Re: [RFC PATCH 0/5] madvise MADV_DOEXEC

On Mon, Jul 27, 2020 at 02:00:17PM -0400, Steven Sistare wrote:

On 7/27/2020 1:07 PM, ebiederm@xmission.com wrote:

quoted

Anthony Yznaga [off-list ref] writes:

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.

What is wrong with using a file descriptor to a possibly deleted file
and re-mmaping it?

There is already MAP_FIXED that allows you to ensure you have the same
address.

MAP_FIXED blows away any existing mapping in that range, which is not the 
desired behavior.  We want to preserve the previously created mapping at

There's also MAP_FIXED_NOREPLACE since v4.17 in case that helps.

Note that this should really go to linux-api too. I won't argue to
resend it since that would mean spamming everyone's inbox with the same
thread again but in case you send a revised version, please ensure to Cc
linux-api. The glibc folks are listening on there too.

Thanks!
Christian