Re: [PATCH v4 3/7] mm/huge_memory: respect MADV_COLLAPSE with PR_THP_DISABLE_EXCEPT_ADVISED

[Zi Yan]

On 13 Aug 2025, at 9:55, Usama Arif wrote:

> From: David Hildenbrand <david@xxxxxxxxxx>
>
> Let's allow for making MADV_COLLAPSE succeed on areas that neither have
> VM_HUGEPAGE nor VM_NOHUGEPAGE when we have THP disabled
> unless explicitly advised (PR_THP_DISABLE_EXCEPT_ADVISED).
>
> MADV_COLLAPSE is a clear advice that we want to collapse.
>
> Note that we still respect the VM_NOHUGEPAGE flag, just like
> MADV_COLLAPSE always does. So consequently, MADV_COLLAPSE is now only
> refused on VM_NOHUGEPAGE with PR_THP_DISABLE_EXCEPT_ADVISED,
> including for shmem.
>
> Co-developed-by: Usama Arif <usamaarif642@xxxxxxxxx>
> Signed-off-by: Usama Arif <usamaarif642@xxxxxxxxx>
> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
> Reviewed-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
> Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
> ---
>  include/linux/huge_mm.h    | 8 +++++++-
>  include/uapi/linux/prctl.h | 2 +-
>  mm/huge_memory.c           | 5 +++--
>  mm/memory.c                | 6 ++++--
>  mm/shmem.c                 | 2 +-
>  5 files changed, 16 insertions(+), 7 deletions(-)
>
> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> index 92ea0b9771fae..1ac0d06fb3c1d 100644
> --- a/include/linux/huge_mm.h
> +++ b/include/linux/huge_mm.h
> @@ -329,7 +329,7 @@ struct thpsize {
>   * through madvise or prctl.
>   */
>  static inline bool vma_thp_disabled(struct vm_area_struct *vma,
> -		vm_flags_t vm_flags)
> +		vm_flags_t vm_flags, bool forced_collapse)
>  {
>  	/* Are THPs disabled for this VMA? */
>  	if (vm_flags & VM_NOHUGEPAGE)
> @@ -343,6 +343,12 @@ static inline bool vma_thp_disabled(struct vm_area_struct *vma,
>  	 */
>  	if (vm_flags & VM_HUGEPAGE)
>  		return false;
> +	/*
> +	 * Forcing a collapse (e.g., madv_collapse), is a clear advice to
> +	 * use THPs.
> +	 */
> +	if (forced_collapse)
> +		return false;
>  	return mm_flags_test(MMF_DISABLE_THP_EXCEPT_ADVISED, vma->vm_mm);
>  }
>
> diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h
> index 150b6deebfb1e..51c4e8c82b1e9 100644
> --- a/include/uapi/linux/prctl.h
> +++ b/include/uapi/linux/prctl.h
> @@ -185,7 +185,7 @@ struct prctl_mm_map {
>  #define PR_SET_THP_DISABLE	41
>  /*
>   * Don't disable THPs when explicitly advised (e.g., MADV_HUGEPAGE /
> - * VM_HUGEPAGE).
> + * VM_HUGEPAGE, MADV_COLLAPSE).
>   */
>  # define PR_THP_DISABLE_EXCEPT_ADVISED	(1 << 1)
>  #define PR_GET_THP_DISABLE	42
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 9c716be949cbf..1eca2d543449c 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -104,7 +104,8 @@ unsigned long __thp_vma_allowable_orders(struct vm_area_struct *vma,
>  {
>  	const bool smaps = type == TVA_SMAPS;
>  	const bool in_pf = type == TVA_PAGEFAULT;
> -	const bool enforce_sysfs = type != TVA_FORCED_COLLAPSE;
> +	const bool forced_collapse = type == TVA_FORCED_COLLAPSE;
> +	const bool enforce_sysfs = !forced_collapse;
>  	unsigned long supported_orders;
>
>  	/* Check the intersection of requested and supported orders. */
> @@ -122,7 +123,7 @@ unsigned long __thp_vma_allowable_orders(struct vm_area_struct *vma,
>  	if (!vma->vm_mm)		/* vdso */
>  		return 0;
>
> -	if (thp_disabled_by_hw() || vma_thp_disabled(vma, vm_flags))
> +	if (thp_disabled_by_hw() || vma_thp_disabled(vma, vm_flags, forced_collapse))
>  		return 0;
>
>  	/* khugepaged doesn't collapse DAX vma, but page fault is fine. */
> diff --git a/mm/memory.c b/mm/memory.c
> index 7b1e8f137fa3f..e4f533655305a 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -5332,9 +5332,11 @@ vm_fault_t do_set_pmd(struct vm_fault *vmf, struct folio *folio, struct page *pa
>  	 * It is too late to allocate a small folio, we already have a large
>  	 * folio in the pagecache: especially s390 KVM cannot tolerate any
>  	 * PMD mappings, but PTE-mapped THP are fine. So let's simply refuse any
> -	 * PMD mappings if THPs are disabled.
> +	 * PMD mappings if THPs are disabled. As we already have a THP ...
> +	 * behave as if we are forcing a collapse.

What does the “...” mean here?

Shouldn’t it be:

As we already have a THP,
behave as if we are forcing a collapse.

>  	 */
> -	if (thp_disabled_by_hw() || vma_thp_disabled(vma, vma->vm_flags))
> +	if (thp_disabled_by_hw() || vma_thp_disabled(vma, vma->vm_flags,
> +						     /* forced_collapse=*/ true))
>  		return ret;
>
>  	if (!thp_vma_suitable_order(vma, haddr, PMD_ORDER))
> diff --git a/mm/shmem.c b/mm/shmem.c
> index e2c76a30802b6..d945de3a7f0e7 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -1817,7 +1817,7 @@ unsigned long shmem_allowable_huge_orders(struct inode *inode,
>  	vm_flags_t vm_flags = vma ? vma->vm_flags : 0;
>  	unsigned int global_orders;
>
> -	if (thp_disabled_by_hw() || (vma && vma_thp_disabled(vma, vm_flags)))
> +	if (thp_disabled_by_hw() || (vma && vma_thp_disabled(vma, vm_flags, shmem_huge_force)))
>  		return 0;
>
>  	global_orders = shmem_huge_global_enabled(inode, index, write_end,
> -- 
> 2.47.3

Otherwise, LGTM. Reviewed-by: Zi Yan <ziy@xxxxxxxxxx>


Best Regards,
Yan, Zi