Merge tag 'mm-stable-2025-05-31-14-50' of git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

Pull MM updates from Andrew Morton:

 - "Add folio_mk_pte()" from Matthew Wilcox simplifies the act of
   creating a pte which addresses the first page in a folio and reduces
   the amount of plumbing which architecture must implement to provide
   this.

 - "Misc folio patches for 6.16" from Matthew Wilcox is a shower of
   largely unrelated folio infrastructure changes which clean things up
   and better prepare us for future work.

 - "memory,x86,acpi: hotplug memory alignment advisement" from Gregory
   Price adds early-init code to prevent x86 from leaving physical
   memory unused when physical address regions are not aligned to memory
   block size.

 - "mm/compaction: allow more aggressive proactive compaction" from
   Michal Clapinski provides some tuning of the (sadly, hard-coded (more
   sadly, not auto-tuned)) thresholds for our invokation of proactive
   compaction. In a simple test case, the reduction of a guest VM's
   memory consumption was dramatic.

 - "Minor cleanups and improvements to swap freeing code" from Kemeng
   Shi provides some code cleaups and a small efficiency improvement to
   this part of our swap handling code.

 - "ptrace: introduce PTRACE_SET_SYSCALL_INFO API" from Dmitry Levin
   adds the ability for a ptracer to modify syscalls arguments. At this
   time we can alter only "system call information that are used by
   strace system call tampering, namely, syscall number, syscall
   arguments, and syscall return value.

   This series should have been incorporated into mm.git's "non-MM"
   branch, but I goofed.

 - "fs/proc: extend the PAGEMAP_SCAN ioctl to report guard regions" from
   Andrei Vagin extends the info returned by the PAGEMAP_SCAN ioctl
   against /proc/pid/pagemap. This permits CRIU to more efficiently get
   at the info about guard regions.

 - "Fix parameter passed to page_mapcount_is_type()" from Gavin Shan
   implements that fix. No runtime effect is expected because
   validate_page_before_insert() happens to fix up this error.

 - "kernel/events/uprobes: uprobe_write_opcode() rewrite" from David
   Hildenbrand basically brings uprobe text poking into the current
   decade. Remove a bunch of hand-rolled implementation in favor of
   using more current facilities.

 - "mm/ptdump: Drop assumption that pxd_val() is u64" from Anshuman
   Khandual provides enhancements and generalizations to the pte dumping
   code. This might be needed when 128-bit Page Table Descriptors are
   enabled for ARM.

 - "Always call constructor for kernel page tables" from Kevin Brodsky
   ensures that the ctor/dtor is always called for kernel pgtables, as
   it already is for user pgtables.

   This permits the addition of more functionality such as "insert hooks
   to protect page tables". This change does result in various
   architectures performing unnecesary work, but this is fixed up where
   it is anticipated to occur.

 - "Rust support for mm_struct, vm_area_struct, and mmap" from Alice
   Ryhl adds plumbing to permit Rust access to core MM structures.

 - "fix incorrectly disallowed anonymous VMA merges" from Lorenzo
   Stoakes takes advantage of some VMA merging opportunities which we've
   been missing for 15 years.

 - "mm/madvise: batch tlb flushes for MADV_DONTNEED and MADV_FREE" from
   SeongJae Park optimizes process_madvise()'s TLB flushing.

   Instead of flushing each address range in the provided iovec, we
   batch the flushing across all the iovec entries. The syscall's cost
   was approximately halved with a microbenchmark which was designed to
   load this particular operation.

 - "Track node vacancy to reduce worst case allocation counts" from
   Sidhartha Kumar makes the maple tree smarter about its node
   preallocation.

   stress-ng mmap performance increased by single-digit percentages and
   the amount of unnecessarily preallocated memory was dramaticelly
   reduced.

 - "mm/gup: Minor fix, cleanup and improvements" from Baoquan He removes
   a few unnecessary things which Baoquan noted when reading the code.

