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? Thanks, -- Peter Xu
