userfaultfd: add UFFDIO_SET_MODE for runtime sync/async toggle

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.

Link: https://lore.kernel.org/20260708111417.173443-14-kirill@shutemov.name
Signed-off-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
Assisted-by: Claude:claude-opus-4-6
Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: David Hildenbrand <david@kernel.org>
Cc: James Houghton <jthoughton@google.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Liam Howlett <liam@infradead.org>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Sean Christopherson <seanjc@google.com>
Cc: SeongJae Park <sj@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

0 files changed, 0 insertions, 0 deletions