page) - 1;
>  
> -	details.check_mapping = mapping;
> +	details.zap_mapping = mapping;
>  	details.single_page = page;
>  
>  	i_mmap_lock_write(mapping);
> @@ -3396,7 +3379,7 @@ void unmap_mapping_pages(struct address_space *mapping, pgoff_t start,
>  	pgoff_t	first_index = start, last_index = start + nr - 1;
>  	struct zap_details details = { };
>  
> -	details.check_mapping = even_cows ? NULL : mapping;
> +	details.zap_mapping = even_cows ? NULL : mapping;
>  	if (last_index < first_index)
>  		last_index = ULONG_MAX;
>  
>