lib: Add epoch-based SMR

mem/page.c

@@ -29,6 +29,15 @@ void page_hold(struct page* page) {
     atomic_fetch_add(&page->holds, 1, ATOMIC_RELAXED);
 }
 
+bool page_try_hold(struct page* page) {
+    size_t holds = atomic_load(&page->holds, ATOMIC_RELAXED);
+    while (holds) {
+        if (atomic_compare_exchange_weak(&page->holds, &holds, holds + 1, ATOMIC_ACQUIRE, ATOMIC_RELAXED))
+            return true;
+    }
+    return false;
+}
+
 void page_unhold(struct page* page) {
     if (atomic_fetch_sub(&page->holds, 1, ATOMIC_RELEASE) != 1)
         return;

mem/vma.c

@@ -445,7 +445,7 @@ static errno_t fault_anon(
         errno_t status = anon_new(anon->page, &copy);
         if (status)
             return status;
-        *radix_slot(&m->amap->anons, slot) = copy;
+        radix_replace(&m->amap->anons, slot, copy);
         anon_unref(anon);
         anon = copy;
         writable = true;

mem/vmo.c

@@ -1,6 +1,7 @@
 // SPDX-License-Identifier: ISC
 
 #include <zinnia/alloc.h>
+#include <zinnia/epoch.h>
 #include <zinnia/errno.h>
 #include <zinnia/print.h>
 #include <zinnia/utils.h>
@@ -143,10 +144,34 @@ static errno_t fill(struct vmo* vmo, uint64_t index, enum vmo_access access, str
     return 0;
 }
 
+static struct page* lookup_fast(struct vmo* vmo, uint64_t index) {
+    epoch_enter();
+    struct page* page = radix_lookup(&vmo->pages, index);
+    if (page && !page_try_hold(page))
+        page = nullptr;
+    const bool valid =
+        page && radix_lookup(&vmo->pages, index) == page && !(atomic_load(&page->flags, ATOMIC_ACQUIRE) & PAGE_BUSY);
+    epoch_exit();
+
+    if (page && !valid) {
+        page_unhold(page);
+        return nullptr;
+    }
+    return page;
+}
+
 errno_t vmo_get_page(struct vmo* vmo, uint64_t index, enum vmo_access access, struct page** out) {
     if (vmo->pager->lookup)
         return ENOTSUP;
 
+    if (access == VMO_READ) {
+        struct page* page = lookup_fast(vmo, index);
+        if (page) {
+            *out = page;
+            return 0;
+        }
+    }
+
     mutex_lock(&vmo->lock);
     while (true) {
         struct page* page = radix_lookup(&vmo->pages, index);