[PATCH v1 2/5] s390/uv: convert gmap_make_secure() to work on folios

[David Hildenbrand]

We have various goals that require gmap_make_secure() to only work on
folios. We want to limit the use of page_mapcount() to the places where it
is absolutely necessary, we want to avoid using page flags of tail
pages, and we want to remove page_has_private().

So, let's convert gmap_make_secure() to folios. 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() -- we might
still find PTE-mapped THPs and could end up working on tail pages of
such large folios for now.

To handle that cleanly, let's simply split any PTE-mapped large folio,
so we can be sure that we are always working with small folios and never
on tail pages.

There is no real change: splitting will similarly fail on unexpected folio
references, just like it would already when we try to freeze the folio
refcount.

Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
---
 arch/s390/include/asm/page.h |  1 +
 arch/s390/kernel/uv.c        | 66 ++++++++++++++++++++++--------------
 2 files changed, 42 insertions(+), 25 deletions(-)

diff --git a/arch/s390/include/asm/page.h b/arch/s390/include/asm/page.h
index 9381879f7ecf..54d015bcd8e3 100644
--- a/arch/s390/include/asm/page.h
+++ b/arch/s390/include/asm/page.h
@@ -215,6 +215,7 @@ static inline unsigned long __phys_addr(unsigned long x, bool is_31bit)
 
 #define phys_to_page(phys)	pfn_to_page(phys_to_pfn(phys))
 #define page_to_phys(page)	pfn_to_phys(page_to_pfn(page))
+#define folio_to_phys(page)	pfn_to_phys(folio_pfn(folio))
 
 static inline void *pfn_to_virt(unsigned long pfn)
 {
diff --git a/arch/s390/kernel/uv.c b/arch/s390/kernel/uv.c
index 7401838b960b..adcbd4b13035 100644
--- a/arch/s390/kernel/uv.c
+++ b/arch/s390/kernel/uv.c
@@ -181,36 +181,36 @@ int uv_convert_owned_from_secure(unsigned long paddr)
 }
 
 /*
- * Calculate the expected ref_count for a page that would otherwise have no
+ * Calculate the expected ref_count for a folio that would otherwise have no
  * further pins. This was cribbed from similar functions in other places in
  * the kernel, but with some slight modifications. We know that a secure
- * page can not be a huge page for example.
+ * folio can only be a small folio for example.
  */
-static int expected_page_refs(struct page *page)
+static int expected_folio_refs(struct folio *folio)
 {
 	int res;
 
-	res = page_mapcount(page);
-	if (PageSwapCache(page)) {
+	res = folio_mapcount(folio);
+	if (folio_test_swapcache(folio)) {
 		res++;
-	} else if (page_mapping(page)) {
+	} else if (folio_mapping(folio)) {
 		res++;
-		if (page_has_private(page))
+		if (folio_has_private(folio))
 			res++;
 	}
 	return res;
 }
 
-static int make_page_secure(struct page *page, struct uv_cb_header *uvcb)
+static int make_folio_secure(struct folio *folio, struct uv_cb_header *uvcb)
 {
 	int expected, cc = 0;
 
-	if (PageWriteback(page))
+	if (folio_test_writeback(folio))
 		return -EAGAIN;
-	expected = expected_page_refs(page);
-	if (!page_ref_freeze(page, expected))
+	expected = expected_folio_refs(folio);
+	if (!folio_ref_freeze(folio, expected))
 		return -EBUSY;
-	set_bit(PG_arch_1, &page->flags);
+	set_bit(PG_arch_1, &folio->flags);
 	/*
 	 * If the UVC does not succeed or fail immediately, we don't want to
 	 * loop for long, or we might get stall notifications.
@@ -220,9 +220,9 @@ static int make_page_secure(struct page *page, struct uv_cb_header *uvcb)
 	 * -EAGAIN and we let the callers deal with it.
 	 */
 	cc = __uv_call(0, (u64)uvcb);
-	page_ref_unfreeze(page, expected);
+	folio_ref_unfreeze(folio, expected);
 	/*
-	 * Return -ENXIO if the page was not mapped, -EINVAL for other errors.
+	 * Return -ENXIO if the folio was not mapped, -EINVAL for other errors.
 	 * If busy or partially completed, return -EAGAIN.
 	 */
 	if (cc == UVC_CC_OK)
@@ -277,7 +277,7 @@ int gmap_make_secure(struct gmap *gmap, unsigned long gaddr, void *uvcb)
 	bool local_drain = false;
 	spinlock_t *ptelock;
 	unsigned long uaddr;
-	struct page *page;
+	struct folio *folio;
 	pte_t *ptep;
 	int rc;
 
@@ -306,33 +306,49 @@ int gmap_make_secure(struct gmap *gmap, unsigned long gaddr, void *uvcb)
 	if (!ptep)
 		goto out;
 	if (pte_present(*ptep) && !(pte_val(*ptep) & _PAGE_INVALID) && pte_write(*ptep)) {
-		page = pte_page(*ptep);
+		folio = page_folio(pte_page(*ptep));
 		rc = -EAGAIN;
-		if (trylock_page(page)) {
+
+		/* We might get PTE-mapped large folios; split them first. */
+		if (folio_test_large(folio)) {
+			rc = -E2BIG;
+		} else if (folio_trylock(folio)) {
 			if (should_export_before_import(uvcb, gmap->mm))
-				uv_convert_from_secure(page_to_phys(page));
-			rc = make_page_secure(page, uvcb);
-			unlock_page(page);
+				uv_convert_from_secure(folio_to_phys(folio));
+			rc = make_folio_secure(folio, uvcb);
+			folio_unlock(folio);
 		}
 
 		/*
-		 * Once we drop the PTL, the page may get unmapped and
+		 * Once we drop the PTL, the folio may get unmapped and
 		 * freed immediately. We need a temporary reference.
 		 */
-		if (rc == -EAGAIN)
-			get_page(page);
+		if (rc == -EAGAIN || rc == -E2BIG)
+			folio_get(folio);
 	}
 	pte_unmap_unlock(ptep, ptelock);
 out:
 	mmap_read_unlock(gmap->mm);
 
+	if (rc == -E2BIG) {
+		/*
+		 * Splitting might fail with -EBUSY due to unexpected folio
+		 * references, just like make_folio_secure(). So handle it
+		 * ahead of time without the PTL being held.
+		 */
+		folio_lock(folio);
+		rc = split_folio(folio);
+		folio_unlock(folio);
+		folio_put(folio);
+	}
+
 	if (rc == -EAGAIN) {
 		/*
 		 * If we are here because the UVC returned busy or partial
 		 * completion, this is just a useless check, but it is safe.
 		 */
-		wait_on_page_writeback(page);
-		put_page(page);
+		folio_wait_writeback(folio);
+		folio_put(folio);
 	} else if (rc == -EBUSY) {
 		/*
 		 * If we have tried a local drain and the page refcount
-- 
2.44.0