review request for 7142641: -Xshared:on fails on ARM

[Dean Long]

Hi Dan,

yes, I was concerned about that as well.  But MAP_SHARED isn't actually 
needed to share read-only pages.  If you trace an app's startup using 
"strace" you can see that the dynamic linker maps shared libraries such 
as libc with MAP_PRIVATE.  Multiple processes still share those pages 
through the page cache.  Furthermore, the linux mmap implementation 
turns off the shared flag if the file was opened read-only (but 
unfortunately it turns off the flag only after enforcing the cache 
coloring constraints.)

dl

On 2/28/2012 2:40 PM, Daniel D. Daugherty wrote:
> Dean,
>
> I'm confused... I thought mapping the read-only CDS archive pages as
> "shared" permits those pages to be shared by other java processes that
> are also using the CDS archive...
>
> Dan
>
> On 2/28/12 3:05 PM, Dean Long wrote:
>> http://cr.openjdk.java.net/~dlong/7142641/webrev.0/
>> Summary of changes:    3 lines changed: 0 ins; 2 del; 1 mod; 5538 unchg
>>
>> The Class Data Sharing file is mapped "shared" on linux, which places 
>> extra constraints on which virtual address a file offset can use.  
>> This is to ensure consistency on certain platforms with aliasing 
>> caches.  However as the CDS file is opened read-only, mapping it 
>> "shared" has no benefit and can cause the mmap to fail.  The simplest 
>> fix is to change the mapping to "private".
>>
>> Tested on linux arm platform with aliasing cache.  Installed CDS with 
>> -Xshare:dump then ran through some vm tests as a sanity check.
>>
>> This bug is on the 7u4 watch list.
>>
>> dl