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

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Message-ID: <6771bdca-b489-42f3-b2fe-5449879e8687@linux.alibaba.com>
Date: Tue, 24 Jun 2025 22:08:43 +0800
From: Baolin Wang <baolin.wang@...ux.alibaba.com>
To: Dev Jain <dev.jain@....com>, akpm@...ux-foundation.org, hughd@...gle.com,
 david@...hat.com
Cc: ziy@...dia.com, lorenzo.stoakes@...cle.com, Liam.Howlett@...cle.com,
 npache@...hat.com, ryan.roberts@....com, baohua@...nel.org,
 linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 1/2] mm: huge_memory: disallow hugepages if the
 system-wide THP sysfs settings are disabled



On 2025/6/24 17:57, Baolin Wang wrote:
> 
> 
> 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.

After thinking more, since collapse_pte_mapped_thp() is only used for 
file/shmem collapse, changing to __thp_vma_allowable_orders() has no 
effect. So I prefer to leave it as is.