Re: [PATCH v5 02/11] mm: add PMD swap entry splitting support

[Usama Arif <usama.arif@linux.dev>]

On 24/07/2026 07:52, Dev Jain wrote:
> 
> 
> On 22/07/26 8:49 pm, Usama Arif wrote:
>> Add a swap branch in __split_huge_pmd_locked() that splits a PMD swap
>> entry into 512 PTE swap entries.  Unlike migration splits, no folio
>> reference is needed because swap entries point to swap slots, not
> 
> Confusing wording. swap entries need reference of swap slots instead
> of folios. I would drop the mention of migration here.

I added about migration because its the other if part of the if else and
was a good comparison. Will remove it. Thanks!


> 
>> pages.  Each PTE inherits the correct sub-slot offset and preserves
>> soft_dirty, uffd_wp, and exclusive flags.
>>
>> The folio_remove_rmap_pmd() gate at the end must inspect old_pmd
>> rather than *pmd: for a present THP split, *pmd has already been
>> cleared by pmdp_invalidate(), and that invalidated bit pattern can
>> decode as a plausible swap entry.
>>
>> This branch is reached from the explicit __split_huge_pmd() callers
>> that hit a non-present PMD: partial-range mprotect / munmap, the
>> wp_huge_pmd() PMD-COW fallback, and the swap-in / swapoff fallbacks
>> added in later patches when the cached folio is no longer PMD-sized.
>> page_vma_mapped_walk() does not iterate PMD swap entries, so
>> try_to_unmap_one() and try_to_migrate_one() do not reach this branch
>> and freeze=true cannot occur in this branch today.  page and folio
>> are therefore left uninitialized in the swap branch; a
>> VM_WARN_ON_ONCE(freeze) catches any future caller that breaks this
>> invariant before the freeze path dereferences page_to_pfn(page + i)
>> or put_page(page).
>>
>> Signed-off-by: Usama Arif <usama.arif@linux.dev>
>> ---
>>  mm/huge_memory.c | 29 ++++++++++++++++++++++++++++-
>>  1 file changed, 28 insertions(+), 1 deletion(-)
>>
>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>> index 04e8a6b55343..9819c0ae228a 100644
>> --- a/mm/huge_memory.c
>> +++ b/mm/huge_memory.c
>> @@ -3210,6 +3210,14 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
>>  			folio_add_anon_rmap_ptes(folio, page, HPAGE_PMD_NR,
>>  						 vma, haddr, rmap_flags);
>>  		}
>> +	} else if (pmd_is_swap_entry(*pmd)) {
>> +		VM_WARN_ON_ONCE(freeze);
>> +		/* Swap entries have no page for the migration freeze path. */
>> +		freeze = false;
>> +		old_pmd = *pmd;
>> +		soft_dirty = pmd_swp_soft_dirty(old_pmd);
>> +		uffd_wp = pmd_swp_uffd(old_pmd);
>> +		anon_exclusive = pmd_swp_exclusive(old_pmd);
>>  	} else {
>>  		/*
>>  		 * Up to this point the pmd is present and huge and userland has
>> @@ -3346,6 +3354,25 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
>>  			VM_WARN_ON(!pte_none(ptep_get(pte + i)));
>>  			set_pte_at(mm, addr, pte + i, entry);
>>  		}
>> +	} else if (pmd_is_swap_entry(old_pmd)) {
>> +		softleaf_t sl_entry = softleaf_from_pmd(old_pmd);
>> +		pte_t swp_pte;
>> +		swp_entry_t sub_entry;
>> +
>> +		for (i = 0, addr = haddr; i < HPAGE_PMD_NR;
>> +		     i++, addr += PAGE_SIZE) {
>> +			sub_entry = swp_entry(swp_type(sl_entry),
>> +					      swp_offset(sl_entry) + i);
>> +			swp_pte = swp_entry_to_pte(sub_entry);
>> +			if (soft_dirty)
>> +				swp_pte = pte_swp_mksoft_dirty(swp_pte);
>> +			if (uffd_wp)
>> +				swp_pte = pte_swp_mkuffd(swp_pte);
>> +			if (anon_exclusive)
>> +				swp_pte = pte_swp_mkexclusive(swp_pte);
>> +			VM_WARN_ON(!pte_none(ptep_get(pte + i)));
>> +			set_pte_at(mm, addr, pte + i, swp_pte);
>> +		}
> 
> All these for loops could later benefit from the set_softleaf_ptes() helper:
> https://lore.kernel.org/all/20260723070905.3422276-6-dev.jain@arm.com/
> 
> 
>>  	} else {
>>  		pte_t entry;
>>  
>> @@ -3373,7 +3400,7 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
>>  	}
>>  	pte_unmap(pte);
>>  
>> -	if (!pmd_is_migration_entry(*pmd))
>> +	if (!pmd_is_migration_entry(old_pmd) && !pmd_is_swap_entry(old_pmd))
>>  		folio_remove_rmap_pmd(folio, page, vma);
>>  	if (freeze)
>>  		put_page(page);
>