PartitionAlloc: add a discardSystemPages() hint API.

Source/wtf/PageAllocator.h

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 89 matching lines @@
 // after recommitting and writing to it. In particlar note that system pages are// not guaranteed to be zero-filled upon re-commit.
 // len must be a multiple of kSystemPageSize bytes.
 WTF_EXPORT void decommitSystemPages(void* addr, size_t len);
 
 // Recommit one or more system pages. Decommitted system pages must be
 // recommitted before they are read are written again.
 // Note that this operation may be a no-op on some platforms.
 // len must be a multiple of kSystemPageSize bytes.
 WTF_EXPORT void recommitSystemPages(void* addr, size_t len);
 
+// Discard one or more system pages. Discarding is a hint to the system that
+// the page is no longer required. The hint may:
+// - Do nothing.
+// - Discard the page immediately, freeing up physical pages.
+// - Discard the page at some time in the future in response to memory pressure.
+// Only committed pages should be discarded. Discarding a page does not
+// decommit it, and it is valid to discard an already-discarded page.

[haraken, 2015/06/18 18:15:40]

I'm a bit confused with "Discarding a page does not decommit it" -- it looks
like you're calling decommitSystemPages in discardSystemPages.

+// A read or write to a discarded page will not fault.

[haraken, 2015/06/18 18:15:40]

Given that you call decommitSystemPages, I guess a read or write to the page can
fault.

+// Reading from a discarded page may return the original page content, or a
+// page full of zeroes.
+// Writing to a discarded page is the only guaranteed way to undo the effect of

[haraken, 2015/06/18 18:15:40]

"undo the effect of discarding it" might be a bit misleading. It's possible that
the page is already discarded and then it can return a page of zeros.

+// discarding it. Once written to, the content of the page is guaranteed stable
+// again. After being written to, the page content may be based on the original
+// page content, or a page of zeroes.
+// len must be a multiple of kSystemPageSize bytes.
+WTF_EXPORT void discardSystemPages(void* addr, size_t len);
+
 } // namespace WTF
 
 #endif // WTF_PageAllocator_h