Re: [PATCH v4 3/4] mm: Change zap_details.zap_mapping into even_cows

From: John Hubbard
Date: Wed Feb 16 2022 - 22:20:05 EST


On 2/16/22 1:48 AM, Peter Xu wrote:
> Currently we have a zap_mapping pointer maintained in zap_details, when it is
> specified we only want to zap the pages that has the same mapping with what the
> caller has specified.
>
> But what we want to do is actually simpler: we want to skip zapping
> private (COW-ed) pages in some cases. We can refer to unmap_mapping_pages()
> callers where we could have passed in different even_cows values. The other
> user is unmap_mapping_folio() where we always want to skip private pages.
>
> According to Hugh, we used a mapping pointer for historical reason, as
> explained here:
>
> https://lore.kernel.org/lkml/391aa58d-ce84-9d4-d68d-d98a9c533255@xxxxxxxxxx/
>
> Quoting partly from Hugh:
>
> Which raises the question again of why I did not just use a boolean flag
> there originally: aah, I think I've found why. In those days there was a
> horrible "optimization", for better performance on some benchmark I guess,
> which when you read from /dev/zero into a private mapping, would map the zero
> page there (look up read_zero_pagealigned() and zeromap_page_range() if you
> dare). So there was another category of page to be skipped along with the
> anon COWs, and I didn't want multiple tests in the zap loop, so checking
> check_mapping against page->mapping did both. I think nowadays you could do
> it by checking for PageAnon page (or genuine swap entry) instead.
>
> This patch replaced the zap_details.zap_mapping pointer into the even_cows
> boolean, then we check it against PageAnon.
>
> Suggested-by: Hugh Dickins <hughd@xxxxxxxxxx>
> Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
> ---
> mm/memory.c | 16 +++++++---------
> 1 file changed, 7 insertions(+), 9 deletions(-)

Reviewed-by: John Hubbard <jhubbard@xxxxxxxxxx>

thanks,
--
John Hubbard
NVIDIA

>
> diff --git a/mm/memory.c b/mm/memory.c
> index 14d8428ff4db..ffa8c7dfe9ad 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -1309,8 +1309,8 @@ copy_page_range(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma)
> * 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 folio *single_folio; /* Locked folio to be unmapped */
> + bool even_cows; /* Zap COWed private pages too? */
> };
>
> /* Whether we should zap all COWed (private) pages too */
> @@ -1321,13 +1321,10 @@ static inline bool should_zap_cows(struct zap_details *details)
> return true;
>
> /* Or, we zap COWed pages only if the caller wants to */
> - return !details->zap_mapping;
> + return details->even_cows;
> }
>
> -/*
> - * We set details->zap_mapping when we want to unmap shared but keep private
> - * pages. Return true if we should zap this page, false otherwise.
> - */
> +/* Decides whether we should zap this page with the page pointer specified */
> static inline bool should_zap_page(struct zap_details *details, struct page *page)
> {
> /* If we can make a decision without *page.. */
> @@ -1338,7 +1335,8 @@ static inline bool should_zap_page(struct zap_details *details, struct page *pag
> if (!page)
> return true;
>
> - return details->zap_mapping == page_rmapping(page);
> + /* Otherwise we should only zap non-anon pages */
> + return !PageAnon(page);
> }
>
> static unsigned long zap_pte_range(struct mmu_gather *tlb,
> @@ -3403,7 +3401,7 @@ void unmap_mapping_folio(struct folio *folio)
> first_index = folio->index;
> last_index = folio->index + folio_nr_pages(folio) - 1;
>
> - details.zap_mapping = mapping;
> + details.even_cows = false;
> details.single_folio = folio;
>
> i_mmap_lock_write(mapping);
> @@ -3432,7 +3430,7 @@ void unmap_mapping_pages(struct address_space *mapping, pgoff_t start,
> pgoff_t first_index = start;
> pgoff_t last_index = start + nr - 1;
>
> - details.zap_mapping = even_cows ? NULL : mapping;
> + details.even_cows = even_cows;
> if (last_index < first_index)
> last_index = ULONG_MAX;
>