Re: [PATCH 02/13] mm: extract ensure_on_mmlist() helper

Re: [PATCH 02/13] mm: extract ensure_on_mmlist() helper

[David Hildenbrand (Arm)]

On 4/27/26 12:01, Usama Arif wrote:
> When a swap entry is installed in a page table, the mm must be added
> to init_mm.mmlist so that swapoff can find and unuse its swap entries.
> This double-checked locking pattern is currently open-coded in
> try_to_unmap_one() and copy_nonpresent_pte().
> 
> Move it into ensure_on_mmlist() in mm/internal.h and convert both
> callers so it can be reused by upcoming PMD-level swap entry code
> paths that also need to register the mm with swapoff.
> 
> copy_nonpresent_pte() previously inserted into &src_mm->mmlist rather
> than &init_mm.mmlist, but the insertion point is irrelevant, mmlist
> is a circular list and swapoff walks it entirely from init_mm.mmlist,
> so only membership matters, not position.
> 
> Signed-off-by: Usama Arif <usama.arif@linux.dev>
> ---
>  mm/internal.h | 13 +++++++++++++
>  mm/memory.c   |  9 +--------
>  mm/rmap.c     |  7 +------
>  3 files changed, 15 insertions(+), 14 deletions(-)
> 
> diff --git a/mm/internal.h b/mm/internal.h
> index 5a2ddcf68e0b..7de489689f54 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -1952,4 +1952,17 @@ static inline int get_sysctl_max_map_count(void)
>  bool may_expand_vm(struct mm_struct *mm, const vma_flags_t *vma_flags,
>  		   unsigned long npages);
>  
> +/*
> + * Ensure @mm is on the init_mm.mmlist so swapoff can find it.
> + */
> +static inline void ensure_on_mmlist(struct mm_struct *mm)
> +{
> +	if (list_empty(&mm->mmlist)) {
> +		spin_lock(&mmlist_lock);
> +		if (list_empty(&mm->mmlist))
> +			list_add(&mm->mmlist, &init_mm.mmlist);
> +		spin_unlock(&mmlist_lock);
> +	}
> +}

Instead of talking about the low level detail ("add to mmlist"), maybe we could
just talk about the high-level goal: make sure that the MM can hold swap entries.


mm_prepare_for_swap()

or sth like that?

-- 
Cheers,

David

Re: [PATCH 02/13] mm: extract ensure_on_mmlist() helper

[Usama Arif]

On 13/05/2026 14:32, David Hildenbrand (Arm) wrote:
> On 4/27/26 12:01, Usama Arif wrote:
>> When a swap entry is installed in a page table, the mm must be added
>> to init_mm.mmlist so that swapoff can find and unuse its swap entries.
>> This double-checked locking pattern is currently open-coded in
>> try_to_unmap_one() and copy_nonpresent_pte().
>>
>> Move it into ensure_on_mmlist() in mm/internal.h and convert both
>> callers so it can be reused by upcoming PMD-level swap entry code
>> paths that also need to register the mm with swapoff.
>>
>> copy_nonpresent_pte() previously inserted into &src_mm->mmlist rather
>> than &init_mm.mmlist, but the insertion point is irrelevant, mmlist
>> is a circular list and swapoff walks it entirely from init_mm.mmlist,
>> so only membership matters, not position.
>>
>> Signed-off-by: Usama Arif <usama.arif@linux.dev>
>> ---
>>  mm/internal.h | 13 +++++++++++++
>>  mm/memory.c   |  9 +--------
>>  mm/rmap.c     |  7 +------
>>  3 files changed, 15 insertions(+), 14 deletions(-)
>>
>> diff --git a/mm/internal.h b/mm/internal.h
>> index 5a2ddcf68e0b..7de489689f54 100644
>> --- a/mm/internal.h
>> +++ b/mm/internal.h
>> @@ -1952,4 +1952,17 @@ static inline int get_sysctl_max_map_count(void)
>>  bool may_expand_vm(struct mm_struct *mm, const vma_flags_t *vma_flags,
>>  		   unsigned long npages);
>>  
>> +/*
>> + * Ensure @mm is on the init_mm.mmlist so swapoff can find it.
>> + */
>> +static inline void ensure_on_mmlist(struct mm_struct *mm)
>> +{
>> +	if (list_empty(&mm->mmlist)) {
>> +		spin_lock(&mmlist_lock);
>> +		if (list_empty(&mm->mmlist))
>> +			list_add(&mm->mmlist, &init_mm.mmlist);
>> +		spin_unlock(&mmlist_lock);
>> +	}
>> +}
> 
> Instead of talking about the low level detail ("add to mmlist"), maybe we could
> just talk about the high-level goal: make sure that the MM can hold swap entries.
> 
> 
> mm_prepare_for_swap()
> 
> or sth like that?
> 

Thanks for the review!

