Re: [RFC PATCH v1 1/6] fs/proc/task_mmu: Fix pte update and tlb maintenance ordering in pagemap_scan_pmd_entry()

[Ryan Roberts <ryan.roberts@arm.com>]

On 30/05/2025 17:26, Jann Horn wrote:
> On Fri, May 30, 2025 at 4:04 PM Ryan Roberts <ryan.roberts@arm.com> wrote:
>> pagemap_scan_pmd_entry() was previously modifying ptes while in lazy mmu
>> mode, then performing tlb maintenance for the modified ptes, then
>> leaving lazy mmu mode. But any pte modifications during lazy mmu mode
>> may be deferred until arch_leave_lazy_mmu_mode(), inverting the required
>> ordering between pte modificaiton and tlb maintenance.
>>
>> Let's fix that by leaving mmu mode, forcing all the pte updates to be
>> actioned, before doing the tlb maintenance.
>>
>> This is a theorectical bug discovered during code review.
>>
>> Fixes: 52526ca7fdb9 ("fs/proc/task_mmu: implement IOCTL to get and optionally clear info about PTEs")
> 
> Hmm... isn't lazy mmu mode supposed to also delay TLB flushes, and
> preserve the ordering of PTE modifications and TLB flushes?
> 
> Looking at the existing implementations of lazy MMU:
> 
>  - In Xen PV implementation of lazy MMU, I see that TLB flush
> hypercalls are delayed as well (xen_flush_tlb(),
> xen_flush_tlb_one_user() and xen_flush_tlb_multi() all use
> xen_mc_issue(XEN_LAZY_MMU) which delays issuing if lazymmu is active).
>  - The sparc version also seems to delay TLB flushes, and sparc's
> arch_leave_lazy_mmu_mode() seems to do TLB flushes via
> flush_tlb_pending() if necessary.
>  - powerpc's arch_leave_lazy_mmu_mode() also seems to do TLB flushes.
> 
> Am I missing something?

I doubt it. I suspect this was just my misunderstanding then. I hadn't
appreciated that lazy mmu is also guarranteed to maintain flush ordering; it's
chronically under-documented. Sorry for the noise here. On that basis, I expect
the first 2 patches can definitely be dropped.

> 
> If arm64 requires different semantics compared to all existing
> implementations and doesn't delay TLB flushes for lazy mmu mode, I
> think the "Fixes" tag should point to your addition of lazy mmu
> support for arm64.

arm64 doesn't require different semantics. arm64 is using lazy mmu in a very
limited manner and it can already tolerate the current code.

I just spotted this during code review and was trying to be a good citizen.
Thanks for setting me straight!

Thanks,
Ryan

> 
>> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
>> ---
>>  fs/proc/task_mmu.c | 3 +--
>>  1 file changed, 1 insertion(+), 2 deletions(-)
>>
>> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
>> index 994cde10e3f4..361f3ffd9a0c 100644
>> --- a/fs/proc/task_mmu.c
>> +++ b/fs/proc/task_mmu.c
>> @@ -2557,10 +2557,9 @@ static int pagemap_scan_pmd_entry(pmd_t *pmd, unsigned long start,
>>         }
>>
>>  flush_and_return:
>> +       arch_leave_lazy_mmu_mode();
>>         if (flush_end)
>>                 flush_tlb_range(vma, start, addr);
>> -
>> -       arch_leave_lazy_mmu_mode();
> 
> I think this ordering was probably intentional, because doing it this
> way around allows Xen PV to avoid one more hypercall, because the TLB
> flush can be batched together with the page table changes?
> 
> 
>>         pte_unmap_unlock(start_pte, ptl);
>>
>>         cond_resched();
>> --
>> 2.43.0
>>