Commit eef88fe4 authored by David Hildenbrand's avatar David Hildenbrand Committed by Alexander Gordeev
Browse files

s390/uv: Split large folios in gmap_make_secure() 



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.

Reviewed-by: default avatarClaudio Imbrenda <imbrenda@linux.ibm.com>
Signed-off-by: default avatarDavid Hildenbrand <david@redhat.com>
Link: https://lore.kernel.org/r/20240508182955.358628-4-david@redhat.com


Signed-off-by: default avatarHeiko Carstens <hca@linux.ibm.com>
Signed-off-by: default avatarAlexander Gordeev <agordeev@linux.ibm.com>
parent 68ad4743

arch/s390/kernel/uv.c

+25 −6
Original line number Diff line number Diff line
@@ -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