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    SubjectRe: Fw: [PATCH] ia64: race flushing icache in do_no_page path
    On Fri, 2007-04-27 at 15:18 +0100, Hugh Dickins wrote:
    > 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.

    That sounds about right. Before installing new mapping, kernel should
    ensure there are no stale contents in caches or TLB.
    lazy_mmu_prot_update needs to be called whenever the permissions on pte
    (about to) change. So if remapping is causing change in protection then
    lazy_mmu_prot_update needs to be called.

    > 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.

    Right. Extra flush_icache_page routines will add cost to archs that
    have non-null definition of this routine. BTW, isn't flush_icache_page
    marked for deprecation?

    > 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.

    lazy_mmu_prot_update was added specifically for notifying change in
    protection. So, in a way it is closer to update_mmu_cache (Which is for
    change in mappings itself). Though for ia64 implementation, this ends
    up flushing the icaches when needed.

    Hopefully my reply is useful.


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