Re: [PATCH v4 4/4] mm: Add zap_skip_check_mapping() helper

[Hugh Dickins <hughd@google.com>]

On Wed, 15 Sep 2021, 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.
> 
> Reviewed-by: Alistair Popple <apopple@nvidia.com>
> Signed-off-by: Peter Xu <peterx@redhat.com>

Again, I won't NAK, but I have no enthusiasm for this at all: our tastes
clearly differ.  I don't find the new name helpful, I don't find the
separated "helper" helpful, and you have hidden the helpful comment
(but I'd be on firmer ground if the unmap_shared_mapping_pages() it
referred to had ever existed! perhaps it was in an intermediate tree).

But then I would feel this way, wouldn't I?
See dd9fd0e03de ("[PATCH] rmap: nonlinear truncation") in
//git.kernel.org/pub/scm/linux/kernel/git/tglx/history.git

I'm glad to see that you have dropped 5/5 for now:
I was not keen on that one either.

Hugh

> ---
>  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 a7e427177817..8db8ce0ca6ce 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)]--;
>  
> @@ -3369,7 +3352,7 @@ 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);
> @@ -3398,7 +3381,7 @@ void unmap_mapping_pages(struct address_space *mapping, pgoff_t start,
>  	pgoff_t	first_index = start;
>  	pgoff_t	last_index = start + nr - 1;
>  
> -	details.check_mapping = even_cows ? NULL : mapping;
> +	details.zap_mapping = even_cows ? NULL : mapping;
>  	if (last_index < first_index)
>  		last_index = ULONG_MAX;
>  
> -- 
> 2.31.1