[ forgive formatting, a series of unfortunate events has me using Outlook for the moment
From: Jan Kara <jack(a)suse.cz>
> > > These flags are device properties that affect the kernel and
> > > userspace's handling of persistence.
> > >
> > That will not handle the scenario with multiple applications using
> > the same fsdax mount point where one is updated to use the new
> > instruction and the other is not.
> Right, it needs to be a global setting / flag day to switch from one
> regime to another. Per-process control is a recipe for disaster.
First I'd like to mention that hopefully the concern is mostly theoretical since
as Aneesh wrote above, real persistent memory never shipped for PPC and
so there are very few apps (if any) using the old way to ensure cache
But I'd like to understand why do you think per-process control is a recipe for
disaster? Because from my POV the sysfs interface you propose is actually
difficult to use in practice. As a distributor, you have hard time picking the
default because you have a choice between picking safe option which is
going to confuse users because of failing MAP_SYNC and unsafe option
where everyone will be happy until someone looses data because of some
ancient application using wrong instructions to persist data. Poor experience
for users in either way. And when distro defaults to "safe option", then the
burden is on the sysadmin to toggle the switch but how is he supposed to
decide when that is safe? First he has to understand what the problem
actually is, then he has to audit all the applications using pmem whether they
use the new instruction - which is IMO a lot of effort if you have a couple of
applications and practically infeasible if you have more of them.
So IMO the burden should be *on the application* to declare that it is aware
of the new instructions to flush pmem on the platform and only to such
application the kernel should give the trust to use MAP_SYNC mappings.
The "disaster" in my mind is this need to globally change the ABI for
persistence semantics for all of Linux because one CPU wants a do over. What does a
generic "MAP_SYNC_ENABLE" knob even mean to the existing deployed base of
persistent memory applications? Yes, sysfs is awkward, but it's trying to provide some
relief without imposing unexplainable semantics on everyone else. I think a comprehensive
(overengineered) solution would involve not introducing another "I know what I'm
doing" flag to the interface, but maybe requiring applications to call a pmem sync
API in something like a vsyscall. Or, also overengineered, some binary translation /
interpretation to actively detect and kill applications that deploy the old instructions.
Something horrid like on first write fault to a MAP_SYNC try to look ahead in the binary
for the correct sync sequence and kill the application otherwise. That would at least
provide some enforcement and safety without requiring other architectures to consider what
MAP_SYNC_ENABLE means to them.