Suggested fix for JDK-4724038 (Add unmap method to MappedByteBuffer)

David Holmes david.holmes at
Fri Sep 11 06:15:40 UTC 2015

On 11/09/2015 12:56 AM, Vitaly Davidovich wrote:
>> Sure, if you like, but that's a detail.  We are arguing about what
>> colour to paint the bike shed.  :)
> The reason it may be more than just a bikeshed is because if this requires
> more cleverness in the segfault handler, then it's yet more work to make
> this happen.  I agree it's an implementation detail, but I don't think it's
> trivial (unless someone comes along and says that modifying the trap
> handler to account for this type of scenario is trivial).  One would also
> need to modify the safepoint code in the VM to allow skipping all the
> housekeeping it does typically, but I suspect this part is fairly easy.

Not sure how the safepoint code is supposed to know this particular 
safepoint is "special".


> At any rate, this is probably the best that can be done if we don't want to
> expose the unsafe method.  Part of me wonders whether this attempt to
> preserve JVM integrity is really worth the cost; there're already ways to
> bring down the JVM (e.g. JNI, existing Unsafe usage, etc), and so putting a
> safety perimeter around this piece seems uninteresting (especially given
> that people today are already using unsafe hacks for this).  I do think,
> however, that avoiding safepoints on systems that allow atomic remapping
> would be needed, although that increases the complexity of the solution
> somewhat.
> On Thu, Sep 10, 2015 at 10:45 AM, Andrew Haley <aph at> wrote:
>> On 09/10/2015 03:35 PM, Vitaly Davidovich wrote:
>>> Well, you'd probably want something other than NPE here -- perhaps a
>>> new dedicated exception to signal this condition.
>> Sure, if you like, but that's a detail.  We are arguing about what
>> colour to paint the bike shed.  :)
>>> And this means the segfault handling now needs to know about this
>>> type of situation as well, rather than just NPEs.
>> Maybe, but perhaps not.  The access to the guard page might just
>> trigger the usual segfault path, which (given an oop map) looks just
>> like a null pointer exception.
>> Andrew.

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