Re: [PATCH v5 10/11] mm: install PMD swap entries on swap-out

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

On 23/07/2026 20:16, Matthew Wilcox wrote:
> On Wed, Jul 22, 2026 at 08:19:41AM -0700, Usama Arif wrote:
>> +++ b/mm/rmap.c
>> @@ -2282,6 +2282,25 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
>>  				goto walk_abort;
>>  			}
>>  
>> +#ifdef CONFIG_THP_SWAP
>> +			/*
>> +			 * If the folio is in the swap cache and we're not
>> +			 * asked to split, install a PMD-level swap entry.
>> +			 */
>> +			if (!(flags & TTU_SPLIT_HUGE_PMD) &&
>> +			    folio_test_anon(folio) &&
>> +			    folio_test_swapcache(folio)) {
>> +				if (set_pmd_swap_entry(&pvmw, folio))
>> +					goto walk_abort;
>> +
>> +				add_mm_counter(mm, MM_ANONPAGES,
>> +					       -HPAGE_PMD_NR);
>> +				add_mm_counter(mm, MM_SWAPENTS,
>> +					       HPAGE_PMD_NR);
>> +				goto walk_done;
>> +			}
>> +#endif
>> +
>>  			if (flags & TTU_SPLIT_HUGE_PMD) {
>>  				/*
>>  				 * We temporarily have to drop the PTL and
> 
> This makes me sad.  It feels like we're bolting more complexity onto
> try_to_unmap() instead of removing TTU_SPLIT_HUGE_PMD entirely.
> 
> Ideally we'd just cope with folios of whatever size (including
> PMD and PUD mapped folios) and only handle hugetlb weirdness when we
> absolutely have to.  Similarly, we could split PUDs to PMDs and
> PMDs to PTEs when we need to, without the caller specifying
> TTU_SPLIT_anything.
> 
> Is there a reason we can't do that?

I will prototype what the code would look like without TTU_SPLIT_HUGE_PMD
and get back to you on this. Looking at the code right now, I feel like I
would be shifting complexity, but let me try and do something about this.