RFR (S): 8131734: Add free_archive_regions support to G1 for -Xshared:auto

Tom Benson tom.benson at oracle.com
Thu Aug 6 14:12:01 UTC 2015


Hi Thomas,

On 8/6/2015 3:48 AM, Thomas Schatzl wrote:
> Looking again at the code starting from line 1144 with the comment,
> there may still be an issue here: consider a VM A having 8M regions 1,
> 2, and 3, each spanning 8M (see figure below; each letter represents 1M
> of space), and a mapping from a VM B with 2M region size with the
> following layout that is loaded into VM A and failed.
>
>      1        2       3       region#
> AAAAAAAA AAAAAAAA AAAAAAAA   VM A regions (8M region size)
>    BB  BB BBBBBBBB BBBB       VM B mapping (2M region size)
>
> E.g. the mapping is from 2-3M, and another mapping from 6-20M. E.g. the
> ranges array contains
>
>    (2, 3), (6, 20)
>
> now, after freeing region 1 from the first mapping (2, 3), start_region
> of the second mapping (=1) equals prev_last_region (=1); now the cursor
> is advanced to the region of the end of the second mapping (=3),
> forgetting to free region 2.
>
> What am I missing here?
The cursor would not be advanced to to the end of the second mapping, 
but rather to the end of region A 1.  So the current range being freed 
would be reduced from (6,20) to (8,20).   Line 1148:

     if (start_region == prev_last_region) {
       start_address = start_region->end();     // Resets start address 
to end of A1, in your example

Tom

>
> Thanks for adding the initialization checks.
>
> Thanks,
>    Thomas
>
>
>



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