Re: [PATCH v4 3/4] mm: Change zap_details.zap_mapping into even_cows

[John Hubbard]

On 2/16/22 1:48 AM, Peter Xu wrote:
> Currently we have a zap_mapping pointer maintained in zap_details, when it is
> specified we only want to zap the pages that has the same mapping with what the
> caller has specified.
> 
> But what we want to do is actually simpler: we want to skip zapping
> private (COW-ed) pages in some cases.  We can refer to unmap_mapping_pages()
> callers where we could have passed in different even_cows values.  The other
> user is unmap_mapping_folio() where we always want to skip private pages.
> 
> According to Hugh, we used a mapping pointer for historical reason, as
> explained here:
> 
>   https://lore.kernel.org/lkml/391aa58d-ce84-9d4-d68d-d98a9c533255@xxxxxxxxxx/
> 
> Quoting partly from Hugh:
> 
>   Which raises the question again of why I did not just use a boolean flag
>   there originally: aah, I think I've found why.  In those days there was a
>   horrible "optimization", for better performance on some benchmark I guess,
>   which when you read from /dev/zero into a private mapping, would map the zero
>   page there (look up read_zero_pagealigned() and zeromap_page_range() if you
>   dare).  So there was another category of page to be skipped along with the
>   anon COWs, and I didn't want multiple tests in the zap loop, so checking
>   check_mapping against page->mapping did both.  I think nowadays you could do
>   it by checking for PageAnon page (or genuine swap entry) instead.
> 
> This patch replaced the zap_details.zap_mapping pointer into the even_cows
> boolean, then we check it against PageAnon.
> 
> Suggested-by: Hugh Dickins <hughd@xxxxxxxxxx>
> Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
> ---
>  mm/memory.c | 16 +++++++---------
>  1 file changed, 7 insertions(+), 9 deletions(-)

Reviewed-by: John Hubbard <jhubbard@xxxxxxxxxx>

thanks,
-- 
John Hubbard
NVIDIA

> 
> diff --git a/mm/memory.c b/mm/memory.c
> index 14d8428ff4db..ffa8c7dfe9ad 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -1309,8 +1309,8 @@ copy_page_range(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma)
>   * Parameter block passed down to zap_pte_range in exceptional cases.
>   */
>  struct zap_details {
> -	struct address_space *zap_mapping;	/* Check page->mapping if set */
>  	struct folio *single_folio;	/* Locked folio to be unmapped */
> +	bool even_cows;			/* Zap COWed private pages too? */
>  };
>  
>  /* Whether we should zap all COWed (private) pages too */
> @@ -1321,13 +1321,10 @@ static inline bool should_zap_cows(struct zap_details *details)
>  		return true;
>  
>  	/* Or, we zap COWed pages only if the caller wants to */
> -	return !details->zap_mapping;
> +	return details->even_cows;
>  }
>  
> -/*
> - * We set details->zap_mapping when we want to unmap shared but keep private
> - * pages. Return true if we should zap this page, false otherwise.
> - */
> +/* Decides whether we should zap this page with the page pointer specified */
>  static inline bool should_zap_page(struct zap_details *details, struct page *page)
>  {
>  	/* If we can make a decision without *page.. */
> @@ -1338,7 +1335,8 @@ static inline bool should_zap_page(struct zap_details *details, struct page *pag
>  	if (!page)
>  		return true;
>  
> -	return details->zap_mapping == page_rmapping(page);
> +	/* Otherwise we should only zap non-anon pages */
> +	return !PageAnon(page);
>  }
>  
>  static unsigned long zap_pte_range(struct mmu_gather *tlb,
> @@ -3403,7 +3401,7 @@ void unmap_mapping_folio(struct folio *folio)
>  	first_index = folio->index;
>  	last_index = folio->index + folio_nr_pages(folio) - 1;
>  
> -	details.zap_mapping = mapping;
> +	details.even_cows = false;
>  	details.single_folio = folio;
>  
>  	i_mmap_lock_write(mapping);
> @@ -3432,7 +3430,7 @@ void unmap_mapping_pages(struct address_space *mapping, pgoff_t start,
>  	pgoff_t	first_index = start;
>  	pgoff_t	last_index = start + nr - 1;
>  
> -	details.zap_mapping = even_cows ? NULL : mapping;
> +	details.even_cows = even_cows;
>  	if (last_index < first_index)
>  		last_index = ULONG_MAX;
>