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/ > > 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, }; -- Thanks, David / dhildenb
