RFR: 8272807: Permit use of memory concurrent with pretouch [v2]

[Aleksey Shipilev]

On Tue, 8 Feb 2022 08:43:40 GMT, Kim Barrett <kbarrett at openjdk.org> wrote:

>> All right, what irked me originally is that `cur == last` break assumes both `cur` and `last` are multiples of `page_size`, otherwise this loops "indefinitely" (until successive overflows help, I guess). This is true now, but it is not even asserted. Maybe it should be `cur >= last`?
>
> I assumed that having both values be initialized as page-aligned just
> before the loop made that obvious.
> 
> A slightly different way to write it might be:
> 
> void os::pretouch_memory(void* start, void* end, size_t page_size) {
>   assert(start <= end, "invalid range: " PTR_FORMAT " -> " PTR_FORMAT, p2i(start), p2i(end));
>   assert(is_power_of_2(page_size), "page size misaligned: %zu", page_size);
>   assert(page_size >= sizeof(int), "page size too small: %zu", page_size);
>   if (start < end) {
>     // We're doing concurrent-safe touch and memory state has page
>     // granularity, so we can touch anywhere in a page.  We need int
>     // alignment for the touch.
>     int* cur = static_cast<int*>(align_down(start, sizeof(int)));
>     void* last = align_down(static_cast<char*>(end) - 1, page_size);
>     assert(cur <= last, "invariant");
>     // Iterate from first page through last (inclusive), being careful to
>     // avoid overflow if the last page abuts the end of the address range.
>     for ( ; true; cur += (page_size / sizeof(int))) {
>       Atomic::add(cur, 0, memory_order_relaxed);
>       if (cur >= last) break;
>     }
>   }
> }
> 
> I don't know if you'd find that clearer.

`int* cur` does not make it cleaner. I am fine with `align_down`-ing both `cur` and `last` to `page_size`, and then using `cur >= last` for extra safety. In other words, just change `==` to `>=` in the breaking condition.

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PR: https://git.openjdk.java.net/jdk/pull/7343