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

On Tue, Feb 11, 2020 at 3:32 PM Brian Geffon [off-list ref] wrote:

Hi Daniel,

quoted

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.

This limits the usefulness of the feature IMO and really is too
specific to that one use case, suppose you want to snapshot a memory
region to disk without having to stop a thread you can
mremap(MREMAP_DONTUNMAP) it to another location and safely write it to
disk knowing the faulting thread will be stopped and you can handle it
later if it was registered with userfaultfd, if we were to also change
it to PROT_NONE that thread would see a SEGV. There are other examples
where you can possibly use this flag instead of VM_UFFD_WP, but
changing the protections of the mapping prevents you from being able
to do this without a funny signal handler dance.

We seem to have identified two use cases which require contradictory
things. We can handle this with an option. Maybe the new region's
protection bits should be specified in the flags argument. The most
general API would accept any set of mmap flags to apply to the
left-behind region, but it's probably hard to squeeze that
functionality into the existing API.