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?
