Re: [PATCH v2 12/14] userfaultfd: add UFFDIO_SET_MODE for runtime sync/async toggle

[Mike Rapoport]

On Fri, May 08, 2026 at 04:55:24PM +0100, Kiryl Shutsemau (Meta) wrote:
> Add an ioctl to toggle async mode at runtime without re-registering
> the userfaultfd. This allows a VMM to switch between sync and async
> RWP modes on-the-fly -- for example, starting in async mode for
> working set scanning, then switching to sync mode to intercept faults
> during page eviction.
> 
> UFFDIO_SET_MODE takes an enable/disable bitmask of UFFD_FEATURE_*
> flags. Only UFFD_FEATURE_RWP_ASYNC is toggleable today; the ioctl
> rejects any other bit with -EINVAL. Enabling RWP_ASYNC also requires
> RWP to have been negotiated at UFFDIO_API time, mirroring the
> UFFDIO_API invariant.
> 
> Fault-path readers of ctx->features run under mmap_read_lock or a
> per-VMA lock; the RMW takes mmap_write_lock and calls
> vma_start_write() on every UFFD-armed VMA, so those readers are fully
> excluded. userfaultfd_show_fdinfo(), however, reads ctx->features
> without any lock, so the RMW is written as a single WRITE_ONCE and
> fdinfo reads it with READ_ONCE. That keeps the lockless observer from
> seeing a mid-RMW intermediate and removes the audit burden when new
> toggleable bits are added later.
> 
> When switching to async, pending sync waiters are woken so they retry
> and auto-resolve under the new mode.
> 
> Signed-off-by: Kiryl Shutsemau (Meta) <kas@xxxxxxxxxx>
> Assisted-by: Claude:claude-opus-4-6

Reviewed-by: Mike Rapoport (Microsoft) <rppt@xxxxxxxxxx>

> ---
>  fs/userfaultfd.c                 | 150 +++++++++++++++++++++++++------
>  include/uapi/linux/userfaultfd.h |  14 +++
>  2 files changed, 136 insertions(+), 28 deletions(-)

-- 
Sincerely yours,
Mike.