[v3 07/24] mm: thp: retry on split failure in change_pmd_range()

Usama Arif
2026-03-27T02:14:59Z

change_pmd_range() splits a huge PMD when mprotect() targets a sub-PMD
range or when VMA flags require per-PTE protection bits that can't be
represented at PMD granularity.

If pte_alloc_one() fails inside __split_huge_pmd(), the huge PMD remains
intact. Without this change, change_pte_range() would return -EAGAIN
because pte_offset_map_lock() returns NULL for a huge PMD, sending the
code back to the 'again' label to retry the split—without ever calling
cond_resched().

Now that __split_huge_pmd() returns an error code, handle it explicitly:
yield the CPU with cond_resched() and retry via goto again, giving other
tasks a chance to free memory.

Trying to return an error all the way to change_protection_range would
not work as it would leave a memory range with new protections, and
others unchanged, with no easy way to roll back the already modified
entries (and previous splits). __split_huge_pmd only requires an
order-0 allocation and is extremely unlikely to fail.

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

diff --git a/mm/mprotect.c b/mm/mprotect.c
index 110d47a36d4bb..e39e96963da8b 100644
--- a/mm/mprotect.c
+++ b/mm/mprotect.c
@@ -477,7 +477,16 @@ static inline long change_pmd_range(struct mmu_gather *tlb,
if (pmd_is_huge(_pmd)) {
if ((next - addr != HPAGE_PMD_SIZE) ||
pgtable_split_needed(vma, cp_flags)) {
- __split_huge_pmd(vma, pmd, addr, false);
+ ret = __split_huge_pmd(vma, pmd, addr, false);
+ if (ret) {
+ /*
+ * Yield and retry. Other tasks
+ * may free memory while we
+ * reschedule.
+ */
+ cond_resched();
+ goto again;
+ }
/*
* For file-backed, the pmd could have been
* cleared; make sure pmd populated if
--
2.52.0