linux-kernel - Re: [PATCH v4 1/2] mm: huge_memory: disallow hugepages if the system-wide THP sysfs settings are disabled

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Message-ID: <ed1be989-4840-4461-bc9f-7f9cdc434d93@arm.com>
Date: Wed, 25 Jun 2025 10:04:15 +0530
From: Dev Jain <dev.jain@....com>
To: Baolin Wang <baolin.wang@...ux.alibaba.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 v4 1/2] mm: huge_memory: disallow hugepages if the
 system-wide THP sysfs settings are disabled


On 25/06/25 7:10 am, 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. Originally,
> we could prevent khugepaged by setting THP_MADVISE and removing MADV_HUGEPAGE
> setting, while madvise_collapse() can still perform THP collapse. However,
> this would cause some test cases to fail because some tests previously set
> MADV_NOHUGEPAGE, and now there is no other way to clear the MADV_NOHUGEPAGE
> flag except for setting MADV_HUGEPAGE. Therefore, it should be changed to
> THP_ALWAYS here to allow madvise_collapse() to perform THP collapse.
>
> Suggested-by: David Hildenbrand <david@...hat.com>
> Suggested-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> Reviewed-by: Zi Yan <ziy@...dia.com>
> Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> Signed-off-by: Baolin Wang <baolin.wang@...ux.alibaba.com>
> ---
>   

Reviewed-by: Dev Jain <dev.jain@....com>

>