Re: [PATCH v6 03/12] mm: add PMD swap entry splitting support

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

On 18/08/2026 18:53, David Hildenbrand (Arm) wrote:
> On 8/18/26 15:09, Usama Arif wrote:
>> Add a swap branch in __split_huge_pmd_locked() that splits a PMD swap
>> entry into 512 PTE swap entries. No folio reference is needed because
>> swap entries point to swap slots rather than 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 1b6b0aa2baa3b..a473e85d30f51 100644
>> --- a/mm/huge_memory.c
>> +++ b/mm/huge_memory.c
>> @@ -3252,6 +3252,14 @@ 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;
> 
> It's odd to VM_WARN_ON_ONCE() and then set freeze=false;
> 
> I'd just add the comment above the VM_WARN_ON_ONCE() and drop the =false.
> 
> freeze=true really only applies during page migration, where swap entries don't
> apply.
> 

Ack

> I think it's time to clean that up ... that is
> 
> a) Expose a helper called split_pmd_to_migration_entries() that is only used by
> code that installs migration entries.
> 
> b) Hide that "freeze" flag from all other file-external functions
> 
> c) Rename the boolean to "use_migration_entries"
> 
> Then it's rather clear in this code that this should never happen.
> 


Thanks! I have done this now for next revision. The migration choice is now private as
use_migration_entries, the normal split APIs no longer expose it, and the rmap migration
path uses split_pmd_to_migration_entries().

m
> 
> 
>> +		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
>> @@ -3388,6 +3396,25 @@ 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);
> 
> No existing code uses "sl_entry". Maybe just call it "pmd_swp_entry"/"swp_entry"
> and below "pte_swp_entry".
> 

Thanks! Will do in next revision

> 
> 
> Apart from that nothing jumped at me :)
> 

Thanks for the review!