Re: [PATCH v2 0/2] Replace and improve "mcsafe" with copy_safe()

[Andy Lutomirski]

> On Apr 30, 2020, at 5:25 PM, Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> 
> 
> It wasn't clear how "copy_to_mc()" could ever fault. Poisoning
> after-the-fact? Why would that be preferable to just mapping a dummy
> page?

If the kernel gets an async memory error and maps a dummy page, then subsequent reads will subsequently succeed and return garbage when they should fail.  If x86 had write-only pages, we could map a dummy write-only page. But we donât, so I think weâre stuck.

As for naming the kind of memory weâre taking about, ISTM there are two classes: DAX and monstrous memory-mapped non-persistent cache devices.  Both could be Optane, I suppose.

But I also think itâs legitimate to use these accessors to increase the chance of surviving a failure of normal memory. If a normal page happens to be page cache when it fails and if page cache access use these fancy accessors, then we might actually survive a failure.

We could be ambitious: declare that all page cache and all get_user_pageâd memory should use the new accessors.  I doubt weâll ever really succeed due to magical things like rseq and anything that thinks that users can set up their own memory as a kernel-accessed ring buffer, but I suppose we could try.