RFR: 8264827: Large mapped buffer/segment crash the VM when calling isLoaded

[Chris Hegarty]

On Wed, 7 Apr 2021 15:45:30 GMT, Chris Hegarty <chegar at openjdk.org> wrote:

> Avoid overflow when calculating the number of pages for large mapped segment sizes.

Note for reviewers on the test. A 3GB file is sufficient to demonstrate the issue in the old code, as follows (with a 4K page size, since the narrowing cast of size is the significant issue):

jshell> int pageSize() { return 4 * 1024; }
|  created method pageSize()

jshell> int pageCount(long size) { return (int)(size + (long)pageSize() - 1L) / pageSize(); }
|  created method pageCount(long)

jshell> pageCount(3L * 1024L * 1024L * 1024L)
$3 ==> -262143

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PR: https://git.openjdk.java.net/jdk/pull/3378