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

[Brian Geffon <bgeffon@google.com>]

Hi Kirill,
If the old_len == new_len then there is no change in the number of
locked pages they just moved, if the new_len < old_len then the
process of unmapping (new_len - old_len) bytes from the old mapping
will handle the locked page accounting. So in this special case where
we're growing the VMA, vma_to_resize() will enforce that growing the
vma doesn't exceed RLIMIT_MEMLOCK, but vma_to_resize() doesn't handle
incrementing mm->locked_bytes which is why we have that special case
incrementing it here.

Thanks,
Brian

On Mon, Feb 10, 2020 at 2:45 AM Kirill A. Shutemov <kirill@shutemov.name> wrote:
>
> 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