Re: [PATCH] mm/huge_memory: bypass THP tuneables for huge pfnmap mappings

["David Hildenbrand (Arm)" <david@kernel.org>]

On 8/27/26 21:55, Lorenzo Stoakes (ARM) wrote:
> The sysfs THP tuneables at /sys/kernel/mm/transparent_huge_pages/ rather
> confusingly only control the behaviour of THP in some instances.
> 
> They are not applicable to MADV_COLLAPSE operations, nor to DAX mappings.
> 
> Long-term, THP is predicated upon compaction being able to obtain large
> folios to populate THP ranges.
> 
> However, vm_normal_folio() returns NULL for PFN map mappings, thus their
> reference count is maintained by the driver, not core mm.
> 
> As a consequence, the folios are not subject to reclaim nor compaction, so
> are not truly part of the THP mechanism at all.
> 
> However, since commit 5dd40721f147 ("mm: allow THP orders for PFNMAPs")
> introduced the ability to establish huge PFN maps, they have been subject
> to THP tuneables.
> 
> This is incorrect - if a huge PFN map is available (defined by
> vma->vm_ops->huge_fault being non-NULL for a VMA_PFNMAP_BIT VMA), then it
> should be mapped huge upon fault-in.
> 
> Correct this by explicitly checking for this while ensuring that smaps
> continues to accurately report THPeligible statistics.
> 
> While here, abstract the entire file-backed THP check in
> vma_can_map_huge_file(), with sensible separation of logic into helper
> functions.
> 
> Note that drm_gem_shmem_mmap() and panthor_gem_mmap() establish huge PFN
> maps of shmem folios, however they are marked unevictable in
> drm_gem_get_pages(), and in any case would fail the reference check in
> __remove_mapping() even if they weren't.
> 
> Failing to map huge PFN maps has resulted in significant real-world
> performance degradation, see links for details.
> 
> Reported-by: Cedric Le Goater <clg@redhat.com>
> Closes: https://lore.kernel.org/linux-mm/20260805055544.1568534-1-clg@redhat.com/
> Reported-by: Saravanan D <saravanand@crusoe.ai>
> Closes: https://lore.kernel.org/linux-mm/20260821070520.25759-1-saravanand@crusoe.ai/
> Fixes: 5dd40721f147 ("mm: allow THP orders for PFNMAPs")
> Cc: stable@vger.kernel.org
> Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
> ---
>  mm/huge_memory.c | 86 +++++++++++++++++++++++++++++++++++++++++---------------
>  1 file changed, 64 insertions(+), 22 deletions(-)
> 
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index afbb5974bd22..4bf7b670586d 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -92,7 +92,7 @@ unsigned long huge_anon_orders_madvise __read_mostly;
>  unsigned long huge_anon_orders_inherit __read_mostly;
>  static bool anon_orders_configured __initdata;
>  
> -static inline bool file_thp_enabled(struct vm_area_struct *vma)
> +static inline bool file_thp_enabled(const struct vm_area_struct *vma)
>  {
>  	struct inode *inode;
>  
> @@ -118,6 +118,67 @@ static bool vma_is_special_huge(const struct vm_area_struct *vma)
>  	return vma_test_any(vma, VMA_PFNMAP_BIT, VMA_MIXEDMAP_BIT);
>  }
>  
> +static bool vma_bypass_thp_tuneables_file(const struct vm_area_struct *vma,
> +		enum tva_type type)
> +{
> +	const bool has_huge_fault = vma->vm_ops->huge_fault;
> +
> +	/* MADV_COLLAPSE ignores tuneables. */
> +	if (type == TVA_FORCED_COLLAPSE)
> +		return true;
> +	/* Huge PFN mappings are uncompactable so the policy doesn't apply. */
> +	if (vma_test(vma, VMA_PFNMAP_BIT) && has_huge_fault)
> +		return true;
> +	return false;
> +}
> +
> +static bool vma_thp_tuneables_allow_file(vm_flags_t vm_flags)
> +{
> +	/* THP=always? */
> +	if (hugepage_global_always())
> +		return true;
> +	/* THP=madvise and marked MADV_HUGEPAGE? */
> +	if (hugepage_global_enabled() && (vm_flags & VM_HUGEPAGE))
> +		return true;
> +	return false;
> +}
> +
> +static bool vma_check_thp_tuneables_file(const struct vm_area_struct *vma,
> +		vm_flags_t vm_flags, enum tva_type type)
> +{
> +	return vma_bypass_thp_tuneables_file(vma, type) ||
> +		vma_thp_tuneables_allow_file(vm_flags);
> +}
> +
> +static bool vma_can_map_huge_file(const struct vm_area_struct *vma,
> +		vm_flags_t vm_flags, enum tva_type type)
> +{
> +	const bool has_huge_fault = vma->vm_ops->huge_fault;
> +
> +	/*
> +	 * Enforce THP collapse requirements as necessary. Anonymous vmas
> +	 * were already handled in thp_vma_allowable_orders().
> +	 */
> +	if (!vma_check_thp_tuneables_file(vma, vm_flags, type))
> +		return false;
> +
> +	switch (type) {
> +	case TVA_PAGEFAULT:
> +		/*
> +		 * Trust that ->huge_fault() handlers know what they are doing
> +		 * in fault path.
> +		 */
> +		return has_huge_fault;
> +	case TVA_SMAPS:
> +		if (has_huge_fault)
> +			return true;
> +		fallthrough;
> +	default:
> +		/* Only regular file is valid in collapse path. */
> +		return file_thp_enabled(vma);
> +	}
> +}
> +
>  unsigned long __thp_vma_allowable_orders(struct vm_area_struct *vma,
>  					 vm_flags_t vm_flags,
>  					 enum tva_type type,
> @@ -190,27 +251,8 @@ unsigned long __thp_vma_allowable_orders(struct vm_area_struct *vma,
>  						   vma, vma_start_pgoff(vma), 0,
>  						   forced_collapse);
>  
> -	if (!vma_is_anonymous(vma)) {
> -		/*
> -		 * Enforce THP collapse requirements as necessary. Anonymous vmas
> -		 * were already handled in thp_vma_allowable_orders().
> -		 */
> -		if (!forced_collapse &&
> -		    (!hugepage_global_enabled() || (!(vm_flags & VM_HUGEPAGE) &&
> -						    !hugepage_global_always())))
> -			return 0;
> -
> -		/*
> -		 * Trust that ->huge_fault() handlers know what they are doing
> -		 * in fault path.
> -		 */
> -		if (((in_pf || smaps)) && vma->vm_ops->huge_fault)
> -			return orders;
> -		/* Only regular file is valid in collapse path */
> -		if (((!in_pf || smaps)) && file_thp_enabled(vma))
> -			return orders;
> -		return 0;
> -	}
> +	if (!vma_is_anonymous(vma))
> +		return vma_can_map_huge_file(vma, vm_flags, type) ? orders : 0;

I don't have the time to find out how these functions ended up being named, I'll
have to trust that it will be alright.

Adding to what Zi say, vma_can_map_huge_file is also confusing, no?

are we mapping a huge file? :)

-- 
Cheers,

David