 - ""Enhance sysfs handling for memory hotplug in weighted interleave"
   from Rakie Kim "enhances the weighted interleave policy in the memory
   management subsystem by improving sysfs handling, fixing memory
   leaks, and introducing dynamic sysfs updates for memory hotplug
   support". Fixes things on error paths which we are unlikely to hit.

 - "mm/damon: auto-tune DAMOS for NUMA setups including tiered memory"
   from SeongJae Park introduces new DAMOS quota goal metrics which
   eliminate the manual tuning which is required when utilizing DAMON
   for memory tiering.

 - "mm/vmalloc.c: code cleanup and improvements" from Baoquan He
   provides cleanups and small efficiency improvements which Baoquan
   found via code inspection.

 - "vmscan: enforce mems_effective during demotion" from Gregory Price
   changes reclaim to respect cpuset.mems_effective during demotion when
   possible. because presently, reclaim explicitly ignores
   cpuset.mems_effective when demoting, which may cause the cpuset
   settings to violated.

   This is useful for isolating workloads on a multi-tenant system from
   certain classes of memory more consistently.

 - "Clean up split_huge_pmd_locked() and remove unnecessary folio
   pointers" from Gavin Guo provides minor cleanups and efficiency gains
   in in the huge page splitting and migrating code.

 - "Use kmem_cache for memcg alloc" from Huan Yang creates a slab cache
   for `struct mem_cgroup', yielding improved memory utilization.

 - "add max arg to swappiness in memory.reclaim and lru_gen" from
   Zhongkun He adds a new "max" argument to the "swappiness=" argument
   for memory.reclaim MGLRU's lru_gen.

   This directs proactive reclaim to reclaim from only anon folios
   rather than file-backed folios.

 - "kexec: introduce Kexec HandOver (KHO)" from Mike Rapoport is the
   first step on the path to permitting the kernel to maintain existing
   VMs while replacing the host kernel via file-based kexec. At this
   time only memblock's reserve_mem is preserved.

 - "mm: Introduce for_each_valid_pfn()" from David Woodhouse provides
   and uses a smarter way of looping over a pfn range. By skipping
   ranges of invalid pfns.

 - "sched/numa: Skip VMA scanning on memory pinned to one NUMA node via
   cpuset.mems" from Libo Chen removes a lot of pointless VMA scanning
   when a task is pinned a single NUMA mode.

   Dramatic performance benefits were seen in some real world cases.

 - "JFS: Implement migrate_folio for jfs_metapage_aops" from Shivank
   Garg addresses a warning which occurs during memory compaction when
   using JFS.

 - "move all VMA allocation, freeing and duplication logic to mm" from
   Lorenzo Stoakes moves some VMA code from kernel/fork.c into the more
   appropriate mm/vma.c.

 - "mm, swap: clean up swap cache mapping helper" from Kairui Song
   provides code consolidation and cleanups related to the folio_index()
   function.

 - "mm/gup: Cleanup memfd_pin_folios()" from Vishal Moola does that.

 - "memcg: Fix test_memcg_min/low test failures" from Waiman Long
   addresses some bogus failures which are being reported by the
   test_memcontrol selftest.

 - "eliminate mmap() retry merge, add .mmap_prepare hook" from Lorenzo
   Stoakes commences the deprecation of file_operations.mmap() in favor
   of the new file_operations.mmap_prepare().

   The latter is more restrictive and prevents drivers from messing with
   things in ways which, amongst other problems, may defeat VMA merging.

 - "memcg: decouple memcg and objcg stocks"" from Shakeel Butt decouples
   the per-cpu memcg charge cache from the objcg's one.

   This is a step along the way to making memcg and objcg charging
   NMI-safe, which is a BPF requirement.

 - "mm/damon: minor fixups and improvements for code, tests, and
   documents" from SeongJae Park is yet another batch of miscellaneous
   DAMON changes. Fix and improve minor problems in code, tests and
   documents.

 - "memcg: make memcg stats irq safe" from Shakeel Butt converts memcg
   stats to be irq safe. Another step along the way to making memcg
   charging and stats updates NMI-safe, a BPF requirement.

 - "Let unmap_hugepage_range() and several related functions take folio
   instead of page" from Fan Ni provides folio conversions in the
   hugetlb code.

