[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 at arm.com
Sat May 31 02:45:31 AEST 2025
On 30/05/2025 17:26, Jann Horn wrote:
> On Fri, May 30, 2025 at 4:04 PM Ryan Roberts <ryan.roberts at 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 at 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
>>
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