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Message-ID: <8912e179-601a-4677-b2f6-14f40d488d98@arm.com>
Date: Tue, 24 Jun 2025 14:11:36 +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 v3 1/2] mm: huge_memory: disallow hugepages if the
system-wide THP sysfs settings are disabled
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".
> "
>
> 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.
>
> Suggested-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> Signed-off-by: Baolin Wang <baolin.wang@...ux.alibaba.com>
> ---
> include/linux/huge_mm.h | 51 ++++++++++++++++++-------
> tools/testing/selftests/mm/khugepaged.c | 6 +--
> 2 files changed, 39 insertions(+), 18 deletions(-)
>
> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> index 4d5bb67dc4ec..ab70ca4e704b 100644
> --- a/include/linux/huge_mm.h
> +++ b/include/linux/huge_mm.h
> @@ -267,6 +267,42 @@ 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)
> +{
> + const unsigned long always = READ_ONCE(huge_anon_orders_always);
> + const unsigned long madvise = READ_ONCE(huge_anon_orders_madvise);
> + const unsigned long inherit = READ_ONCE(huge_anon_orders_inherit);
> + const unsigned long never = ~(always | madvise | inherit);
> + const bool inherit_never = !hugepage_global_enabled();
> +
> + /* Disallow orders that are set to NEVER directly ... */
> + orders &= ~never;
> +
> + /* ... or through inheritance (global == NEVER). */
> + if (inherit_never)
> + orders &= ~inherit;
> +
> + /*
> + * 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;
> +
> + /* We already excluded never inherit above. */
> + if (vm_flags & VM_HUGEPAGE)
> + return orders & (always | madvise | inherit);
> +
> + if (hugepage_global_always())
> + return orders & (always | inherit);
> +
> + return orders & always;
> +}
> +
> /**
> * thp_vma_allowable_orders - determine hugepage orders that are allowed for vma
> * @vma: the vm area to check
> @@ -289,19 +325,8 @@ 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 (vm_flags & VM_HUGEPAGE)
> - mask |= READ_ONCE(huge_anon_orders_madvise);
> - if (hugepage_global_always() ||
> - ((vm_flags & VM_HUGEPAGE) && hugepage_global_enabled()))
> - mask |= READ_ONCE(huge_anon_orders_inherit);
> -
> - orders &= mask;
> - if (!orders)
> - return 0;
> - }
> + if (vma_is_anonymous(vma))
> + orders = __thp_mask_anon_orders(vm_flags, tva_flags, orders);
>
> return __thp_vma_allowable_orders(vma, vm_flags, tva_flags, orders);
> }
> diff --git a/tools/testing/selftests/mm/khugepaged.c b/tools/testing/selftests/mm/khugepaged.c
> index 4341ce6b3b38..85bfff53dba6 100644
> --- a/tools/testing/selftests/mm/khugepaged.c
> +++ b/tools/testing/selftests/mm/khugepaged.c
> @@ -501,11 +501,7 @@ static void __madvise_collapse(const char *msg, char *p, int nr_hpages,
>
> printf("%s...", msg);
>
> - /*
> - * Prevent khugepaged interference and tests that MADV_COLLAPSE
> - * ignores /sys/kernel/mm/transparent_hugepage/enabled
> - */
> - settings.thp_enabled = THP_NEVER;
> + settings.thp_enabled = THP_ALWAYS;
> settings.shmem_enabled = SHMEM_NEVER;
> thp_push_settings(&settings);
>
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