Re: [PATCH v2 4/5] mm: Add zap_skip_check_mapping() helper

Alistair Popple
2021-09-03T00:59:02Z

On Friday, 3 September 2021 6:18:19 AM AEST Peter Xu wrote:
> Use the helper for the checks. Rename "check_mapping" into "zap_mapping"
> because "check_mapping" looks like a bool but in fact it stores the mapping
> itself. When it's set, we check the mapping (it must be non-NULL). When it's
> cleared we skip the check, which works like the old way.
>
> Move the duplicated comments to the helper too.
>
> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---
> include/linux/mm.h | 15 ++++++++++++++-
> mm/memory.c | 29 ++++++-----------------------
> 2 files changed, 20 insertions(+), 24 deletions(-)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 69259229f090..81e402a5fbc9 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -1720,10 +1720,23 @@ extern void user_shm_unlock(size_t, struct ucounts *);
> * 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; /* Check page->mapping if set */
> struct page *single_page; /* Locked page to be unmapped */
> };
>
> +/*
> + * We set details->zap_mappings when we want to unmap shared but keep private
> + * pages. Return true if skip zapping this page, false otherwise.
> + */
> +static inline bool
> +zap_skip_check_mapping(struct zap_details *details, struct page *page)
> +{
> + if (!details || !page)
> + return false;
> +
> + return details->zap_mapping != page_rmapping(page);

Shouldn't this check still be
details->zap_mapping && details->zap_mapping != page_rmapping(page)?

Previously we wouldn't skip zapping pages if even_cows == true (ie.
details->check_mapping == NULL). With this change the check when
even_cows == true becomes NULL != page_rmapping(page). Doesn't this mean we
will now skip zapping any pages with a mapping when even_cows == true?

> +}
> +
> 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 6bba3b9fef7c..e5ee8399d270 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -1333,16 +1333,8 @@ 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_skip_check_mapping(details, page)))
> + continue;
>
> ptent = ptep_get_and_clear_full(mm, addr, pte,
> tlb->fullmm);
> tlb_remove_tlb_entry(tlb, pte, addr);
> @@ -1375,17 +1367,8 @@ 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_skip_check_mapping(details, page)))
> + continue;
> pte_clear_not_present_full(mm, addr, pte, tlb->fullmm);
> rss[mm_counter(page)]--;
>
> @@ -3368,7 +3351,7 @@ void unmap_mapping_page(struct page *page)
> first_index = page->index;
> last_index = page->index + thp_nr_pages(page) - 1;
>
> - details.check_mapping = mapping;
> + details.zap_mapping = mapping;
> details.single_page = page;
>
> i_mmap_lock_write(mapping);
> @@ -3396,7 +3379,7 @@ void unmap_mapping_pages(struct address_space *mapping, pgoff_t start,
> pgoff_t first_index = start, last_index = start + nr - 1;
> struct zap_details details = { };
>
> - details.check_mapping = even_cows ? NULL : mapping;
> + details.zap_mapping = even_cows ? NULL : mapping;
> if (last_index < first_index)
> last_index = ULONG_MAX;
>
>