* tag 'mm-stable-2025-05-31-14-50' of git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm: (285 commits)
  mm: pcp: increase pcp->free_count threshold to trigger free_high
  mm/hugetlb: convert use of struct page to folio in __unmap_hugepage_range()
  mm/hugetlb: refactor __unmap_hugepage_range() to take folio instead of page
  mm/hugetlb: refactor unmap_hugepage_range() to take folio instead of page
  mm/hugetlb: pass folio instead of page to unmap_ref_private()
  memcg: objcg stock trylock without irq disabling
  memcg: no stock lock for cpu hot-unplug
  memcg: make __mod_memcg_lruvec_state re-entrant safe against irqs
  memcg: make count_memcg_events re-entrant safe against irqs
  memcg: make mod_memcg_state re-entrant safe against irqs
  memcg: move preempt disable to callers of memcg_rstat_updated
  memcg: memcg_rstat_updated re-entrant safe against irqs
  mm: khugepaged: decouple SHMEM and file folios' collapse
  selftests/eventfd: correct test name and improve messages
  alloc_tag: check mem_profiling_support in alloc_tag_init
  Docs/damon: update titles and brief introductions to explain DAMOS
  selftests/damon/_damon_sysfs: read tried regions directories in order
  mm/damon/tests/core-kunit: add a test for damos_set_filters_default_reject()
  mm/damon/paddr: remove unused variable, folio_list, in damon_pa_stat()
  mm/damon/sysfs-schemes: fix wrong comment on damons_sysfs_quota_goal_metric_strs
  ...

1 files changed, 129 insertions, 118 deletions

diff --git a/rust/kernel/task.rs b/rust/kernel/task.rs
index 9e6f6854948d..927413d85484 100644
--- a/rust/kernel/task.rs
+++ b/rust/kernel/task.rs
@@ -7,6 +7,7 @@
 use crate::{
     bindings,
     ffi::{c_int, c_long, c_uint},
+    mm::MmWithUser,
     pid_namespace::PidNamespace,
     types::{ARef, NotThreadSafe, Opaque},
 };
@@ -33,22 +34,20 @@ pub const TASK_NORMAL: c_uint = bindings::TASK_NORMAL as c_uint;
 #[macro_export]
 macro_rules! current {
     () => {
-        // SAFETY: Deref + addr-of below create a temporary `TaskRef` that cannot outlive the
-        // caller.
+        // SAFETY: This expression creates a temporary value that is dropped at the end of the
+        // caller's scope. The following mechanisms ensure that the resulting `&CurrentTask` cannot
+        // leave current task context:
+        //
+        // * To return to userspace, the caller must leave the current scope.
+        // * Operations such as `begin_new_exec()` are necessarily unsafe and the caller of
+        //   `begin_new_exec()` is responsible for safety.
+        // * Rust abstractions for things such as a `kthread_use_mm()` scope must require the
+        //   closure to be `Send`, so the `NotThreadSafe` field of `CurrentTask` ensures that the
+        //   `&CurrentTask` cannot cross the scope in either direction.
         unsafe { &*$crate::task::Task::current() }
     };
 }
 
-/// Returns the currently running task's pid namespace.
-#[macro_export]
-macro_rules! current_pid_ns {
-    () => {
-        // SAFETY: Deref + addr-of below create a temporary `PidNamespaceRef` that cannot outlive
-        // the caller.
-        unsafe { &*$crate::task::Task::current_pid_ns() }
-    };
-}
-
 /// Wraps the kernel's `struct task_struct`.
 ///
 /// # Invariants
@@ -87,7 +86,7 @@ macro_rules! current_pid_ns {
 /// impl State {
 ///     fn new() -> Self {
 ///         Self {
-///             creator: current!().into(),
+///             creator: ARef::from(&**current!()),
 ///             index: 0,
 ///         }
 ///     }
@@ -107,6 +106,44 @@ unsafe impl Send for Task {}
 // synchronised by C code (e.g., `signal_pending`).
 unsafe impl Sync for Task {}
 
