[PATCH v3 03/10] s390/uv: split large folios in gmap_make_secure()

David Hildenbrand
2024-05-08T18:30:16Z

While s390x makes sure to never have PMD-mapped THP in processes that use
KVM -- by remapping them using PTEs in
thp_split_walk_pmd_entry()->split_huge_pmd() -- there is still the
possibility of having PTE-mapped THPs (large folios) mapped into guest
memory.

This would happen if user space allocates memory before calling
KVM_CREATE_VM (which would call s390_enable_sie()). With upstream QEMU,
this currently doesn't happen, because guest memory is setup and
conditionally preallocated after KVM_CREATE_VM.

Could it happen with shmem/file-backed memory when another process
allocated memory in the pagecache? Likely, although currently not a
common setup.

Trying to split any PTE-mapped large folios sounds like the right and
future-proof thing to do here. So let's call split_folio() and handle the
return values accordingly.

Signed-off-by: David Hildenbrand <david@redhat.com>
---
arch/s390/kernel/uv.c | 31 +++++++++++++++++++++++++------
1 file changed, 25 insertions(+), 6 deletions(-)

diff --git a/arch/s390/kernel/uv.c b/arch/s390/kernel/uv.c
index 25fe28d189df..3c6d86e3e828 100644
--- a/arch/s390/kernel/uv.c
+++ b/arch/s390/kernel/uv.c
@@ -338,11 +338,10 @@ int gmap_make_secure(struct gmap *gmap, unsigned long gaddr, void *uvcb)
goto out;
if (pte_present(*ptep) && !(pte_val(*ptep) & _PAGE_INVALID) && pte_write(*ptep)) {
folio = page_folio(pte_page(*ptep));
- rc = -EINVAL;
- if (folio_test_large(folio))
- goto unlock;
rc = -EAGAIN;
- if (folio_trylock(folio)) {
+ if (folio_test_large(folio)) {
+ rc = -E2BIG;
+ } else if (folio_trylock(folio)) {
if (should_export_before_import(uvcb, gmap->mm))
uv_convert_from_secure(PFN_PHYS(folio_pfn(folio)));
rc = make_folio_secure(folio, uvcb);
@@ -353,15 +352,35 @@ int gmap_make_secure(struct gmap *gmap, unsigned long gaddr, void *uvcb)
* Once we drop the PTL, the folio may get unmapped and
* freed immediately. We need a temporary reference.
*/
- if (rc == -EAGAIN)
+ if (rc == -EAGAIN || rc == -E2BIG)
folio_get(folio);
}
-unlock:
pte_unmap_unlock(ptep, ptelock);
out:
mmap_read_unlock(gmap->mm);

switch (rc) {
+ case -E2BIG:
+ folio_lock(folio);
+ rc = split_folio(folio);
+ folio_unlock(folio);
+ folio_put(folio);
+
+ switch (rc) {
+ case 0:
+ /* Splitting succeeded, try again immediately. */
+ goto again;
+ case -EAGAIN:
+ /* Additional folio references. */
+ if (drain_lru(&drain_lru_called))
+ goto again;
+ return -EAGAIN;
+ case -EBUSY:
+ /* Unexpected race. */
+ return -EAGAIN;
+ }
+ WARN_ON_ONCE(1);
+ return -ENXIO;
case -EAGAIN:
/*
* If we are here because the UVC returned busy or partial
--
2.45.0