Re: [PATCH v3 5/5] mm: Add ZAP_FLAG_SKIP_SWAP and zap_flags

[Alistair Popple <apopple@nvidia.com>]

On Thursday, 9 September 2021 2:36:28 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, there's no explicit reason why passing in the details pointer should
> 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
> erroneously 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 should still
> be the major case here).
> 
> We may want to look into when exactly we need ZAP_FLAG_SKIP_SWAP and we should
> have it in a synchronous manner, e.g., currently even if ZAP_FLAG_SKIP_SWAP is
> set we'll still look into swap pmds no matter what.  But that should be a
> separate effort of this patch.

I didn't really follow what you mean by "synchronous" here, although the
explanation about pmds makes sense so it's probably just terminology.
 
> The flag introduced in this patch will be a preparation for more bits defined
> in the future, e.g., for a new bit in flag to show whether to persist the
> upcoming uffd-wp bit in pgtable entries.

That's kind of the problem. The patch itself looks correct to me however as
mentioned it is mostly reverting a previous cleanup and it's hard to tell why
that's justified without the subsequent patches. Perhaps it makes the usage of
zap_details a bit clearer, but a comment also would with less code.

I know you want to try and shrink the uffd-wp series but I think this patch
might be easier to review if it was included as part of that series.

> Cc: Kirill A. Shutemov <kirill@shutemov.name>
> Cc: Hugh Dickins <hughd@google.com>
> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---
>  include/linux/mm.h | 16 ++++++++++++++++
>  mm/memory.c        |  6 +++---
>  2 files changed, 19 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index ed44f31615d9..beb784ce35b9 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -1717,12 +1717,18 @@ static inline bool can_do_mlock(void) { return false; }
>  extern int user_shm_lock(size_t, struct ucounts *);
>  extern void user_shm_unlock(size_t, struct ucounts *);
>  
> +typedef unsigned int __bitwise zap_flags_t;
> +
> +/* Whether to skip zapping swap entries */
> +#define  ZAP_FLAG_SKIP_SWAP  ((__force zap_flags_t) BIT(0))
> +
>  /*
>   * 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 page *single_page;		/* Locked page to be unmapped */
> +	zap_flags_t zap_flags;			/* Extra flags for zapping */
>  };
>  
>  /*
> @@ -1739,6 +1745,16 @@ zap_skip_check_mapping(struct zap_details *details, struct page *page)
>  	    (details->zap_mapping != page_rmapping(page));
>  }
>  
> +/* Return true if skip swap entries, false otherwise */
> +static inline bool
> +zap_skip_swap(struct zap_details *details)
> +{
> +	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 e5ee8399d270..26e37bef1888 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -1379,8 +1379,7 @@ 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))
> @@ -3353,6 +3352,7 @@ void unmap_mapping_page(struct page *page)
>  
>  	details.zap_mapping = mapping;
>  	details.single_page = page;
> +	details.zap_flags = ZAP_FLAG_SKIP_SWAP;
>  
>  	i_mmap_lock_write(mapping);
>  	if (unlikely(!RB_EMPTY_ROOT(&mapping->i_mmap.rb_root)))
> @@ -3377,7 +3377,7 @@ 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_flags = ZAP_FLAG_SKIP_SWAP };
>  
>  	details.zap_mapping = even_cows ? NULL : mapping;
>  	if (last_index < first_index)
>