+/// Represents the [`Task`] in the `current` global.
+///
+/// This type exists to provide more efficient operations that are only valid on the current task.
+/// For example, to retrieve the pid-namespace of a task, you must use rcu protection unless it is
+/// the current task.
+///
+/// # Invariants
+///
+/// Each value of this type must only be accessed from the task context it was created within.
+///
+/// Of course, every thread is in a different task context, but for the purposes of this invariant,
+/// these operations also permanently leave the task context:
+///
+/// * Returning to userspace from system call context.
+/// * Calling `release_task()`.
+/// * Calling `begin_new_exec()` in a binary format loader.
+///
+/// Other operations temporarily create a new sub-context:
+///
+/// * Calling `kthread_use_mm()` creates a new context, and `kthread_unuse_mm()` returns to the
+///   old context.
+///
+/// This means that a `CurrentTask` obtained before a `kthread_use_mm()` call may be used again
+/// once `kthread_unuse_mm()` is called, but it must not be used between these two calls.
+/// Conversely, a `CurrentTask` obtained between a `kthread_use_mm()`/`kthread_unuse_mm()` pair
+/// must not be used after `kthread_unuse_mm()`.
+#[repr(transparent)]
+pub struct CurrentTask(Task, NotThreadSafe);
+
+// Make all `Task` methods available on `CurrentTask`.
+impl Deref for CurrentTask {
+    type Target = Task;
+    #[inline]
+    fn deref(&self) -> &Task {
+        &self.0
+    }
+}
+
 /// The type of process identifiers (PIDs).
 pub type Pid = bindings::pid_t;
 
@@ -133,119 +170,29 @@ impl Task {
     ///
     /// # Safety
     ///
-    /// Callers must ensure that the returned object doesn't outlive the current task/thread.
-    pub unsafe fn current() -> impl Deref<Target = Task> {
-        struct TaskRef<'a> {
-            task: &'a Task,
-            _not_send: NotThreadSafe,
+    /// Callers must ensure that the returned object is only used to access a [`CurrentTask`]
+    /// within the task context that was active when this function was called. For more details,
+    /// see the invariants section for [`CurrentTask`].
+    pub unsafe fn current() -> impl Deref<Target = CurrentTask> {
+        struct TaskRef {
+            task: *const CurrentTask,
         }
 
-        impl Deref for TaskRef<'_> {
-            type Target = Task;
+        impl Deref for TaskRef {
+            type Target = CurrentTask;
 
             fn deref(&self) -> &Self::Target {
-                self.task
+                // SAFETY: The returned reference borrows from this `TaskRef`, so it cannot outlive
+                // the `TaskRef`, which the caller of `Task::current()` has promised will not
+                // outlive the task/thread for which `self.task` is the `current` pointer. Thus, it
+                // is okay to return a `CurrentTask` reference here.
+                unsafe { &*self.task }
             }
         }
 
