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

[Daniel Colascione <dancol@google.com>]

On Tue, Feb 11, 2020 at 3:32 PM Brian Geffon <bgeffon@google.com> wrote:
>
> Hi Daniel,
>
> > 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.