diff --git a/util/posix/scoped_mmap.cc b/util/posix/scoped_mmap.cc
index 0965fd67..fbf47fae 100644
--- a/util/posix/scoped_mmap.cc
+++ b/util/posix/scoped_mmap.cc
@@ -33,6 +33,11 @@ bool Munmap(uintptr_t addr, size_t len) {
   return true;
 }
 
+size_t RoundPage(size_t size) {
+  const size_t kPageMask = base::checked_cast<size_t>(getpagesize()) - 1;
+  return (size + kPageMask) & ~kPageMask;
+}
+
 }  // namespace
 
 namespace crashpad {
@@ -41,7 +46,7 @@ ScopedMmap::ScopedMmap() {}
 
 ScopedMmap::~ScopedMmap() {
   if (is_valid()) {
-    Munmap(reinterpret_cast<uintptr_t>(addr_), len_);
+    Munmap(reinterpret_cast<uintptr_t>(addr_), RoundPage(len_));
   }
 }
 
@@ -51,29 +56,28 @@ bool ScopedMmap::Reset() {
 
 bool ScopedMmap::ResetAddrLen(void* addr, size_t len) {
   const uintptr_t new_addr = reinterpret_cast<uintptr_t>(addr);
+  const size_t new_len_round = RoundPage(len);
 
   if (addr == MAP_FAILED) {
     DCHECK_EQ(len, 0u);
   } else {
-    // Round |len| up to the next page.
-    const size_t kPageMask = base::checked_cast<size_t>(getpagesize()) - 1;
-    len = (len + kPageMask) & ~kPageMask;
-
     DCHECK_NE(len, 0u);
     DCHECK_EQ(new_addr % getpagesize(), 0u);
-    DCHECK((base::CheckedNumeric<uintptr_t>(new_addr) + (len - 1)).IsValid());
+    DCHECK((base::CheckedNumeric<uintptr_t>(new_addr) + (new_len_round - 1))
+               .IsValid());
   }
 
   bool result = true;
 
   if (is_valid()) {
     const uintptr_t old_addr = reinterpret_cast<uintptr_t>(addr_);
+    const size_t old_len_round = RoundPage(len_);
     if (old_addr < new_addr) {
-      result &= Munmap(old_addr, std::min(len_, new_addr - old_addr));
+      result &= Munmap(old_addr, std::min(old_len_round, new_addr - old_addr));
     }
-    if (old_addr + len_ > new_addr + len) {
-      uintptr_t unmap_start = std::max(old_addr, new_addr + len);
-      result &= Munmap(unmap_start, old_addr + len_ - unmap_start);
+    if (old_addr + old_len_round > new_addr + new_len_round) {
+      uintptr_t unmap_start = std::max(old_addr, new_addr + new_len_round);
+      result &= Munmap(unmap_start, old_addr + old_len_round - unmap_start);
     }
   }
 
@@ -108,7 +112,7 @@ bool ScopedMmap::ResetMmap(void* addr,
 }
 
 bool ScopedMmap::Mprotect(int prot) {
-  if (mprotect(addr_, len_, prot) < 0) {
+  if (mprotect(addr_, RoundPage(len_), prot) < 0) {
     PLOG(ERROR) << "mprotect";
     return false;
   }
diff --git a/util/posix/scoped_mmap.h b/util/posix/scoped_mmap.h
index b0ff3dc0..9f22372b 100644
--- a/util/posix/scoped_mmap.h
+++ b/util/posix/scoped_mmap.h
@@ -91,6 +91,11 @@ class ScopedMmap {
   }
 
