Re: [PATCH v3 1/2] mm: huge_memory: disallow hugepages if the system-wide THP sysfs settings are disabled

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 2025/6/23 18:26, Lorenzo Stoakes wrote:
> On Mon, Jun 23, 2025 at 04:28:08PM +0800, Baolin Wang wrote:
>> When invoking thp_vma_allowable_orders(), the TVA_ENFORCE_SYSFS flag is not
>> specified, we will ignore the THP sysfs settings. Whilst it makes sense for the
>> callers who do not specify this flag, it creates a odd and surprising situation
>> where a sysadmin specifying 'never' for all THP sizes still observing THP pages
>> being allocated and used on the system.
>>
>> The motivating case for this is MADV_COLLAPSE. 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.
>>
>> Currently, besides MADV_COLLAPSE not setting TVA_ENFORCE_SYSFS, there is only
>> one other instance where TVA_ENFORCE_SYSFS is not set, which is in the
>> collapse_pte_mapped_thp() function, but I believe this is reasonable from its
>> comments:
>>
>> "
>> /*
>>   * If we are here, we've succeeded in replacing all the native pages
>>   * in the page cache with a single hugepage. If a mm were to fault-in
>>   * this memory (mapped by a suitably aligned VMA), we'd get the hugepage
>>   * and map it by a PMD, regardless of sysfs THP settings. As such, let's
>>   * analogously elide sysfs THP settings here.
>>   */
>> if (!thp_vma_allowable_order(vma, vma->vm_flags, 0, PMD_ORDER))
>> "
>>
>> 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, the current strategy should be:
>>
>> If no hugepage modes are enabled for the desired orders, nor can we enable them
>> by inheriting from a 'global' enabled setting - then it must be the case that
>> all desired orders either specify or inherit 'NEVER' - and we must abort.
>>
>> Meanwhile, we should fix the khugepaged selftest for MADV_COLLAPSE by enabling
>> THP.
> 
> Thanks! Sounds good.
>>
>> Suggested-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> 
> Appreciate it though I'm not so bothered about attribution :) but just to say,
> of course the 'never' stuff is David's idea (and a good one!) :)

Yes, I should also add:

Suggested-by: David Hildenbrand <david@redhat.com>

>> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
> 
> LGTM so:
> 
> Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>

Thanks.