On 28.01.22 10:17, Peter Xu wrote: > On Fri, Jan 28, 2022 at 10:03:20AM +0100, David Hildenbrand wrote: >> On 28.01.22 05:54, 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/ >>> >>> Quotting partly from Hugh: >> >> s/Quotting/Quoting/ > > Will fix. > >> >>> >>> 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(-) >>> >>> 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; >> >> Already initialized to 0 via struct zap_details details = { }; >> >> We could think about >> >> struct zap_details details = { >> .single_folio = folio, >> }; >> >>> 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; >>> >> >> Eventually >> >> struct zap_details details = { >> .even_cows = even_cows, >> }; > > I think in the very initial version I have had that C99 init format but I > dropped it for some reason, perhaps when rebasing to the single_page work to > avoid touching the existing code. > > Since as you mentioned single_folio is another.. let's do the cleanup on top? Sure, why not. -- Thanks, David / dhildenb
