My book has a fairly detailed discussion of how these operations were supposed to work and what the reasoning behind them was. Unfortunately, I don't have time to really participate this discussion at the moment, but I hope somebody else has access to the book and would (re-)read it for some background (not to claim that it got everything right 100% but to ensure that earlier mistakes are not repeated...). --david On 4/27/07, Hugh Dickins <hugh@xxxxxxxxxxx> wrote:
On Fri, 27 Apr 2007, Nick Piggin wrote: > > But that's because of ia64's cache coherency implementation. I don't really > follow the documentation to know whether it should be one way or the other, > but surely it should be done either before or after the set_pte_at, not both. > > Anyway, how about fremap or mprotect, for example? > ... > > OK, I'm still not sure that I understand why lazy_mmu_prot_update should be > used rather than flush_icache_page (in concept, not ia64 implementation). > Sure, flush_icache_page isn't given the pte, but let's assume we can change > that. You're asking lots of good questions. I wish the ia64 people would know the answers, but from the length of time the "lazy_mmu_prot_update" stuff took to get into the tree, and the length of time it's taken to be found defective, I suspect they don't, and we'll have to guess for them. Some guesses I'm working with... I presume Mike and Anil are correct, that it needs to be done before putting pte into page table, not left until after: but as you've guessed, that needs to be done everywhere, not just in the two places so far identified. When it was discussed last year (in connection with Peter's page cleaning patches) it was thought to be a variant of update_mmu_cache() (after setting pte), and we added the fremap one to accompany it; but now it looks to be a variant of flush_icache_page() (before setting pte). I believe lazy_mmu_prot_update(pteval) came into existence primarily for mprotect's change_pte_range() case. If ia64 filled in its flush_icache_page(vma, page), that could have been used there (checking 'vm_flags & VM_EXEC' instead of pte_exec): but that would involve a relatively expensive(?) pte_page() in a place which doesn't need to know the struct page for other cases. Well, not pte_page(), it needs to be vm_normal_page() doesn't it? and ia64's current lazy_mmu_prot_update is unsafe when !pfn_valid. Some flush_icache_pages are already in place, others are not: do we need to add some? But those architectures which have a non-empty flush_icache_page seem to have survived without the additional calls - so they might be unnecessarily slowed down by additional calls. I believe that was the secondary reason for lazy_mmu_prot_update(), perhaps better called ia64_flush_icache_page(): to allow calls to be added where ia64 was (mistakenly) thought to want them, without needing a protracted audit of how other architectures might be impacted. But I'm still trying to make sense of it. Hugh - To unsubscribe from this list: send the line "unsubscribe linux-ia64" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
-- Mosberger Consulting LLC, http://www.mosberger-consulting.com/ - To unsubscribe from this list: send the line "unsubscribe linux-ia64" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
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