Re: [PATCH v2] mremap.2: Add information for MREMAP_DONTUNMAP.

["Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>]

Hello Brian and Loresh,

>>>> +Possible applications for this behavior might be garbage collection or
>>>
>>> Can you elaborate the garbage collection use case a little, please?
>>
>> Lokesh, who is CCed, can probably expand better than I can, Lokesh
>> would you mind elaborating on how the JVM plans to use this.
>>
> There are many GC algorithms in literature which use PROT_NONE+SIGSEGV
> trick to implement concurrent compaction of java heap. In Android
> Runtime we plan to use userfaultfd instead. But this requires a
> stop-the-world, wherein Java threads are paused, right before starting
> the compaction phase. Within this pause, the physical pages in the
> Java heap will be moved to another area, so that the Java heap, which
> is already registered with userfaultfd, can start 'userfaulting' (as
> Java heap pages are missing) once application threads are resumed.
> 
> In the absence of MREMAP_DONTUNMAP, I'd have to do it by first doing
> mremap, and then mmaping Java heap, as its virtual mapping would be
> removed by the preceding mremap. This not only causes performance
> issues as two system calls need to be made instead of one, but it also
> leaves a window open for a native thread, which is not paused, to
> create a virtual mapping for its own usage right where Java heap is
> supposed to be.

Thank you both for your explanations.

I added some text to the page. Does the following look okay?

   MREMAP_DONTUNMAP use cases
       Possible applications for MREMAP_DONTUNMAP include:

       *  Non-cooperative userfaultfd(2): an application can yank  out  a
          virtual  address range using MREMAP_DONTUNMAP and then employ a
          userfaultfd(2) handler to handle the page  faults  that  subse‐
          quently  occur  as  other threads in the process touch pages in
          the yanked range.

       *  Garbage collection: MREMAP_DONTUNMAP can be used in conjunction
          with  userfaultfd(2) to implement garbage collection algorithms
          (e.g., in a Java virtual machine).  Such an implementation  can
          be  cheaper  (and simpler) than conventional garbage collection
          techniques that involve marking pages with protection PROT_NONE
          in  conjunction with the of a SIGSEGV handler to catch accesses
          to those pages.

Cheers,

Michael

-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/