> 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.
