Re: [PATCH v4] mm: Add MREMAP_DONTUNMAP to mremap().

Kirill A. Shutemov
2020-02-10T10:45:07Z

On Fri, Feb 07, 2020 at 12:18:56PM -0800, Brian Geffon wrote:
> 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. Because MREMAP_DONTUNMAP always results in moving
> a VMA you MUST use the MREMAP_MAYMOVE flag. The final result is two
> equally sized VMAs where the destination contains the PTEs of the source.
>
> We hope to use this in Chrome OS where with userfaultfd we could write
> an anonymous mapping to disk without having to STOP the process or worry
> about VMA permission changes.
>
> 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."
>
> Signed-off-by: Brian Geffon <bgeffon@google.com>
> ---
> include/uapi/linux/mman.h | 5 +-
> mm/mremap.c | 98 ++++++++++++++++++++++++++++++---------
> 2 files changed, 80 insertions(+), 23 deletions(-)
>
> diff --git a/include/uapi/linux/mman.h b/include/uapi/linux/mman.h
> index fc1a64c3447b..923cc162609c 100644
> --- a/include/uapi/linux/mman.h
> +++ b/include/uapi/linux/mman.h
> @@ -5,8 +5,9 @@
> #include <asm/mman.h>
> #include <asm-generic/hugetlb_encode.h>
>
> -#define MREMAP_MAYMOVE 1
> -#define MREMAP_FIXED 2
> +#define MREMAP_MAYMOVE 1
> +#define MREMAP_FIXED 2
> +#define MREMAP_DONTUNMAP 4
>
> #define OVERCOMMIT_GUESS 0
> #define OVERCOMMIT_ALWAYS 1
> diff --git a/mm/mremap.c b/mm/mremap.c
> index 122938dcec15..9f4aa17f178b 100644
> --- a/mm/mremap.c
> +++ b/mm/mremap.c
> @@ -318,8 +318,8 @@ unsigned long move_page_tables(struct vm_area_struct *vma,
> static unsigned long move_vma(struct vm_area_struct *vma,
> unsigned long old_addr, unsigned long old_len,
> unsigned long new_len, unsigned long new_addr,
> - bool *locked, struct vm_userfaultfd_ctx *uf,
> - struct list_head *uf_unmap)
> + bool *locked, unsigned long flags,
> + struct vm_userfaultfd_ctx *uf, struct list_head *uf_unmap)
> {
> struct mm_struct *mm = vma->vm_mm;
> struct vm_area_struct *new_vma;
> @@ -408,11 +408,41 @@ static unsigned long move_vma(struct vm_area_struct *vma,
> if (unlikely(vma->vm_flags & VM_PFNMAP))
> untrack_pfn_moved(vma);
>
> + if (unlikely(!err && (flags & MREMAP_DONTUNMAP))) {
> + if (vm_flags & VM_ACCOUNT) {
> + /* Always put back VM_ACCOUNT since we won't unmap */
> + vma->vm_flags |= VM_ACCOUNT;
> +
> + vm_acct_memory(vma_pages(new_vma));
> + }
> +
> + /*
> + * locked_vm accounting: if the mapping remained the same size
> + * it will have just moved and we don't need to touch locked_vm
> + * because we skip the do_unmap. If the mapping shrunk before
> + * being moved then the do_unmap on that portion will have
> + * adjusted vm_locked. Only if the mapping grows do we need to
> + * do something special; the reason is locked_vm only accounts
> + * for old_len, but we're now adding new_len - old_len locked
> + * bytes to the new mapping.
> + */
> + if (new_len > old_len)
> + mm->locked_vm += (new_len - old_len) >> PAGE_SHIFT;

Hm. How do you enforce that we're not over RLIMIT_MEMLOCK?

--
Kirill A. Shutemov