On Mon, 2018-07-30 at 12:49 -0700, Andy Lutomirski wrote: > > I think it's a big step in the right direction, but it still makes be > nervous. I'd be more comfortable with it if you at least had a > functional set of patches that result in active_mm being gone, > because > that will mean that you actually audited the whole mess and fixed > anything that might rely on active_mm pointing somewhere or that > might > be putting a value you didn't take into account into active_mm. IOW > I'm not totally thrilled by applying the patches as is if we're still > a bit unsure as to what might have gotten missed. > > I don't think it's at all necessary to redo the patches. > > Does that seem reasonable?
Absolutely. I tried to keep ->active_mm very similar to before for exactly that reason. Lets go through all the places where it is used, in x86 and architecture independent code. I have not checked other architectures. It looks like we should be able to get rid of ->active_mm at some point, but a lot of it depends on other architecture maintainers. arch/x86/events/core.c: - get_segment_base: get current->active_mm->context.ldt, this appears to be for TIF_IA32 user programs only, so we should be able to use current->mm here arch/x86/kernel/cpu/common.c: - current task's ->active_mm assigned in two places, never read arch/x86/lib/insn-eval.c: - get_desc() gets current->active_mm->context.ldt, this appears to be only for user space programs arch/x86/mm/tlb.c: - this series adds two places where current->active_mm is written, it is never read arch/x86/platform/efi/efi_64.c: - current->active_mm is set to efi_mm for a little bit, with irqs disabled, and then changed back, with irqs still disabled; we should be able to get rid of ->active_mm here - in the init code, ->active_mm is set to efi_mm as well, presumably the kernel automatically switches that back on the next context switch; this may be buggy, since preemption is enabled and a GFP_KERNEL allocation is just a few lines below arch/x86/power/cpu.c: - fix_processor_context() calls load_mm_ldt(current->active_mm);, we should be able to use cpu_tlbstate.loaded_mm instead drivers/cpufreq/pmac32-cpufreq.c: - pmu_set_cpu_speed() restores current->active_mm - don't know if anyone still cares about 32 bit PPC :) drivers/firmware/efi/arm-runtime.c: - efi_virtmap_unload switches back the pgd to current->active_mm from &efi_mm; that mm could be stored elsewhere if we excised ->active_mm everywhere drivers/macintosh/via-pmu.c: - same deal as pmap32-cpufreq.c above mm/mmu_context.c: - use_mm() tracks the ->active_mm a kernel thread is pointing to, but the mm is also tracked in ->mm - unuse_mm() is the same deal as use_mm(), we should be able to get rid of ->active_mm if everybody stops using it, and we no longer refcount it anywhere init/init_task.c: - init_task.active_mm = &init_mm fs/exec.c: - exec_mmap() juggles both ->mm and ->active_mm, in order to get refcounting right; without refcounting we can lose ->active_mm -- All Rights Reversed.
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