Re: [PATCH v8 03/10] mm: thp: Introduce multi-size THP sysfs interface

[David Hildenbrand <david@redhat.com>]

On 07.12.23 12:22, Ryan Roberts wrote:
> On 07/12/2023 11:13, David Hildenbrand wrote:
>>>>
>>>>> +
>>>>>         if (!vma->vm_mm)        /* vdso */
>>>>> -        return false;
>>>>> +        return 0;
>>>>>           /*
>>>>>          * Explicitly disabled through madvise or prctl, or some
>>>>> @@ -88,16 +141,16 @@ bool hugepage_vma_check(struct vm_area_struct *vma,
>>>>> unsigned long vm_flags,
>>>>>          * */
>>>>>         if ((vm_flags & VM_NOHUGEPAGE) ||
>>>>>             test_bit(MMF_DISABLE_THP, &vma->vm_mm->flags))
>>>>> -        return false;
>>>>> +        return 0;
>>>>>         /*
>>>>>          * If the hardware/firmware marked hugepage support disabled.
>>>>>          */
>>>>>         if (transparent_hugepage_flags & (1 <<
>>>>> TRANSPARENT_HUGEPAGE_UNSUPPORTED))
>>>>> -        return false;
>>>>> +        return 0;
>>>>>           /* khugepaged doesn't collapse DAX vma, but page fault is fine. */
>>>>>         if (vma_is_dax(vma))
>>>>> -        return in_pf;
>>>>> +        return in_pf ? orders : 0;
>>>>>           /*
>>>>>          * khugepaged special VMA and hugetlb VMA.
>>>>> @@ -105,17 +158,29 @@ bool hugepage_vma_check(struct vm_area_struct *vma,
>>>>> unsigned long vm_flags,
>>>>>          * VM_MIXEDMAP set.
>>>>>          */
>>>>>         if (!in_pf && !smaps && (vm_flags & VM_NO_KHUGEPAGED))
>>>>> -        return false;
>>>>> +        return 0;
>>>>>           /*
>>>>> -     * Check alignment for file vma and size for both file and anon vma.
>>>>> +     * Check alignment for file vma and size for both file and anon vma by
>>>>> +     * filtering out the unsuitable orders.
>>>>>          *
>>>>>          * Skip the check for page fault. Huge fault does the check in fault
>>>>> -     * handlers. And this check is not suitable for huge PUD fault.
>>>>> +     * handlers.
>>>>>          */
>>>>> -    if (!in_pf &&
>>>>> -        !transhuge_vma_suitable(vma, (vma->vm_end - HPAGE_PMD_SIZE)))
>>>>> -        return false;
>>>>> +    if (!in_pf) {
>>>>> +        int order = first_order(orders);
>>>>> +        unsigned long addr;
>>>>> +
>>>>> +        while (orders) {
>>>>> +            addr = vma->vm_end - (PAGE_SIZE << order);
>>>>> +            if (thp_vma_suitable_orders(vma, addr, BIT(order)))
>>>>> +                break;
>>>>
>>>> Comment: you'd want a "thp_vma_suitable_order" helper here. But maybe the
>>>> compiler is smart enough to optimize the loop and everyything else out.
>>>
>>> I'm happy to refactor so that thp_vma_suitable_order() is the basic primitive,
>>> then make thp_vma_suitable_orders() a loop that calls thp_vma_suitable_order()
>>> (that's basically how it is laid out already, just all in one function). Is that
>>> what you are requesting?
>>
>> You got the spirit, yes.
>>
>>>>
>>>> [...]
>>>>
>>>>> +
>>>>> +static ssize_t thpsize_enabled_store(struct kobject *kobj,
>>>>> +                     struct kobj_attribute *attr,
>>>>> +                     const char *buf, size_t count)
>>>>> +{
>>>>> +    int order = to_thpsize(kobj)->order;
>>>>> +    ssize_t ret = count;
>>>>> +
>>>>> +    if (sysfs_streq(buf, "always")) {
>>>>> +        set_bit(order, &huge_anon_orders_always);
>>>>> +        clear_bit(order, &huge_anon_orders_inherit);
>>>>> +        clear_bit(order, &huge_anon_orders_madvise);
>>>>> +    } else if (sysfs_streq(buf, "inherit")) {
>>>>> +        set_bit(order, &huge_anon_orders_inherit);
>>>>> +        clear_bit(order, &huge_anon_orders_always);
>>>>> +        clear_bit(order, &huge_anon_orders_madvise);
>>>>> +    } else if (sysfs_streq(buf, "madvise")) {
>>>>> +        set_bit(order, &huge_anon_orders_madvise);
>>>>> +        clear_bit(order, &huge_anon_orders_always);
>>>>> +        clear_bit(order, &huge_anon_orders_inherit);
>>>>> +    } else if (sysfs_streq(buf, "never")) {
>>>>> +        clear_bit(order, &huge_anon_orders_always);
>>>>> +        clear_bit(order, &huge_anon_orders_inherit);
>>>>> +        clear_bit(order, &huge_anon_orders_madvise);
>>>>
>>>> Note: I was wondering for a second if some concurrent cames could lead to an
>>>> inconsistent state. I think in the worst case we'll simply end up with "never"
>>>> on races.
>>>
>>> You mean if different threads try to write different values to this file
>>> concurrently? Or if there is a concurrent fault that tries to read the flags
>>> while they are being modified?
>>
>> I thought about what you said first, but what you said last might also apply. As
>> long as "nothing breaks", all good.
>>
>>>
>>> I thought about this for a long time too and wasn't sure what was best. The
>>> existing global enabled store impl clears the bits first then sets the bit. With
>>> this approach you can end up with multiple bits set if there is a race to set
>>> diffierent values, and you can end up with a faulting thread seeing never if it
>>> reads the bits after they have been cleared but before setting them.
>>
>> Right, but user space is playing stupid games and can win stupid prices. As long
>> as nothing breaks, we're good.
>>
>>>
>>> I decided to set the new bit before clearing the old bits, which is different; A
>>> racing fault will never see "never" but as you say, a race to set the file could
>>> result in "never" being set.
>>>
>>> On reflection, it's probably best to set the bit *last* like the global control
>>> does?
>>
>> Probably might just slap a simple spinlock in there, so at least the writer side
>> is completely serialized. Then you can just set the bit last. It's unlikely that
>> readers will actually run into issues, and if they ever would, we could use some
>> rcu magic to let them read a consistent state.
> 
> I'd prefer to leave it as it is now; clear first, set last without any explicit
> serialization. I've convinced myself that nothing breaks and its the same
> pattern used by the global control so its consistent. Unless you're insisting on
> the spin lock?

No, not at all. But it would certainly remove any possible concerns :)

-- 
Cheers,

David / dhildenb