Re: [RFC PATCH 0/4] Allow persistent data on DAX device being used as KMEM

[Srinivas Aji <srinivas.aji@memverge.com>]

On Fri, Aug 05, 2022 at 02:46:26PM +0200, David Hildenbrand wrote:
> Can you explain how "zero copy snapshots of processes" would work, both
> 
> a) From a user space POV
> b) From a kernel-internal POV
> 
> Especially, what I get is that you have a filesystem on that memory
> region, and all memory that is not used for filesystem blocks can be
> used as ordinary system RAM (a little like shmem, but restricted to dax
> memory regions?).
> 
> But how does this interact with zero-copy snapshots?
> 
> I feel like I am missing one piece where we really need system RAM as
> part of the bigger picture. Hopefully it's not some hack that converts
> system RAM to file system blocks :)

My proposal probably falls into this category. The idea is that if we
have the persistent filesystem in the same space as system RAM, we
could make most of the process pages part of a snapshot file by
holding references to the these pages and making the pages
copy-on-write for the process, in about the same way a forked child
would. (I still don't have this piece fully worked out. May be there
are reasons why this won't work or will make something else difficult,
and that is why you are advising against it.)

Regarding the userspace and kernel POV:

The userspace operation would be that the process tries to save or
restore its pages using vmsplice(). In the kernel, this would be
implemented using a filesystem which shares pages with system RAM and
uses a zero-copy COW mechanism for those process pages which can be
shared with the filesystem.

I had earlier been thinking of having a different interface to the
kernel, which creates a file with only those memory pages which can be
saved using COW and also indicates to the caller which pages have
actually been saved.  But having a vmsplice implementation which does
COW as far as possible keeps the userspace process indicating the
desired function (saving or restoring memory pages) and the kernel
implementation handling the zero copy as an optimization where
possible.

Thanks,
Srinivas