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    SubjectRe: [PATCH 1/2] KVM: async_pf: kill the unnecessary use_mm/unuse_mm async_pf_execute()
    On 04/22, Christian Borntraeger wrote:
    >
    > On 22/04/14 22:15, Christian Borntraeger wrote:
    > > On 21/04/14 15:25, Oleg Nesterov wrote:
    > >> async_pf_execute() has no reasons to adopt apf->mm, gup(current, mm)
    > >> should work just fine even if current has another or NULL ->mm.
    > >>
    > >> Recently kvm_async_page_present_sync() was added insedie the "use_mm"
    > >> section, but it seems that it doesn't need current->mm too.
    > >>
    > >> Signed-off-by: Oleg Nesterov <oleg@redhat.com>
    > >
    > > Indeed, use/unuse_mm should only be necessary for copy_to/from_user etc.
    > > This is fine for s390, but it seems that x86 kvm_arch_async_page_not_present
    > > might call apf_put_user which might call copy_to_user, so this is not ok, I guess.
    >
    > wanted to say kvm_arch_async_page_not_present, but I have to correct myself.
    > x86 does the "page is there" in the cpu loop, not in the worker. The cpu look
    > d oes have a valid mm. So this patch should be also ok.

    Thanks ;)

    Btw, I forgot to mention this in the changelog, but

    > >> @@ -80,12 +80,10 @@ static void async_pf_execute(struct work_struct *work)
    > >>
    > >> might_sleep();
    > >>
    > >> - use_mm(mm);
    > >> down_read(&mm->mmap_sem);
    > >> get_user_pages(current, mm, addr, 1, 1, 0, NULL, NULL);
    > >> up_read(&mm->mmap_sem);
    > >> kvm_async_page_present_sync(vcpu, apf);
    > >> - unuse_mm(mm);

    it can actually do

    get_user_pages(NULL, mm, addr, 1, 1, 0, NULL, NULL);

    "task" is only used to increment task_struct->xxx_flt. I don't think
    async_pf_execute() actually needs this (current is PF_WQ_WORKER after
    all), but I didn't dare to do another change in the code I can hardly
    understand.

    Oleg.



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