From: Peter Xu <peterx@xxxxxxxxxx> Subject: mm: change zap_details.zap_mapping into even_cows 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 replaces the zap_details.zap_mapping pointer into the even_cows boolean, then we check it against PageAnon. Link: https://lkml.kernel.org/r/20220216094810.60572-4-peterx@xxxxxxxxxx Signed-off-by: Peter Xu <peterx@xxxxxxxxxx> Suggested-by: Hugh Dickins <hughd@xxxxxxxxxx> Reviewed-by: John Hubbard <jhubbard@xxxxxxxxxx> Cc: David Hildenbrand <david@xxxxxxxxxx> Cc: Alistair Popple <apopple@xxxxxxxxxx> Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx> Cc: "Kirill A . Shutemov" <kirill@xxxxxxxxxxxxx> Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx> Cc: Vlastimil Babka <vbabka@xxxxxxx> Cc: Yang Shi <shy828301@xxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- mm/memory.c | 16 +++++++--------- 1 file changed, 7 insertions(+), 9 deletions(-) --- a/mm/memory.c~mm-change-zap_detailszap_mapping-into-even_cows +++ a/mm/memory.c @@ -1309,8 +1309,8 @@ copy_page_range(struct vm_area_struct *d * 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(struc 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(struc 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, @@ -3398,7 +3396,7 @@ void unmap_mapping_folio(struct folio *f 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); @@ -3427,7 +3425,7 @@ void unmap_mapping_pages(struct address_ 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; _
