KVM: s390: vsie: stop messing with page refcount

Let's stop messing with the page refcount, and use a flag that is set /
cleared atomically to remember whether a vsie page is currently in use.

Note that we could use a page flag, or a lower bit of the scb_gpa. Let's
keep it simple for now, we have sufficient space.

While at it, stop passing "struct kvm *" to put_vsie_page(), it's
unused.

Signed-off-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Reviewed-by: Christoph Schlameuss <schlameuss@linux.ibm.com>
Tested-by: Christoph Schlameuss <schlameuss@linux.ibm.com>
Message-ID: <20250107154344.1003072-4-david@redhat.com>
Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>

diff --git a/arch/s390/kvm/vsie.c b/arch/s390/kvm/vsie.c
index 3874a1b49dd54cb906910ae56c33ab6d32637274..424f80f5f6b2d0825943ea4900b9ca6e54f2bca9 100644 (file)
--- a/arch/s390/kvm/vsie.c
+++ b/arch/s390/kvm/vsie.c
@@ -23,6 +23,10 @@
 #include "kvm-s390.h"
 #include "gaccess.h"
 
+enum vsie_page_flags {
+       VSIE_PAGE_IN_USE = 0,
+};
+
 struct vsie_page {
        struct kvm_s390_sie_block scb_s;        /* 0x0000 */
        /*
@@ -52,7 +56,12 @@ struct vsie_page {
         * radix tree.
         */
        gpa_t scb_gpa;                          /* 0x0258 */
-       __u8 reserved[0x0700 - 0x0260];         /* 0x0260 */
+       /*
+        * Flags: must be set/cleared atomically after the vsie page can be
+        * looked up by other CPUs.
+        */
+       unsigned long flags;                    /* 0x0260 */
+       __u8 reserved[0x0700 - 0x0268];         /* 0x0268 */
        struct kvm_s390_crypto_cb crycb;        /* 0x0700 */
        __u8 fac[S390_ARCH_FAC_LIST_SIZE_BYTE]; /* 0x0800 */
 };
@@ -1351,6 +1360,20 @@ static int vsie_run(struct kvm_vcpu *vcpu, struct vsie_page *vsie_page)
        return rc;
 }
 
+/* Try getting a given vsie page, returning "true" on success. */
+static inline bool try_get_vsie_page(struct vsie_page *vsie_page)
+{
+       if (test_bit(VSIE_PAGE_IN_USE, &vsie_page->flags))
+               return false;
+       return !test_and_set_bit(VSIE_PAGE_IN_USE, &vsie_page->flags);
+}
+
+/* Put a vsie page acquired through get_vsie_page / try_get_vsie_page. */
+static void put_vsie_page(struct vsie_page *vsie_page)
+{
+       clear_bit(VSIE_PAGE_IN_USE, &vsie_page->flags);
+}
+
 /*
  * Get or create a vsie page for a scb address.
  *
@@ -1369,15 +1392,15 @@ static struct vsie_page *get_vsie_page(struct kvm *kvm, unsigned long addr)
        rcu_read_unlock();
        if (page) {
                vsie_page = page_to_virt(page);
-               if (page_ref_inc_return(page) == 2) {
+               if (try_get_vsie_page(vsie_page)) {
                        if (vsie_page->scb_gpa == addr)
                                return vsie_page;
                        /*
                         * We raced with someone reusing + putting this vsie
                         * page before we grabbed it.
                         */
+                       put_vsie_page(vsie_page);
                }
-               page_ref_dec(page);
        }
 
        /*
@@ -1394,7 +1417,7 @@ static struct vsie_page *get_vsie_page(struct kvm *kvm, unsigned long addr)
                        return ERR_PTR(-ENOMEM);
                }
                vsie_page = page_to_virt(page);
-               page_ref_inc(page);
+               __set_bit(VSIE_PAGE_IN_USE, &vsie_page->flags);
                kvm->arch.vsie.pages[kvm->arch.vsie.page_count] = page;
                kvm->arch.vsie.page_count++;
        } else {
@@ -1402,9 +1425,8 @@ static struct vsie_page *get_vsie_page(struct kvm *kvm, unsigned long addr)
                while (true) {
                        page = kvm->arch.vsie.pages[kvm->arch.vsie.next];
                        vsie_page = page_to_virt(page);
-                       if (page_ref_inc_return(page) == 2)
+                       if (try_get_vsie_page(vsie_page))
                                break;
-                       page_ref_dec(page);
                        kvm->arch.vsie.next++;
                        kvm->arch.vsie.next %= nr_vcpus;
                }
@@ -1417,7 +1439,7 @@ static struct vsie_page *get_vsie_page(struct kvm *kvm, unsigned long addr)
 
        /* Double use of the same address or allocation failure. */
        if (radix_tree_insert(&kvm->arch.vsie.addr_to_page, addr >> 9, page)) {
-               page_ref_dec(page);
+               put_vsie_page(vsie_page);
                mutex_unlock(&kvm->arch.vsie.mutex);
                return NULL;
        }
@@ -1431,14 +1453,6 @@ static struct vsie_page *get_vsie_page(struct kvm *kvm, unsigned long addr)
        return vsie_page;
 }
 
-/* put a vsie page acquired via get_vsie_page */
-static void put_vsie_page(struct kvm *kvm, struct vsie_page *vsie_page)
-{
-       struct page *page = pfn_to_page(__pa(vsie_page) >> PAGE_SHIFT);
-
-       page_ref_dec(page);
-}
-
 int kvm_s390_handle_vsie(struct kvm_vcpu *vcpu)
 {
        struct vsie_page *vsie_page;
@@ -1489,7 +1503,7 @@ out_unshadow:
 out_unpin_scb:
        unpin_scb(vcpu, vsie_page, scb_addr);
 out_put:
-       put_vsie_page(vcpu->kvm, vsie_page);
+       put_vsie_page(vsie_page);
 
        return rc < 0 ? rc : 0;
 }