I apologize, this patch didn't include my signed off by, here it is: Signed-off-by: Brian Geffon <bgeffon@xxxxxxxxxx> On Wed, Mar 3, 2021 at 9:53 AM Brian Geffon <bgeffon@xxxxxxxxxx> wrote: > > Currently MREMAP_DONTUNMAP only accepts private anonymous mappings. This change > will widen the support to include shmem mappings. The primary use case > is to support MREMAP_DONTUNMAP on mappings which may have been created from > a memfd. > > Lokesh Gidra who works on the Android JVM, provided an explanation of how such > a feature will improve Android JVM garbage collection: > "Android is developing a new garbage collector (GC), based on userfaultfd. The > garbage collector will use userfaultfd (uffd) on the java heap during compaction. > On accessing any uncompacted page, the application threads will find it missing, > at which point the thread will create the compacted page and then use UFFDIO_COPY > ioctl to get it mapped and then resume execution. Before starting this compaction, > in a stop-the-world pause the heap will be mremap(MREMAP_DONTUNMAP) so that the > java heap is ready to receive UFFD_EVENT_PAGEFAULT events after resuming execution. > > To speedup mremap operations, pagetable movement was optimized by moving PUD entries > instead of PTE entries [1]. It was necessary as mremap of even modest sized memory > ranges also took several milliseconds, and stopping the application for that long > isn't acceptable in response-time sensitive cases. With UFFDIO_CONTINUE feature [2], > it will be even more efficient to implement this GC, particularly the 'non-moveable' > portions of the heap. It will also help in reducing the need to copy (UFFDIO_COPY) > the pages. However, for this to work, the java heap has to be on a 'shared' vma. > Currently MREMAP_DONTUNMAP only supports private anonymous mappings, this patch will > enable using UFFDIO_CONTINUE for the new userfaultfd-based heap compaction." > > [1] https://lore.kernel.org/linux-mm/20201215030730.NC3CU98e4%25akpm@xxxxxxxxxxxxxxxxxxxx/ > [2] https://lore.kernel.org/linux-mm/20210302000133.272579-1-axelrasmussen@xxxxxxxxxx/ > --- > mm/mremap.c | 3 +-- > 1 file changed, 1 insertion(+), 2 deletions(-) > > diff --git a/mm/mremap.c b/mm/mremap.c > index ec8f840399ed..6934d199da54 100644 > --- a/mm/mremap.c > +++ b/mm/mremap.c > @@ -653,8 +653,7 @@ static struct vm_area_struct *vma_to_resize(unsigned long addr, > return ERR_PTR(-EINVAL); > } > > - if (flags & MREMAP_DONTUNMAP && (!vma_is_anonymous(vma) || > - vma->vm_flags & VM_SHARED)) > + if (flags & MREMAP_DONTUNMAP && !(vma_is_anonymous(vma) || vma_is_shmem(vma))) > return ERR_PTR(-EINVAL); > > if (is_vm_hugetlb_page(vma)) > -- > 2.31.0.rc0.254.gbdcc3b1a9d-goog >
