[PA] Make De/RecommitSystemPages consistent between Fuchsia and POSIX

De/RecommitSystemPages() have similar properties under POSIX and
Fuchsia, since both use over-commit, but the API implementations differ
in their optimizations. Update the Fuchsia implementations with the same
optimizations as provided for POSIX.

Bug: 766882
Change-Id: I34b98db27ed85543e53b3231ec118d0a2409b454
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2558122
Commit-Queue: Bartek Nowierski <bartekn@chromium.org>
Reviewed-by: Chris Palmer <palmer@chromium.org>
Reviewed-by: Wez <wez@chromium.org>
Auto-Submit: Bartek Nowierski <bartekn@chromium.org>
Cr-Commit-Position: refs/heads/master@{#834172}

base/allocator/partition_allocator/page_allocator.h

@@ -31,7 +31,8 @@ enum PageAccessibilityDisposition {
   // Enforces permission update (Decommit will set to PageInaccessible;
   // Recommit will set to whatever was requested, other than PageInaccessible).
   PageUpdatePermissions,
-  // Will not update permissions, if the platform supports that (POSIX only).
+  // Will not update permissions, if the platform supports that (POSIX & Fuchsia
+  // only).
   PageKeepPermissionsIfPossible,
 };
 
@@ -105,8 +106,8 @@ BASE_EXPORT void SetSystemPagesAccess(
 //
 // |accessibility_disposition| allows to specify whether the pages should be
 // made inaccessible (PageUpdatePermissions), or left as is
-// (PageKeepPermissionsIfPossible, POSIX only). The latter should only be used
-// as an optimization if you really know what you're doing.
+// (PageKeepPermissionsIfPossible, POSIX & Fuchsia only). The latter should only
+// be used as an optimization if you really know what you're doing.
 // TODO(bartekn): Ideally, all callers should use PageUpdatePermissions,
 // for better security, but that may lead to a perf regression. Tracked at
 // http://crbug.com/766882.
@@ -122,7 +123,7 @@ BASE_EXPORT void SetSystemPagesAccess(
 //
 // Note: "Committed memory" is a Windows Memory Subsystem concept that ensures
 // processes will not fault when touching a committed memory region. There is
-// no analogue in the POSIX memory API where virtual memory pages are
+// no analogue in the POSIX & Fuchsia memory API where virtual memory pages are
 // best-effort allocated resources on the first touch. If PageUpdatePermissions
 // disposition is used, this API behaves in a platform-agnostic way by
 // simulating the Windows "decommit" state by both discarding the region
@@ -142,8 +143,8 @@ BASE_EXPORT void DecommitSystemPages(
 //
 // |accessibility_disposition| allows to specify whether the page permissions
 // should be set to |page_accessibility| (PageUpdatePermissions), or left as is
-// (PageKeepPermissionsIfPossible, POSIX only). The latter can only be used if
-// the pages were previously accessible and decommitted with
+// (PageKeepPermissionsIfPossible, POSIX & Fuchsia only). The latter can only be
+// used if the pages were previously accessible and decommitted with
 // PageKeepPermissionsIfPossible. It is ok, however, to recommit with
 // PageUpdatePermissions even if pages were decommitted with
 // PageKeepPermissionsIfPossible (merely losing an optimization).

base/allocator/partition_allocator/page_allocator_internals_fuchsia.h

@@ -184,10 +184,9 @@ void DecommitSystemPagesInternal(
     void* address,
     size_t length,
     PageAccessibilityDisposition accessibility_disposition) {
-  // TODO(bartekn): Ignoring accessibility_disposition preserves the behavior
-  // the API had since its conception, but consider similar optimization to
-  // POSIX.
-  SetSystemPagesAccessInternal(address, length, PageInaccessible);
+  if (accessibility_disposition == PageUpdatePermissions) {
+    SetSystemPagesAccess(address, length, PageInaccessible);
+  }
 
   // TODO(https://crbug.com/1022062): Review whether this implementation is
   // still appropriate once DiscardSystemPagesInternal() migrates to a "lazy"
@@ -200,10 +199,12 @@ void RecommitSystemPagesInternal(
     size_t length,
     PageAccessibilityConfiguration accessibility,
     PageAccessibilityDisposition accessibility_disposition) {
-  // TODO(bartekn): Ignoring accessibility_disposition preserves the behavior
-  // the API had since its conception, but consider similar optimization to
-  // POSIX.
-  SetSystemPagesAccessInternal(address, length, accessibility);
+  // On Fuchsia systems, the caller needs to simply read the memory to recommit
+  // it. However, if decommit changed the permissions, recommit has to change
+  // them back.
+  if (accessibility_disposition == PageUpdatePermissions) {
+    SetSystemPagesAccess(address, length, accessibility);
+  }
 }
 
 }  // namespace base

base/allocator/partition_allocator/page_allocator_internals_posix.h

@@ -268,11 +268,9 @@ void RecommitSystemPagesInternal(
     size_t length,
     PageAccessibilityConfiguration accessibility,
     PageAccessibilityDisposition accessibility_disposition) {
-  // On POSIX systems, the caller need simply read the memory to recommit it.
-  // This has the correct behavior because the API requires the permissions to
-  // be the same as before decommitting and all configurations can read.
-  // However, if decommit changed the permissions, recommit has to change them
-  // back.
+  // On POSIX systems, the caller needs to simply read the memory to recommit
+  // it. However, if decommit changed the permissions, recommit has to change
+  // them back.
   if (accessibility_disposition == PageUpdatePermissions) {
     SetSystemPagesAccess(address, length, accessibility);
   }