Re: [PATCH v1 6/9] mm/mmu_gather: define ENCODED_PAGE_FLAG_DELAY_RMAP

[Ryan Roberts]

On 29/01/2024 14:32, David Hildenbrand wrote:
> Nowadays, encoded pages are only used in mmu_gather handling. Let's
> update the documentation, and define ENCODED_PAGE_BIT_DELAY_RMAP. While at
> it, rename ENCODE_PAGE_BITS to ENCODED_PAGE_BITS.
> 
> If encoded page pointers would ever be used in other context again, we'd
> likely want to change the defines to reflect their context (e.g.,
> ENCODED_PAGE_FLAG_MMU_GATHER_DELAY_RMAP). For now, let's keep it simple.
> 
> This is a preparation for using the remaining spare bit to indicate that
> the next item in an array of encoded pages is a "nr_pages" argument and
> not an encoded page.
> 
> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>

Reviewed-by: Ryan Roberts <ryan.roberts@xxxxxxx>

> ---
>  include/linux/mm_types.h | 17 +++++++++++------
>  mm/mmu_gather.c          |  5 +++--
>  2 files changed, 14 insertions(+), 8 deletions(-)
> 
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 8b611e13153e..1b89eec0d6df 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -210,8 +210,8 @@ struct page {
>   *
>   * An 'encoded_page' pointer is a pointer to a regular 'struct page', but
>   * with the low bits of the pointer indicating extra context-dependent
> - * information. Not super-common, but happens in mmu_gather and mlock
> - * handling, and this acts as a type system check on that use.
> + * information. Only used in mmu_gather handling, and this acts as a type
> + * system check on that use.
>   *
>   * We only really have two guaranteed bits in general, although you could
>   * play with 'struct page' alignment (see CONFIG_HAVE_ALIGNED_STRUCT_PAGE)
> @@ -220,21 +220,26 @@ struct page {
>   * Use the supplied helper functions to endcode/decode the pointer and bits.
>   */
>  struct encoded_page;
> -#define ENCODE_PAGE_BITS 3ul
> +
> +#define ENCODED_PAGE_BITS			3ul
> +
> +/* Perform rmap removal after we have flushed the TLB. */
> +#define ENCODED_PAGE_BIT_DELAY_RMAP		1ul
> +
>  static __always_inline struct encoded_page *encode_page(struct page *page, unsigned long flags)
>  {
> -	BUILD_BUG_ON(flags > ENCODE_PAGE_BITS);
> +	BUILD_BUG_ON(flags > ENCODED_PAGE_BITS);
>  	return (struct encoded_page *)(flags | (unsigned long)page);
>  }
>  
>  static inline unsigned long encoded_page_flags(struct encoded_page *page)
>  {
> -	return ENCODE_PAGE_BITS & (unsigned long)page;
> +	return ENCODED_PAGE_BITS & (unsigned long)page;
>  }
>  
>  static inline struct page *encoded_page_ptr(struct encoded_page *page)
>  {
> -	return (struct page *)(~ENCODE_PAGE_BITS & (unsigned long)page);
> +	return (struct page *)(~ENCODED_PAGE_BITS & (unsigned long)page);
>  }
>  
>  /*
> diff --git a/mm/mmu_gather.c b/mm/mmu_gather.c
> index ac733d81b112..6540c99c6758 100644
> --- a/mm/mmu_gather.c
> +++ b/mm/mmu_gather.c
> @@ -53,7 +53,7 @@ static void tlb_flush_rmap_batch(struct mmu_gather_batch *batch, struct vm_area_
>  	for (int i = 0; i < batch->nr; i++) {
>  		struct encoded_page *enc = batch->encoded_pages[i];
>  
> -		if (encoded_page_flags(enc)) {
> +		if (encoded_page_flags(enc) & ENCODED_PAGE_BIT_DELAY_RMAP) {
>  			struct page *page = encoded_page_ptr(enc);
>  			folio_remove_rmap_pte(page_folio(page), page, vma);
>  		}
> @@ -119,6 +119,7 @@ static void tlb_batch_list_free(struct mmu_gather *tlb)
>  bool __tlb_remove_page_size(struct mmu_gather *tlb, struct page *page,
>  		bool delay_rmap, int page_size)
>  {
> +	int flags = delay_rmap ? ENCODED_PAGE_BIT_DELAY_RMAP : 0;
>  	struct mmu_gather_batch *batch;
>  
>  	VM_BUG_ON(!tlb->end);
> @@ -132,7 +133,7 @@ bool __tlb_remove_page_size(struct mmu_gather *tlb, struct page *page,
>  	 * Add the page and check if we are full. If so
>  	 * force a flush.
>  	 */
> -	batch->encoded_pages[batch->nr++] = encode_page(page, delay_rmap);
> +	batch->encoded_pages[batch->nr++] = encode_page(page, flags);
>  	if (batch->nr == batch->max) {
>  		if (!tlb_next_batch(tlb))
>  			return true;