Re: [PATCH v3 3/6] mm/huge_memory: respect MADV_COLLAPSE with PR_THP_DISABLE_EXCEPT_ADVISED

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

On Mon, Aug 04, 2025 at 04:40:46PM +0100, Usama Arif wrote:
> From: David Hildenbrand <david@redhat.com>
>
> 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.

Hm feels like 'including for shmem' is a bit brief here :)

But fine probably ok.

>
> Co-developed-by: Usama Arif <usamaarif642@gmail.com>
> Signed-off-by: Usama Arif <usamaarif642@gmail.com>
> Signed-off-by: David Hildenbrand <david@redhat.com>

LGTM, so:

Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>

> ---
>  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 bd4f9e6327e0..1fd06ecbde72 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 test_bit(MMF_DISABLE_THP_EXCEPT_ADVISED, &vma->vm_mm->flags);
>  }
>
> diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h
> index 9c1d6e49b8a9..cdda963a039a 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 85252b468f80..ef5ccb0ec5d5 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;

I guess as discussed we'll return to this.

>  	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 be761753f240..bd04212d6f79 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -5186,9 +5186,11 @@ 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.
>  	 */
> -	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 e6cdfda08aed..30609197a266 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -1816,7 +1816,7 @@ 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
>