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?
