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

Ryan Roberts
2024-01-30T09:03:41Z

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@redhat.com>

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

> ---
> 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;