On Tue, Mar 19, 2024 at 06:38:23PM +0000, Matthew Wilcox wrote:

quoted

The syscall design is mostly being posted right now to collaborate via
public channels, but if the idea is so fundamentally offensive then i'll
drop it and relay the opinion accordingly.

The syscall design is wrong.  Exposing physical addresses to userspace
is never the right answer.  Think rowhammer.

1) The syscall does not expose physical addresses information, it
   consumes it.

2) The syscall does not allow the user to select target physical address
   only the target node. Now, that said, if source-pages are zeroed on
   migration, that's definitely a concern.  I did not see this to be the
   case, however, and the frequency of write required to make use of
   that for rowhammer seems to be a mitigating factor.

3) there exist 4 interfaces which do expose physical address information
   - /proc/pid/pagemap
   - perf / IBS and PEBs
   - zoneinfo
   - /sys/kerne/mm/page_idle (PFNs)

4) The syscall requires CAP_SYS_ADMIN because these other sources
   require the same, though as v1/v2 discussed there could be an
   argument for CAP_SYS_NIDE.

I'm vehemently opposed to all of the bullshit around CXL.  However, if you
are going to propose something, it should be based around an abstraction.
Say "We have 8 pools of memory.  This VMA is backed by memory from pools
3 & 6.  The relative hotness of the 8 pools are <vector>.  The quantities
of memory in the 8 ppols are <vector>".  And then you can say "migrate
this range of memory to pool 2".

That's just an initial response to the idea.  I refuse to invest a
serious amount of time in a dead-end idea like CXL memory pooling.

Who said anything about pools? Local memory expanders are capable of
hosting hotness tracking offload.

~Gregory