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Message-ID: <97a67b74-d473-455e-a05e-c85fe45da008@linux.alibaba.com>
Date: Thu, 12 Jun 2025 20:45:11 +0800
From: Baolin Wang <baolin.wang@...ux.alibaba.com>
To: David Hildenbrand <david@...hat.com>, akpm@...ux-foundation.org,
hughd@...gle.com
Cc: lorenzo.stoakes@...cle.com, Liam.Howlett@...cle.com, npache@...hat.com,
ryan.roberts@....com, dev.jain@....com, ziy@...dia.com, linux-mm@...ck.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] mm: huge_memory: disallow hugepages if the
system-wide THP sysfs settings are disabled
On 2025/6/12 16:51, David Hildenbrand wrote:
> On 12.06.25 09:51, Baolin Wang wrote:
>>
>>
>> On 2025/6/11 20:34, David Hildenbrand wrote:
>>> On 05.06.25 10:00, Baolin Wang wrote:
>>>> The MADV_COLLAPSE will ignore the system-wide Anon THP sysfs settings,
>>>> which
>>>> means that even though we have disabled the Anon THP configuration,
>>>> MADV_COLLAPSE
>>>> will still attempt to collapse into a Anon THP. This violates the rule
>>>> we have
>>>> agreed upon: never means never.
>>>>
>>>> Another rule for madvise, referring to David's suggestion: “allowing
>>>> for collapsing
>>>> in a VM without VM_HUGEPAGE in the "madvise" mode would be fine".
>>>>
>>>> To address this issue, should check whether the Anon THP configuration
>>>> is disabled
>>>> in thp_vma_allowable_orders(), even when the TVA_ENFORCE_SYSFS flag is
>>>> set.
>>>>
>>>> In summary, the current strategy is:
>>>>
>>>> 1. If always & orders == 0, and madvise & orders == 0, and
>>>> hugepage_global_enabled() == false
>>>> (global THP settings are not enabled), it means mTHP of that orders
>>>> are prohibited
>>>> from being used, then madvise_collapse() is forbidden for that orders.
>>>>
>>>> 2. If always & orders == 0, and madvise & orders == 0, and
>>>> hugepage_global_enabled() == true
>>>> (global THP settings are enabled), and inherit & orders == 0, it means
>>>> mTHP of that
>>>> orders are still prohibited from being used, thus madvise_collapse()
>>>> is not allowed
>>>> for that orders.
>>>>
>>>> Reviewed-by: Zi Yan <ziy@...dia.com>
>>>> Signed-off-by: Baolin Wang <baolin.wang@...ux.alibaba.com>
>>>> ---
>>>> include/linux/huge_mm.h | 23 +++++++++++++++++++----
>>>> 1 file changed, 19 insertions(+), 4 deletions(-)
>>>>
>>>> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
>>>> index 2f190c90192d..199ddc9f04a1 100644
>>>> --- a/include/linux/huge_mm.h
>>>> +++ b/include/linux/huge_mm.h
>>>> @@ -287,20 +287,35 @@ unsigned long thp_vma_allowable_orders(struct
>>>> vm_area_struct *vma,
>>>> unsigned long orders)
>>>> {
>>>> /* Optimization to check if required orders are enabled
>>>> early. */
>>>> - if ((tva_flags & TVA_ENFORCE_SYSFS) && vma_is_anonymous(vma)) {
>>>> - unsigned long mask = READ_ONCE(huge_anon_orders_always);
>>>> + if (vma_is_anonymous(vma)) {
>>>> + unsigned long always = READ_ONCE(huge_anon_orders_always);
>>>> + unsigned long madvise = READ_ONCE(huge_anon_orders_madvise);
>>>> + unsigned long inherit = READ_ONCE(huge_anon_orders_inherit);
>>>> + unsigned long mask = always | madvise;
>>>> +
>>>> + /*
>>>> + * If the system-wide THP/mTHP sysfs settings are disabled,
>>>> + * then we should never allow hugepages.
>>> > + */> + if (!(mask & orders) &&
>>> !(hugepage_global_enabled() && (inherit & orders)))
>>>> + return 0;
>>>
>>> I'm still trying to digest that. Isn't there a way for us to work with
>>> the orders,
>>> essentially masking off all orders that are forbidden globally. Similar
>>> to below, if !orders, then return 0?
