Re: [PATCH v4] mm: Add MREMAP_DONTUNMAP to mremap().

[Brian Geffon]

Thank you Andrew. I'll get working on some self-tests.

Brian

On Sun, Feb 9, 2020 at 5:21 PM Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> On Fri,  7 Feb 2020 12:18:56 -0800 Brian Geffon <bgeffon@xxxxxxxxxx> wrote:
>
> > When remapping an anonymous, private mapping, if MREMAP_DONTUNMAP is
> > set, the source mapping will not be removed. Instead it will be
> > cleared as if a brand new anonymous, private mapping had been created
> > atomically as part of the mremap() call.  If a userfaultfd was watching
> > the source, it will continue to watch the new mapping.  For a mapping
> > that is shared or not anonymous, MREMAP_DONTUNMAP will cause the
> > mremap() call to fail. Because MREMAP_DONTUNMAP always results in moving
> > a VMA you MUST use the MREMAP_MAYMOVE flag. The final result is two
> > equally sized VMAs where the destination contains the PTEs of the source.
> >
> > We hope to use this in Chrome OS where with userfaultfd we could write
> > an anonymous mapping to disk without having to STOP the process or worry
> > about VMA permission changes.
> >
> > This feature also has a use case in Android, Lokesh Gidra has said
> > that "As part of using userfaultfd for GC, We'll have to move the physical
> > pages of the java heap to a separate location. For this purpose mremap
> > will be used. Without the MREMAP_DONTUNMAP flag, when I mremap the java
> > heap, its virtual mapping will be removed as well. Therefore, we'll
> > require performing mmap immediately after. This is not only time consuming
> > but also opens a time window where a native thread may call mmap and
> > reserve the java heap's address range for its own usage. This flag
> > solves the problem."
>
> This seems useful and reasonably mature, so I'll queue it for
> additional testing and shall await review feedback.
>
> Could we please get some self-test code for this feature in
> tools/testing/selftests/vm?  Perhaps in userfaultfd.c?
>