Andy, Thanks, yes, that's a much clearer description of the feature. I'll make sure to update the description with subsequent patches and with later man page updates. Brian On Wed, Jan 22, 2020 at 7:02 PM Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote: > > > > > On Jan 22, 2020, at 5:46 PM, Brian Geffon <bgeffon@xxxxxxxxxx> wrote: > > > > MREMAP_DONTUNMAP is an additional flag that can be used with > > MREMAP_FIXED to move a mapping to a new address. Normally, mremap(2) > > would then tear down the old vma so subsequent accesses to the vma > > cause a segfault. However, with this new flag it will keep the old > > vma with zapping PTEs so any access to the old VMA after that point > > will result in a pagefault. > > This needs a vastly better description. Perhaps: > > 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. > > Or is it something else? > > > > > This feature will find a use in ChromeOS along with userfaultfd. > > Specifically we will want to register a VMA with userfaultfd and then > > pull it out from under a running process. By using MREMAP_DONTUNMAP we > > don't have to worry about mprotecting and then potentially racing with > > VMA permission changes from a running process. > > Does this mean you yank it out but you want to replace it simultaneously? > > > > > 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." > > Cute.