   //! \brief Returns the size of the memory-mapped region.
+  //!
+  //! This is the value originally passed to ResetAddrLen() or ResetMmap(), or
+  //! after Reset(), `0`. It may not be a round number of pages. Providing the
+  //! passed-in value is intended to ease tracking the intended lengths of
+  //! memory-mapped regions backed by files whose sizes are not whole pages.
   size_t len() const { return len_; }
 
  private:
diff --git a/util/posix/scoped_mmap_test.cc b/util/posix/scoped_mmap_test.cc
index e32fa159..5279fdb8 100644
--- a/util/posix/scoped_mmap_test.cc
+++ b/util/posix/scoped_mmap_test.cc
@@ -309,7 +309,7 @@ TEST(ScopedMmapDeathTest, NotIntegralNumberOfPages) {
   ASSERT_TRUE(ScopedMmapResetMmap(&mapping, kHalfPageSize));
   EXPECT_TRUE(mapping.is_valid());
   EXPECT_NE(mapping.addr(), MAP_FAILED);
-  EXPECT_EQ(mapping.len(), kPageSize);
+  EXPECT_EQ(mapping.len(), kHalfPageSize);
 
   TestCookie cookie;
   cookie.SetUp(mapping.addr_as<uint64_t*>());
@@ -319,7 +319,7 @@ TEST(ScopedMmapDeathTest, NotIntegralNumberOfPages) {
   ASSERT_TRUE(mapping.ResetAddrLen(orig_addr, kHalfPageSize));
   EXPECT_TRUE(mapping.is_valid());
   EXPECT_EQ(mapping.addr(), orig_addr);
-  EXPECT_EQ(mapping.len(), kPageSize);
+  EXPECT_EQ(mapping.len(), kHalfPageSize);
 
   EXPECT_EQ(cookie.Observed(), cookie.Expected());
 
@@ -328,14 +328,14 @@ TEST(ScopedMmapDeathTest, NotIntegralNumberOfPages) {
   ASSERT_TRUE(mapping.ResetAddrLen(orig_addr, 1));
   EXPECT_TRUE(mapping.is_valid());
   EXPECT_EQ(mapping.addr(), orig_addr);
-  EXPECT_EQ(mapping.len(), kPageSize);
+  EXPECT_EQ(mapping.len(), 1u);
 
   EXPECT_EQ(cookie.Observed(), cookie.Expected());
 
   ASSERT_TRUE(mapping.ResetAddrLen(orig_addr, kPageSize - 1));
   EXPECT_TRUE(mapping.is_valid());
   EXPECT_EQ(mapping.addr(), orig_addr);
-  EXPECT_EQ(mapping.len(), kPageSize);
+  EXPECT_EQ(mapping.len(), kPageSize - 1);
 
   EXPECT_EQ(cookie.Observed(), cookie.Expected());
 
@@ -356,7 +356,7 @@ TEST(ScopedMmapDeathTest, NotIntegralNumberOfPages) {
   ASSERT_TRUE(mapping.ResetAddrLen(orig_addr, kHalfPageSize));
   EXPECT_TRUE(mapping.is_valid());
   EXPECT_EQ(mapping.addr(), orig_addr);
-  EXPECT_EQ(mapping.len(), kPageSize);
+  EXPECT_EQ(mapping.len(), kHalfPageSize);
 
   EXPECT_EQ(two_cookies[0].Observed(), two_cookies[0].Expected());
   EXPECT_DEATH_CRASH(two_cookies[1].Check(), "");
@@ -376,7 +376,7 @@ TEST(ScopedMmapDeathTest, NotIntegralNumberOfPages) {
   ASSERT_TRUE(mapping.ResetAddrLen(orig_addr, kPageSize + kHalfPageSize));
   EXPECT_TRUE(mapping.is_valid());
   EXPECT_EQ(mapping.addr(), orig_addr);
-  EXPECT_EQ(mapping.len(), 2 * kPageSize);
+  EXPECT_EQ(mapping.len(), kPageSize + kHalfPageSize);
 
   EXPECT_EQ(two_cookies[0].Observed(), two_cookies[0].Expected());
   EXPECT_EQ(two_cookies[1].Observed(), two_cookies[1].Expected());