Messages in this thread | | | Date | Wed, 23 Apr 2014 21:32:28 +0200 | From | Oleg Nesterov <> | Subject | Re: [PATCH 1/2] KVM: async_pf: kill the unnecessary use_mm/unuse_mm async_pf_execute() |
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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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