On Mon, Dec 04, 2023 at 11:11:26AM +0000, Ryan Roberts wrote: > To be honest, while I understand pte_cont() and friends, I don't understand > their relevance (or at least potential future relevance) to GUP? GUP in general can be smarter to recognize if a pte/pmd is a cont_pte and fetch the whole pte/pmd range if the caller specified. Now it loops over each pte/pmd. Fast-gup is better as it at least doesn't take pgtable lock, for cont_pte it looks inside gup_pte_range() which is good enough, but it'll still do folio checks for each sub-pte, even though the 2nd+ folio checks should be mostly the same (if to ignore races when the folio changed within the time of processing the cont_pte chunk). Slow-gup (as of what this series is about so far) doesn't do that either, for each cont_pte whole entry it'll loop N times, frequently taking and releasing the pgtable lock. A smarter slow-gup can fundamentallly setup follow_page_context.page_mask if it sees a cont_pte. There might be a challenge on whether holding the head page's refcount would stablize the whole folio, but that may be another question to ask. I think I also overlooked that PPC_8XX also has cont_pte support, so we actually have three users indeed, if not counting potential future archs adding support to also get that same tlb benefit. Thanks, -- Peter Xu
