Re: [PATCH v4 0/4] arm64/mm: Enable userfaultfd write-protect

[Will Deacon]

Hi Ryan,

On Fri, May 03, 2024 at 03:45:58PM +0100, Ryan Roberts wrote:
> This series adds uffd write-protect support for arm64.
> 
> Previous attempts to add uffd-wp (and soft-dirty) have failed because of a
> perceived lack of available PTE SW bits. However it actually turns out that
> there are 2 available but they are hidden. PTE_PROT_NONE was previously
> occupying a SW bit, but can be moved, freeing up the SW bit. Bit 63 is marked as
> "IGNORED" in the Arm ARM, but it does not currently indicate "reserved for SW
> use" like it does for the other SW bits. I've confirmed with the spec owner that
> this is an oversight; the bit is intended to be reserved for SW use and the spec
> will clarify this in a future update.
> 
> So now we have two spare bits; patch 4 enables uffd-wp on arm64, using the SW
> bit freed up by moving PTE_PROT_NONE. This leaves bit 63 spare for future use
> (e.g. soft-dirty - see RFC at [4] - or some other usage).
> 
> ---
> 
> This applies on top of v6.9-rc5.

I chucked this into the CI on Friday and it looks to have survived the
long weekend, so I've gone ahead and merged it into for-next/core. Short
of any last minute failures (touch wood), this should land in 6.10.

Thanks!

Will