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

On Fri, Feb 7, 2020 at 12:19 PM Brian Geffon [off-list ref] 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.

The left-behind mapping (the "as if a brand new anonymous, private
mapping" map) is immediately writable, right? If so, we need to
account the additional commit charge. What about making the
left-behind mapping PROT_NONE? This way, we'll still solve the
address-space race in Lokesh's use case (because even a PROT_NONE
mapping reserves address space) but won't incur any additional commit
until someone calls mprotect(PROT_WRITE) on the left-behind mapping.
Imagine having two equal-sized mappings and wanting to use mremap() to
swap them: you can implement this swap by carving off a third region
of address space and making two mremap() calls. But without the
PROT_NONE, you pay additional commit for that third region even if you
don't need it.