>>> /* Orders disabled directly. */
>>> orders &= ~TODO;
>>> /* Orders disabled by inheriting from the global toggle. */
>>> if (!hugepage_global_enabled())
>>> orders &= ~READ_ONCE(huge_anon_orders_inherit);
>>>
>>> TODO is probably a -1ULL and then clearing always/madvise/inherit. Could
>>> add a simple helper for that
>>>
>>> huge_anon_orders_never
>>
>> I followed Lorenzo's suggestion to simplify the logic. Does that look
>> more readable?
>>
>> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
>> index 2f190c90192d..3087ac7631e0 100644
>> --- a/include/linux/huge_mm.h
>> +++ b/include/linux/huge_mm.h
>> @@ -265,6 +265,43 @@ unsigned long __thp_vma_allowable_orders(struct
>> vm_area_struct *vma,
>> unsigned long tva_flags,
>> unsigned long orders);
>>
>> +/* Strictly mask requested anonymous orders according to sysfs
>> settings. */
>> +static inline unsigned long __thp_mask_anon_orders(unsigned long
>> vm_flags,
>> + unsigned long tva_flags, unsigned long
>> orders)
>> +{
>> + unsigned long always = READ_ONCE(huge_anon_orders_always);
>> + unsigned long madvise = READ_ONCE(huge_anon_orders_madvise);
>> + unsigned long inherit = READ_ONCE(huge_anon_orders_inherit);
>> + bool inherit_enabled = hugepage_global_enabled();
>> + bool has_madvise = vm_flags & VM_HUGEPAGE;
>> + unsigned long mask = always | madvise;
>> +
>> + mask = always | madvise;
>> + if (inherit_enabled)
>> + mask |= inherit;
>> +
>> + /* All set to/inherit NEVER - never means never globally,
>> abort. */
>> + if (!(mask & orders))
>> + return 0;
>
> Still confusing. I am not sure if we would properly catch when someone
> specifies e.g., 2M and 1M, while we only have 2M disabled.
IIUC, Yes. In your case, we will only allow order 8 (1M mTHP).
> I would rewrite the function to only ever substract from "orders".
>
> ...
>
> /* Disallow orders that are set to NEVER directly ... */
> order &= (always | madvise | inherit);
>
> /* ... or through inheritance. */
> if (inherit_enabled)
> orders &= ~inherit;
Sorry, I didn't get you here.
If orders = THP_ORDERS_ALL_ANON, inherit = 0x200 (order 9), always and
madvise are 0, and inherit_enabled = true. Then orders will be 0 with
your logic. But we should allow order 9, right?
>
> /*
> * Otherwise, we only enforce sysfs settings if asked. In addition,
> * if the user sets a sysfs mode of madvise and if TVA_ENFORCE_SYSFS
> * is not set, we don't bother checking whether the VMA has VM_HUGEPAGE
> * set.
> */
> if (!orders || !(tva_flags & TVA_ENFORCE_SYSFS))
> return orders;
>
>> +
>> + /*
>> + * Otherwise, we only enforce sysfs settings if asked. In
>> addition,
>> + * if the user sets a sysfs mode of madvise and if
>> TVA_ENFORCE_SYSFS
>> + * is not set, we don't bother checking whether the VMA has
>> VM_HUGEPAGE
>> + * set.
>> + */
>> + if (!(tva_flags & TVA_ENFORCE_SYSFS))
>> + return orders;
>> +
>> + mask = always;
>> + if (has_madvise)
>> + mask |= madvise;
>> + if (hugepage_global_always() || (has_madvise && inherit_enabled))
>> + mask |= inherit;
>
> Similarly, this can maybe become (not 100% sure if I got it right, the
> condition above is confusing)
IMO, this is the original logic.
> if (!has_madvise) {
> /*
> * Without VM_HUGEPAGE, only allow orders that are set to
> * ALWAYS directly ...
> */
> order &= (always | inherit);
> /* ... or through inheritance. */
> if (!hugepage_global_always())
> orders &= ~inherit;
Ditto. I can not understand this too.
> }
>
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