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s390/uv: Split large folios in gmap_make_secure()

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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: Claudio Imbrenda <imbrenda@linux.ibm.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Link: https://lore.kernel.org/r/20240508182955.358628-4-david@redhat.com
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>
This commit is contained in:
David Hildenbrand 2024-05-08 20:29:48 +02:00 • committed by Alexander Gordeev
parent 68ad4743be
commit eef88fe45a
1 changed files with 25 additions and 6 deletions
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31
arch/s390/kernel/uv.c
View file

@ -338,11 +338,10 @@ again:
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 @@ again:
* 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