[Fuchsia] Implement de-commit and discard for PageAllocator.

Some allocators (e.g. the Blink heap) call DecommitSystemPages() to
return pages in their free memory pools to the system, or
DiscardSystemPages() to make the pages available to the kernel
without necessarily de-committing them, rather than
actually freeing them.

At present Fuchsia does not implement page swapping, nor does it
provide a "mark discardable" API, so we implement both APIs using
vmar_op_range(DECOMMIT) for now.

Bug: 927411, 1022062
Change-Id: Icc61e7b46ebdc4d3fa5308e1b318a7685a6ceb19
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1892295Reviewed-by: Albert J. Wong <ajwong@chromium.org>
Reviewed-by: Chris Palmer <palmer@chromium.org>
Commit-Queue: Wez <wez@chromium.org>
Auto-Submit: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#713238}

base/allocator/partition_allocator/page_allocator_internals_fuchsia.h

@@ -169,8 +169,21 @@ void FreePagesInternal(void* address, size_t length) {
   ZX_CHECK(status == ZX_OK, status);
 }
 
+void DiscardSystemPagesInternal(void* address, size_t length) {
+  // TODO(https://crbug.com/1022062): Mark pages as discardable, rather than
+  // forcibly de-committing them immediately, when Fuchsia supports it.
+  uint64_t address_int = reinterpret_cast<uint64_t>(address);
+  zx_status_t status = zx::vmar::root_self()->op_range(
+      ZX_VMO_OP_DECOMMIT, address_int, length, nullptr, 0);
+  ZX_CHECK(status == ZX_OK, status);
+}
+
 void DecommitSystemPagesInternal(void* address, size_t length) {
-  DiscardSystemPages(address, length);
+  // TODO(https://crbug.com/1022062): Review whether this implementation is
+  // still appropriate once DiscardSystemPagesInternal() migrates to a "lazy"
+  // discardable API.
+  DiscardSystemPagesInternal(address, length);
+
   SetSystemPagesAccessInternal(address, length, PageInaccessible);
 }
 
@@ -181,10 +194,6 @@ bool RecommitSystemPagesInternal(void* address,
   return true;
 }
 
-void DiscardSystemPagesInternal(void* address, size_t length) {
-  NOTIMPLEMENTED_LOG_ONCE();
-}
-
 }  // namespace base
 
 #endif  // BASE_ALLOCATOR_PARTITION_ALLOCATOR_PAGE_ALLOCATOR_INTERNALS_FUCHSIA_H_