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

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

On 07/12/2023 11:25, David Hildenbrand wrote:
> 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 :)

OK fine, you win :). I'll add a spin lock on the writer side.