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

[Daniel D. Daugherty]

Wow! Ya learn something new every day!

Thanks for the background info. And you can count me as
a reviewer...

Since change affects all Linuxes, you should move the
bug to jvm/hotspot/runtime_cds...

Dan

On 2/28/12 4:43 PM, Dean Long wrote:
> 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