Re: [v2 11/16] mm: handle PMD swap entries in non-present PMD walkers

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

On 12/06/2026 07:45, Lance Yang wrote:
> +Cc linux-mm
> 
> Please Cc linux-mm next time. Pretty clearly MM work ...

Yes, thanks for this! I forgot, will be careful in v3.

> 
> On Tue, Jun 02, 2026 at 07:24:19AM -0700, Usama Arif wrote:
> [...]
>> diff --git a/mm/mincore.c b/mm/mincore.c
>> index e5d13eea9234..3fee8a7b9d9d 100644
>> --- a/mm/mincore.c
>> +++ b/mm/mincore.c
>> @@ -172,7 +172,19 @@ static int mincore_pte_range(pmd_t *pmd, unsigned long addr, unsigned long end,
>>
>> 	ptl = pmd_trans_huge_lock(pmd, vma);
>> 	if (ptl) {
>> -		memset(vec, 1, nr);
>> +		if (pmd_present(*pmd)) {
>> +			memset(vec, 1, nr);
>> +		} else {
>> +			/*
>> +			 * Non-present PMD: migration, device-private, or PMD
>> +			 * swap entry. Route through mincore_swap() the same way
>> +			 * the PTE path does -- the swap entry covers all 512
>> +			 * slots, so the whole vec gets the same answer.
>> +			 */
>> +			softleaf_t entry = softleaf_from_pmd(*pmd);
>> +
>> +			memset(vec, mincore_swap(entry, false), nr);
> 
> Looks buggy ...
> 
> That assumes one swap-cache lookup is enough for whole PMD-sized range.
> I don't think that always holds ...
> 
> See do_huge_pmd_swap_page():
> 
> ---8<---
> 	folio = swap_cache_get_folio(swp_entry);
> [...]
> 	/*
> 	 * Folio should be PMD-sized; if not (e.g. split in swap cache),
> 	 * split the PMD swap entry and retry at PTE level.
> 	 */
> 	if (folio_nr_pages(folio) != HPAGE_PMD_NR) {
> 		folio_unlock(folio);
> 		folio_put(folio);
> 		goto split_fallback;
> 	}
> ---
> 
> it handles the case where swap_cache_get_folio() returns a folio that
> is no longer PMD-sized. E.g. because it was split in the swap cache
> while the PMD swap entry was installed. Then it split the PMD swap entry
> and retries at PTE level :)
> 
> unuse_pmd_entry() has the same fallback. Can mincore hit that case?
> 
> Maybe the comment right above should say something like:
> 
> "
> One lookup is enough for a PMD-sized swapcache folio. If the swapcache
> was split, check the per-page swap slots.
> "
> 
> Hopefully, I'm not missing something here :D
> 
> Cheers, Lance

Good catch! Thanks for pointing this out.

I think the below diff over this commit should be ok. I will add
it to the next revision. Its slower, but it shouldn't be an issue
as its just mincore:


diff --git a/mm/mincore.c b/mm/mincore.c
index 3fee8a7b9d9d..975513fff336 100644
--- a/mm/mincore.c
+++ b/mm/mincore.c
@@ -175,15 +175,42 @@ static int mincore_pte_range(pmd_t *pmd, unsigned long addr, unsigned long end,
                if (pmd_present(*pmd)) {
                        memset(vec, 1, nr);
                } else {
-                       /*
-                        * Non-present PMD: migration, device-private, or PMD
-                        * swap entry. Route through mincore_swap() the same way
-                        * the PTE path does -- the swap entry covers all 512
-                        * slots, so the whole vec gets the same answer.
-                        */
                        softleaf_t entry = softleaf_from_pmd(*pmd);
 
-                       memset(vec, mincore_swap(entry, false), nr);
+                       /*
+                        * Non-present PMD: migration, device-private, or
+                        * PMD swap entry. Migration / device-private cover
+                        * the whole PMD range with a single answer.
+                        */
+                       if (!softleaf_is_swap(entry)) {
+                               memset(vec, mincore_swap(entry, false), nr);
+                       } else {
+                               struct folio *folio = swap_cache_get_folio(entry);
+
+                               /*
+                                * One lookup is enough for a PMD-sized
+                                * swapcache folio. If the swapcache was split
+                                * (e.g. by deferred_split_scan() or
+                                * memory_failure()) while the PMD swap entry
+                                * was installed, check the per-page swap slots.
+                                */
+                               if (folio && folio_nr_pages(folio) == HPAGE_PMD_NR) {
+                                       memset(vec, folio_test_uptodate(folio), nr);
+                                       folio_put(folio);
+                               } else {
+                                       unsigned long haddr = addr & HPAGE_PMD_MASK;
+                                       pgoff_t off = swp_offset(entry) +
+                                               ((addr - haddr) >> PAGE_SHIFT);
+
+                                       if (folio)
+                                               folio_put(folio);
+                                       for (i = 0; i < nr; i++)
+                                               vec[i] = mincore_swap(
+                                                       swp_entry(swp_type(entry),
+                                                                 off + i),
+                                                       false);
+                               }
+                       }
                }
                spin_unlock(ptl);
                goto out;