Re: [PATCH v6 1/2] mm: Add MREMAP_DONTUNMAP to mremap().

From: Lokesh Gidra
Date: Wed Feb 19 2020 - 16:38:32 EST

I've validated the change, that it works for me, through manual
testing. The android runtime changes will follow shortly.

Tested-by: Lokesh Gidra <lokeshgidra@xxxxxxxxxx>

On Wed, Feb 19, 2020 at 12:39 PM Andrew Morton
<akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Tue, 18 Feb 2020 09:32:20 -0800 Brian Geffon <bgeffon@xxxxxxxxxx> wrote:
> > When remapping an anonymous, private mapping, if MREMAP_DONTUNMAP is
> > set, the source mapping will not be removed. The remap operation
> > will be performed as it would have been normally by moving over the
> > page tables to the new mapping. The old vma will have any locked
> > flags cleared, have no pagetables, and any userfaultfds that were
> > watching that range will continue watching it.
> >
> > 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."
> Thanks.
> We're a bit thin on review activity on this one. Has Lokesh been able
> to review and preferably test the code? Are you able to identify other
> potential users? perhaps even glibc?