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

[Andy Lutomirski]

> On Jan 22, 2020, at 5:46 PM, Brian Geffon <bgeffon@xxxxxxxxxx> wrote:
> 
> ïMREMAP_DONTUNMAP is an additional flag that can be used with
> MREMAP_FIXED to move a mapping to a new address. Normally, mremap(2)
> would then tear down the old vma so subsequent accesses to the vma
> cause a segfault. However, with this new flag it will keep the old
> vma with zapping PTEs so any access to the old VMA after that point
> will result in a pagefault.

This needs a vastly better description. Perhaps:

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.

Or is it something else?

> 
> This feature will find a use in ChromeOS along with userfaultfd.
> Specifically we will want to register a VMA with userfaultfd and then
> pull it out from under a running process. By using MREMAP_DONTUNMAP we
> don't have to worry about mprotecting and then potentially racing with
> VMA permission changes from a running process.

Does this mean you yank it out but you want to replace it simultaneously?

> 
> 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."

Cute.