[PATCH v8 18/30] mm/hmm: fault PMD swap entries on demand

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

hmm_vma_handle_absent_pmd() reports anything non-present that is not
device-private as HMM_PFN_ERROR, so a swapped-out THP would look like a
permanent failure rather than memory that needs paging in.

Treat it like a PTE swap entry: record the fault when one was asked for,
and otherwise report the range as non-resident with no flags, which is what
hmm_vma_handle_pte() already does in snapshot mode.

Signed-off-by: Usama Arif <usama.arif@linux.dev>
---
 mm/hmm.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/mm/hmm.c b/mm/hmm.c
index 2f1e98c6b6440..95575ac378888 100644
--- a/mm/hmm.c
+++ b/mm/hmm.c
@@ -377,12 +377,21 @@ static int hmm_vma_handle_absent_pmd(struct mm_walk *walk, unsigned long start,
 	required_fault = hmm_range_need_fault(hmm_vma_walk, hmm_pfns,
 					      npages, 0);
 	if (required_fault) {
-		if (softleaf_is_device_private(entry))
+		if (softleaf_is_device_private(entry) ||
+		    softleaf_is_swap(entry))
 			return hmm_record_fault(addr, end, required_fault, walk);
 		else
 			return -EFAULT;
 	}
 
+	/*
+	 * A swapped-out THP is not resident. Report it as not-valid,
+	 * matching what hmm_vma_handle_pte() does for a PTE swap entry when
+	 * no fault was requested.
+	 */
+	if (softleaf_is_swap(entry))
+		return hmm_pfns_fill(start, end, range, 0);
+
 	return hmm_pfns_fill(start, end, range, HMM_PFN_ERROR);
 }
 #else
-- 
2.53.0-Meta