Re: TLB and PTE coherency during munmap

[Peter Zijlstra <peterz@infradead.org>]

On Tue, May 28, 2013 at 11:10:25AM +0400, Max Filippov wrote:
> On Sun, May 26, 2013 at 6:50 AM, Max Filippov <jcmvbkbc@gmail.com> wrote:
> > Hello arch and mm people.
> >
> > Is it intentional that threads of a process that invoked munmap syscall
> > can see TLB entries pointing to already freed pages, or it is a bug?
> >
> > I'm talking about zap_pmd_range and zap_pte_range:
> >
> >       zap_pmd_range
> >         zap_pte_range
> >           arch_enter_lazy_mmu_mode
> >             ptep_get_and_clear_full
> >             tlb_remove_tlb_entry
> >             __tlb_remove_page
> >           arch_leave_lazy_mmu_mode
> >         cond_resched
> >
> > With the default arch_{enter,leave}_lazy_mmu_mode, tlb_remove_tlb_entry
> > and __tlb_remove_page there is a loop in the zap_pte_range that clears
> > PTEs and frees corresponding pages, but doesn't flush TLB, and
> > surrounding loop in the zap_pmd_range that calls cond_resched. If a thread
> > of the same process gets scheduled then it is able to see TLB entries
> > pointing to already freed physical pages.
> >
> > I've noticed that with xtensa arch when I added a test before returning to
> > userspace checking that TLB contents agrees with page tables of the
> > current mm. This check reliably fires with the LTP test mtest05 that
> > maps, unmaps and accesses memory from multiple threads.
> >
> > Is there anything wrong in my description, maybe something specific to
> > my arch, or this issue really exists?
> 
> Hi,
> 
> I've made similar checking function for MIPS (because qemu is my only choice
> and it simulates MIPS TLB) and ran my tests on mips-malta machine in qemu.
> With MIPS I can also see this issue. I hope I did it right, the patch at the
> bottom is for the reference. The test I run and the diagnostic output are as
> follows:
> 
> To me it looks like the cond_resched in the zap_pmd_range is the root cause
> of this issue (let alone SMP case for now). It was introduced in the commit
> 
> commit 97a894136f29802da19a15541de3c019e1ca147e
> Author: Peter Zijlstra <a.p.zijlstra@chello.nl>
> Date:   Tue May 24 17:12:04 2011 -0700
> 
>     mm: Remove i_mmap_lock lockbreak
> 
> Peter, Kamezawa, other reviewers of that commit, could you please comment?

Are you all running UP systems? I suppose the preemptible muck
invalidated the assumption that UP systems are 'easy'.

If you make tlb_fast_mode() return an unconditional false, does it all
work again?

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