On Thu, Jul 30, 2020 at 05:27:05PM +0200, Christian Brauner wrote:

On Thu, Jul 30, 2020 at 04:22:50PM +0100, Matthew Wilcox wrote:

quoted

On Mon, Jul 27, 2020 at 10:11:22AM -0700, Anthony Yznaga wrote:

quoted

This patchset adds support for preserving an anonymous memory range across
exec(3) using a new madvise MADV_DOEXEC argument.  The primary benefit for
sharing memory in this manner, as opposed to re-attaching to a named shared
memory segment, is to ensure it is mapped at the same virtual address in
the new process as it was in the old one.  An intended use for this is to
preserve guest memory for guests using vfio while qemu exec's an updated
version of itself.  By ensuring the memory is preserved at a fixed address,
vfio mappings and their associated kernel data structures can remain valid.
In addition, for the qemu use case, qemu instances that back guest RAM with
anonymous memory can be updated.

I just realised that something else I'm working on might be a suitable
alternative to this.  Apologies for not realising it sooner.

http://www.wil.cx/~willy/linux/sileby.html

Just skimming: make it O_CLOEXEC by default. ;)

I appreciate the suggestion, and it makes sense for many 'return an fd'
interfaces, but the point of mshare() is to, well, share.  So sharing
the fd with a child is a common usecase, unlike say sharing a timerfd.
The only other reason to use mshare() is to pass the fd over a unix
socket to a non-child, and I submit that is far less common than wanting
to share with a child.