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 2025/6/24 16:41, Dev Jain wrote:
> 
> On 23/06/25 1:58 pm, 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))
> 
> So the behaviour now is: First check whether THP settings converge to 
> never.
> Then, if enforce_sysfs is not set, return immediately. So in this 
> khugepaged
> code will it be better to call __thp_vma_allowable_orders()? If the sysfs
> settings are changed to never before hitting collapse_pte_mapped_thp(),
> then right now we will return SCAN_VMA_CHECK from here, whereas, the 
> comment
> says "regardless of sysfs THP settings", which should include "regardless
> of whether the sysfs settings say never".

Sounds reasonable to me. Thanks.

I will change thp_vma_allowable_order() to __thp_vma_allowable_orders() 
in the collapse_pte_mapped_thp() function to maintain consistency with 
the original logic.

Lorenzo and David, how do you think? Thanks.