Re: TLB and PTE coherency during munmap

[Max Filippov <jcmvbkbc@gmail.com>]

On Wed, May 29, 2013 at 4:27 PM, Peter Zijlstra <peterz@infradead.org> wrote:
> 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

Yes, xtensa SMP support is in the middle of forward porting/cleanup process.

> invalidated the assumption that UP systems are 'easy'.
>
> If you make tlb_fast_mode() return an unconditional false, does it all
> work again?

Actually the only thing that was visibly broken is the test for
TLB/PTE coherency,
but that incoherency appears to be intentional (and the test is too
simple for that).
Unconditionally returning false from tlb_fast_mode() doesn't fix that test, but
AFAICS it fixes underlying page freeing.

-- 
Thanks.
-- Max

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