If Andrew is happy with such a long patch name, okay; but personally I'd prefer brevity to all that detail: mm: Extend MREMAP_DONTUNMAP to non-anonymous mappings On Wed, 17 Mar 2021, Brian Geffon wrote: > Currently MREMAP_DONTUNMAP only accepts private anonymous mappings. This > change will widen the support to include any mappings which are not > VM_DONTEXPAND or VM_PFNMAP. The primary use case is to support > MREMAP_DONTUNMAP on mappings which may have been created from a memfd. > > This change will result in mremap(MREMAP_DONTUNMAP) returning -EINVAL > if VM_DONTEXPAND or VM_PFNMAP mappings are specified. > > 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/ > > Signed-off-by: Brian Geffon <bgeffon@xxxxxxxxxx> Acked-by: Hugh Dickins <hughd@xxxxxxxxxx> Thanks Brian, just what I wanted :) You wondered in another mail about this returning -EINVAL whereas the VM_DONTEXPAND size error returns -EFAULT: I've pondered, and I've read the manpage, and I'm sure it would be wrong to change the old -EFAULT to -EINVAL now; and I don't see good reason to change your -EINVAL to -EFAULT either. Let them differ, that's okay (and it's only in special corner cases that either of these fail anyway). > --- > mm/mremap.c | 4 ++-- > 1 file changed, 2 insertions(+), 2 deletions(-) > > diff --git a/mm/mremap.c b/mm/mremap.c > index ec8f840399ed..db5b8b28c2dd 100644 > --- a/mm/mremap.c > +++ b/mm/mremap.c > @@ -653,8 +653,8 @@ 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->vm_flags & (VM_DONTEXPAND | VM_PFNMAP))) > return ERR_PTR(-EINVAL); > > if (is_vm_hugetlb_page(vma)) > -- > 2.31.0.rc2.261.g7f71774620-goog