Ah so basically rename the function to mm_prepare_for_swap(). I felt like it
makes the function sound more important than it is? But it is a better
name than ensure_on_mmlist(). Maybe mm_prepare_for_swapoff()? As the mmlist
is only used for swapoff.

Re: [PATCH 02/13] mm: extract ensure_on_mmlist() helper

[David Hildenbrand (Arm)]

On 5/13/26 19:21, Usama Arif wrote:
> 
> 
> On 13/05/2026 14:32, David Hildenbrand (Arm) wrote:
>> On 4/27/26 12:01, Usama Arif wrote:
>>> When a swap entry is installed in a page table, the mm must be added
>>> to init_mm.mmlist so that swapoff can find and unuse its swap entries.
>>> This double-checked locking pattern is currently open-coded in
>>> try_to_unmap_one() and copy_nonpresent_pte().
>>>
>>> Move it into ensure_on_mmlist() in mm/internal.h and convert both
>>> callers so it can be reused by upcoming PMD-level swap entry code
>>> paths that also need to register the mm with swapoff.
>>>
>>> copy_nonpresent_pte() previously inserted into &src_mm->mmlist rather
>>> than &init_mm.mmlist, but the insertion point is irrelevant, mmlist
>>> is a circular list and swapoff walks it entirely from init_mm.mmlist,
>>> so only membership matters, not position.
>>>
>>> Signed-off-by: Usama Arif <usama.arif@linux.dev>
>>> ---
>>>  mm/internal.h | 13 +++++++++++++
>>>  mm/memory.c   |  9 +--------
>>>  mm/rmap.c     |  7 +------
>>>  3 files changed, 15 insertions(+), 14 deletions(-)
>>>
>>> diff --git a/mm/internal.h b/mm/internal.h
>>> index 5a2ddcf68e0b..7de489689f54 100644
>>> --- a/mm/internal.h
>>> +++ b/mm/internal.h
>>> @@ -1952,4 +1952,17 @@ static inline int get_sysctl_max_map_count(void)
>>>  bool may_expand_vm(struct mm_struct *mm, const vma_flags_t *vma_flags,
>>>  		   unsigned long npages);
>>>  
>>> +/*
>>> + * Ensure @mm is on the init_mm.mmlist so swapoff can find it.
>>> + */
>>> +static inline void ensure_on_mmlist(struct mm_struct *mm)
>>> +{
>>> +	if (list_empty(&mm->mmlist)) {
>>> +		spin_lock(&mmlist_lock);
>>> +		if (list_empty(&mm->mmlist))
>>> +			list_add(&mm->mmlist, &init_mm.mmlist);
>>> +		spin_unlock(&mmlist_lock);
>>> +	}
>>> +}
>>
>> Instead of talking about the low level detail ("add to mmlist"), maybe we could
>> just talk about the high-level goal: make sure that the MM can hold swap entries.
>>
>>
>> mm_prepare_for_swap()
>>
>> or sth like that?
>>
> 
> Thanks for the review!

Planning on doing more ...

> 
> Ah so basically rename the function to mm_prepare_for_swap(). I felt like it
> makes the function sound more important than it is? But it is a better
> name than ensure_on_mmlist(). Maybe mm_prepare_for_swapoff()? As the mmlist
> is only used for swapoff.

Maybe the clearest thing is mm_prepare_for_swap_entries(), because that's what
we're about to establish in these code paths: swap entries.

