Re: [PATCH v7 1/2] mm: Add MREMAP_DONTUNMAP to mremap().

On Fri, Feb 21, 2020 at 09:42:46AM -0800, Brian Geffon wrote:

When remapping an anonymous, private mapping, if MREMAP_DONTUNMAP is
set, the source mapping will not be removed. The remap operation
will be performed as it would have been normally by moving over the
page tables to the new mapping. The old vma will have any locked
flags cleared, have no pagetables, and any userfaultfds that were
watching that range will continue watching it.

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, it's not possible to resize
a VMA while also moving with MREMAP_DONTUNMAP so old_len must always
be equal to the new_len otherwise it will return -EINVAL.

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."

  v6 -> v7:
    - Don't allow resizing VMA as part of MREMAP_DONTUNMAP.
      There is no clear use case at the moment and it can be added
      later as it simplifies the implementation for now.

  v5 -> v6:
    - Code cleanup suggested by Kirill.

  v4 -> v5:
    - Correct commit message to more accurately reflect the behavior.
    - Clear VM_LOCKED and VM_LOCKEDONFAULT on the old vma.
           
Signed-off-by: Brian Geffon <redacted>
Reviewed-by: Minchan Kim <minchan@kernel.org>
Tested-by: Lokesh Gidra <redacted>

Acked-by: Kirill A. Shutemov <redacted>

-- 
 Kirill A. Shutemov