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

[Kirill A. Shutemov]

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 <bgeffon@xxxxxxxxxx>
> Reviewed-by: Minchan Kim <minchan@xxxxxxxxxx>
> Tested-by: Lokesh Gidra <lokeshgidra@xxxxxxxxxx>

Acked-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>

-- 
 Kirill A. Shutemov