Re: [PATCH v3 08/27] mm: Introduce zap_details.zap_flags

[Alistair Popple <apopple@nvidia.com>]

On Friday, 28 May 2021 6:21:30 AM AEST Peter Xu wrote:
> Instead of trying to introduce one variable for every new zap_details fields,
> let's introduce a flag so that it can start to encode true/false informations.
> 
> Let's start to use this flag first to clean up the only check_mapping variable.
> Firstly, the name "check_mapping" implies this is a "boolean", but actually it
> stores the mapping inside, just in a way that it won't be set if we don't want
> to check the mapping.
> 
> To make things clearer, introduce the 1st zap flag ZAP_FLAG_CHECK_MAPPING, so
> that we only check against the mapping if this bit set.  At the same time, we
> can rename check_mapping into zap_mapping and set it always.
> 
> Since at it, introduce another helper zap_check_mapping_skip() and use it in
> zap_pte_range() properly.
> 
> Some old comments have been removed in zap_pte_range() because they're
> duplicated, and since now we're with ZAP_FLAG_CHECK_MAPPING flag, it'll be very
> easy to grep this information by simply grepping the flag.
> 
> It'll also make life easier when we want to e.g. pass in zap_flags into the
> callers like unmap_mapping_pages() (instead of adding new booleans besides the
> even_cows parameter).
> 
> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---
>  include/linux/mm.h | 19 ++++++++++++++++++-
>  mm/memory.c        | 31 ++++++++-----------------------
>  2 files changed, 26 insertions(+), 24 deletions(-)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index db155be8e66c..52d3ef2ed753 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -1721,13 +1721,30 @@ static inline bool can_do_mlock(void) { return false; }
>  extern int user_shm_lock(size_t, struct user_struct *);
>  extern void user_shm_unlock(size_t, struct user_struct *);
>  
> +/* Whether to check page->mapping when zapping */
> +#define  ZAP_FLAG_CHECK_MAPPING             BIT(0)
> +
>  /*
>   * 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;
> +	unsigned long zap_flags;
>  };
>  
> +/* Return true if skip zapping this page, false otherwise */
> +static inline bool
> +zap_check_mapping_skip(struct zap_details *details, struct page *page)
> +{
> +	if (!details || !page)
> +		return false;
> +
> +	if (!(details->zap_flags & ZAP_FLAG_CHECK_MAPPING))
> +		return false;
> +
> +	return details->zap_mapping != page_rmapping(page);

I doubt this matters in practice, but there is a slight behaviour change
here that might be worth checking. Previously this check was equivalent
to:

details->zap_mapping && details->zap_mapping != page_rmapping(page)

Otherwise I think this looks good.

> +}
> +
>  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 27cf8a6375c6..c9dc4e9e05b5 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -1330,16 +1330,8 @@ 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_check_mapping_skip(details, page)))
> +				continue;
>  			ptent = ptep_get_and_clear_full(mm, addr, pte,
>  							tlb->fullmm);
>  			tlb_remove_tlb_entry(tlb, pte, addr);
> @@ -1372,17 +1364,8 @@ 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_check_mapping_skip(details, page)))
> +				continue;
>  			pte_clear_not_present_full(mm, addr, pte, tlb->fullmm);
>  			rss[mm_counter(page)]--;
>  
> @@ -3345,9 +3328,11 @@ void unmap_mapping_pages(struct address_space *mapping, pgoff_t start,
>  		pgoff_t nr, bool even_cows)
>  {
>  	pgoff_t	first_index = start, last_index = start + nr - 1;
> -	struct zap_details details = { };
> +	struct zap_details details = { .zap_mapping = mapping };
> +
> +	if (!even_cows)
> +		details.zap_flags |= ZAP_FLAG_CHECK_MAPPING;
>  
> -	details.check_mapping = even_cows ? NULL : mapping;
>  	if (last_index < first_index)
>  		last_index = ULONG_MAX;
>  
>