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? >
