Re: [PATCH v3 09/27] mm: Introduce ZAP_FLAG_SKIP_SWAP

[Alistair Popple <apopple@nvidia.com>]

On Friday, 28 May 2021 6:21:35 AM AEST Peter Xu wrote:
> Firstly, the comment in zap_pte_range() is misleading because it checks against
> details rather than check_mappings, so it's against what the code did.
> 
> Meanwhile, it's confusing too on not explaining why passing in the details
> pointer would mean to skip all swap entries.  New user of zap_details could
> very possibly miss this fact if they don't read deep until zap_pte_range()
> because there's no comment at zap_details talking about it at all, so swap
> entries could be errornously skipped without being noticed.
> 
> This partly reverts 3e8715fdc03e ("mm: drop zap_details::check_swap_entries"),
> but introduce ZAP_FLAG_SKIP_SWAP flag, which means the opposite of previous
> "details" parameter: the caller should explicitly set this to skip swap
> entries, otherwise swap entries will always be considered (which is still the
> major case here).
> 
> Cc: Kirill A. Shutemov <kirill@shutemov.name>
> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---
>  include/linux/mm.h | 12 ++++++++++++
>  mm/memory.c        |  8 +++++---
>  2 files changed, 17 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 52d3ef2ed753..1adf313a01fe 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -1723,6 +1723,8 @@ extern void user_shm_unlock(size_t, struct user_struct *);
>  
>  /* Whether to check page->mapping when zapping */
>  #define  ZAP_FLAG_CHECK_MAPPING             BIT(0)
> +/* Whether to skip zapping swap entries */
> +#define  ZAP_FLAG_SKIP_SWAP                 BIT(1)
>  
>  /*
>   * Parameter block passed down to zap_pte_range in exceptional cases.
> @@ -1745,6 +1747,16 @@ zap_check_mapping_skip(struct zap_details *details, struct page *page)
>  	return details->zap_mapping != page_rmapping(page);
>  }
>  
> +/* Return true if skip swap entries, false otherwise */
> +static inline bool
> +zap_skip_swap(struct zap_details *details)

Minor nit-pick but imho it would be nice if the naming was consistent between
this and check mapping. Ie. zap_skip_swap()/zap_skip_check_mapping() or
zap_swap_skip()/zap_check_mapping_skip().

> +{
> +	if (!details)
> +		return false;
> +
> +	return details->zap_flags & ZAP_FLAG_SKIP_SWAP;
> +}
> +
>  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 c9dc4e9e05b5..8a3751be87ba 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -1376,8 +1376,7 @@ static unsigned long zap_pte_range(struct mmu_gather *tlb,
>  			continue;
>  		}
>  
> -		/* If details->check_mapping, we leave swap entries. */
> -		if (unlikely(details))
> +		if (unlikely(zap_skip_swap(details)))
>  			continue;
>  
>  		if (!non_swap_entry(entry))
> @@ -3328,7 +3327,10 @@ 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 = { .zap_mapping = mapping };
> +	struct zap_details details = {
> +		.zap_mapping = mapping,

I meant to comment on this in the previous patch, but it might be nice to set
.zap_mapping in the !even_cows case below to make it very obvious it only
applies to ZAP_FLAG_CHECK_MAPPING.

Otherwise I think this is a good clean up which makes things clearer. I double
checked that unmap_mapping_pages() was the only place in the existing code that
needs ZAP_FLAG_SKIP_SWAP and that appears to be the case so there shouldn't be
any behaviour changes from this.

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

> +		.zap_flags = ZAP_FLAG_SKIP_SWAP,
> +	};
>  
>  	if (!even_cows)
>  		details.zap_flags |= ZAP_FLAG_CHECK_MAPPING;
>