| Message ID | 20210317214147.682687-1-bgeffon@google.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | [v3,1/2] mm: Allow non-VM_DONTEXPAND and VM_PFNMAP mappings with MREMAP_DONTUNMAP | expand |
On Wed, 17 Mar 2021 14:41:46 -0700 Brian Geffon <bgeffon@google.com> 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. Please update changelog to explain why these two were omitted? > 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." Is a manpage update planned? It's appropriate to add this to the series so folks can check it over. Can we please get the appropriate updates into tools/testing/selftests/vm/mremap_test.c for this? > Signed-off-by: Brian Geffon <bgeffon@google.com> v3 is getting up there. Has there been much review activity?
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@linux-foundation.org/ > [2] https://lore.kernel.org/linux-mm/20210302000133.272579-1-axelrasmussen@google.com/ > > Signed-off-by: Brian Geffon <bgeffon@google.com> Acked-by: Hugh Dickins <hughd@google.com> 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
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))
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@linux-foundation.org/ [2] https://lore.kernel.org/linux-mm/20210302000133.272579-1-axelrasmussen@google.com/ Signed-off-by: Brian Geffon <bgeffon@google.com> --- mm/mremap.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-)