I wouldn't mention swapoff, but both are better than ensure_on_mmlist ;)

-- 
Cheers,

David

Re: [PATCH 03/13] fs/proc: use softleaf_has_pfn() in pagemap PMD walker

[David Hildenbrand (Arm)]

On 4/27/26 12:01, Usama Arif wrote:
> pagemap_pmd_range_thp() assumes that every non-present PMD is a
> migration entry and unconditionally calls softleaf_to_page().  This
> will crash on any non-present PMD type that does not encode a PFN,
> such as the upcoming PMD-level swap entries.
> 
> Guard the page lookup with softleaf_has_pfn(), matching how
> pte_to_pagemap_entry() already handles non-present PTEs.
> 
> Signed-off-by: Usama Arif <usama.arif@linux.dev>
> ---

Yes, just like pte_to_pagemap_entry() does

Acked-by: David Hildenbrand (Arm) <david@kernel.org>

-- 
Cheers,

David

Re: [PATCH 01/13] mm: add softleaf_to_pmd() and convert existing callers

[David Hildenbrand (Arm)]

On 4/27/26 12:01, Usama Arif wrote:
> Add softleaf_to_pmd() as the PMD counterpart to softleaf_to_pte(),
> completing the symmetry of the softleaf abstraction for page table
> leaf entries.
> 
> The upcoming PMD swap entry support needs to construct PMD entries
> from swap entries. Converting existing swp_entry_to_pmd() callers
> to softleaf_to_pmd() in a prep patch keeps the feature patches
> focused on new functionality rather than mixing refactoring with
> new code.
> 
> Signed-off-by: Usama Arif <usama.arif@linux.dev>
> ---
>  include/linux/leafops.h | 20 ++++++++++++++++++++
>  mm/huge_memory.c        | 12 ++++++------
>  2 files changed, 26 insertions(+), 6 deletions(-)
> 
> diff --git a/include/linux/leafops.h b/include/linux/leafops.h
> index 992cd8bd8ed0..803d312437df 100644
> --- a/include/linux/leafops.h
> +++ b/include/linux/leafops.h
> @@ -108,6 +108,21 @@ static inline softleaf_t softleaf_from_pmd(pmd_t pmd)
>  	return swp_entry(__swp_type(arch_entry), __swp_offset(arch_entry));
>  }
>  
> +/**
> + * softleaf_to_pmd() - Obtain a PMD entry from a leaf entry.
> + * @entry: Leaf entry.
> + *
> + * This generates an architecture-specific PMD entry that can be utilised to
> + * encode the metadata the leaf entry encodes.
> + *
> + * Returns: Architecture-specific PMD entry encoding leaf entry.
> + */
> +static inline pmd_t softleaf_to_pmd(softleaf_t entry)
> +{
> +	/* Temporary until swp_entry_t eliminated. */
> +	return swp_entry_to_pmd(entry);
> +}
> +

Yeah, just what we do for ptes.

Acked-by: David Hildenbrand (Arm) <david@kernel.org>

-- 
Cheers,

David

Re: [PATCH 04/13] mm/huge_memory: move softleaf_to_folio() inside migration branch

[David Hildenbrand (Arm)]

On 4/27/26 12:01, Usama Arif wrote:
> change_non_present_huge_pmd() calls softleaf_to_folio() unconditionally
> at the top of the function.  softleaf_to_folio() extracts a PFN from
> the entry and converts it to a folio pointer, which is only meaningful
> for migration and device_private entries that encode a real PFN.
> 
> A swap entry encodes a swap offset instead, so softleaf_to_folio()
> would produce a bogus pointer and crash on mprotect() when a PMD swap
> entry is present.
> 
> Move the call into the migration_write branch where the folio is
> actually used, so the function is safe for any non-present PMD type.
> 
> Signed-off-by: Usama Arif <usama.arif@linux.dev>
> ---
>  mm/huge_memory.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 49da0746b8ca..d82a19b5e276 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -2619,11 +2619,12 @@ static void change_non_present_huge_pmd(struct mm_struct *mm,
>  		bool uffd_wp_resolve)
>  {
>  	softleaf_t entry = softleaf_from_pmd(*pmd);
> -	const struct folio *folio = softleaf_to_folio(entry);
>  	pmd_t newpmd;
>  
>  	VM_WARN_ON(!pmd_is_valid_softleaf(*pmd));
>  	if (softleaf_is_migration_write(entry)) {
> +		const struct folio *folio = softleaf_to_folio(entry);
> +
>  		/*
>  		 * A protection check is difficult so
>  		 * just be safe and disable write

Right, and the folio is only required on that path

Acked-by: David Hildenbrand (Arm) <david@kernel.org>

-- 
Cheers,

David