-        let current = Task::current_raw();
         TaskRef {
-            // SAFETY: If the current thread is still running, the current task is valid. Given
-            // that `TaskRef` is not `Send`, we know it cannot be transferred to another thread
-            // (where it could potentially outlive the caller).
-            task: unsafe { &*current.cast() },
-            _not_send: NotThreadSafe,
-        }
-    }
-
-    /// Returns a PidNamespace reference for the currently executing task's/thread's pid namespace.
-    ///
-    /// This function can be used to create an unbounded lifetime by e.g., storing the returned
-    /// PidNamespace in a global variable which would be a bug. So the recommended way to get the
-    /// current task's/thread's pid namespace is to use the [`current_pid_ns`] macro because it is
-    /// safe.
-    ///
-    /// # Safety
-    ///
-    /// Callers must ensure that the returned object doesn't outlive the current task/thread.
-    pub unsafe fn current_pid_ns() -> impl Deref<Target = PidNamespace> {
-        struct PidNamespaceRef<'a> {
-            task: &'a PidNamespace,
-            _not_send: NotThreadSafe,
-        }
-
-        impl Deref for PidNamespaceRef<'_> {
-            type Target = PidNamespace;
-
-            fn deref(&self) -> &Self::Target {
-                self.task
-            }
-        }
-
-        // The lifetime of `PidNamespace` is bound to `Task` and `struct pid`.
-        //
-        // The `PidNamespace` of a `Task` doesn't ever change once the `Task` is alive. A
-        // `unshare(CLONE_NEWPID)` or `setns(fd_pidns/pidfd, CLONE_NEWPID)` will not have an effect
-        // on the calling `Task`'s pid namespace. It will only effect the pid namespace of children
-        // created by the calling `Task`. This invariant guarantees that after having acquired a
-        // reference to a `Task`'s pid namespace it will remain unchanged.
-        //
-        // When a task has exited and been reaped `release_task()` will be called. This will set
-        // the `PidNamespace` of the task to `NULL`. So retrieving the `PidNamespace` of a task
-        // that is dead will return `NULL`. Note, that neither holding the RCU lock nor holding a
-        // referencing count to
-        // the `Task` will prevent `release_task()` being called.
-        //
-        // In order to retrieve the `PidNamespace` of a `Task` the `task_active_pid_ns()` function
-        // can be used. There are two cases to consider:
-        //
-        // (1) retrieving the `PidNamespace` of the `current` task
-        // (2) retrieving the `PidNamespace` of a non-`current` task
-        //
-        // From system call context retrieving the `PidNamespace` for case (1) is always safe and
-        // requires neither RCU locking nor a reference count to be held. Retrieving the
-        // `PidNamespace` after `release_task()` for current will return `NULL` but no codepath
-        // like that is exposed to Rust.
-        //
-        // Retrieving the `PidNamespace` from system call context for (2) requires RCU protection.
-        // Accessing `PidNamespace` outside of RCU protection requires a reference count that
-        // must've been acquired while holding the RCU lock. Note that accessing a non-`current`
-        // task means `NULL` can be returned as the non-`current` task could have already passed
-        // through `release_task()`.
-        //
-        // To retrieve (1) the `current_pid_ns!()` macro should be used which ensure that the
-        // returned `PidNamespace` cannot outlive the calling scope. The associated
-        // `current_pid_ns()` function should not be called directly as it could be abused to
-        // created an unbounded lifetime for `PidNamespace`. The `current_pid_ns!()` macro allows
-        // Rust to handle the common case of accessing `current`'s `PidNamespace` without RCU
-        // protection and without having to acquire a reference count.
-        //
-        // For (2) the `task_get_pid_ns()` method must be used. This will always acquire a
-        // reference on `PidNamespace` and will return an `Option` to force the caller to
-        // explicitly handle the case where `PidNamespace` is `None`, something that tends to be
-        // forgotten when doing the equivalent operation in `C`. Missing RCU primitives make it
-        // difficult to perform operations that are otherwise safe without holding a reference
-        // count as long as RCU protection is guaranteed. But it is not important currently. But we
-        // do want it in the future.
-        //
-        // Note for (2) the required RCU protection around calling `task_active_pid_ns()`
-        // synchronizes against putting the last reference of the associated `struct pid` of
-        // `task->thread_pid`. The `struct pid` stored in that field is used to retrieve the
-        // `PidNamespace` of the caller. When `release_task()` is called `task->thread_pid` will be
-        // `NULL`ed and `put_pid()` on said `struct pid` will be delayed in `free_pid()` via
-        // `call_rcu()` allowing everyone with an RCU protected access to the `struct pid` acquired
-        // from `task->thread_pid` to finish.
-        //
-        // SAFETY: The current task's pid namespace is valid as long as the current task is running.
-        let pidns = unsafe { bindings::task_active_pid_ns(Task::current_raw()) };
-        PidNamespaceRef {
-            // SAFETY: If the current thread is still running, the current task and its associated
-            // pid namespace are valid. `PidNamespaceRef` is not `Send`, so we know it cannot be
-            // transferred to another thread (where it could potentially outlive the current
-            // `Task`). The caller needs to ensure that the PidNamespaceRef doesn't outlive the
-            // current task/thread.
-            task: unsafe { PidNamespace::from_ptr(pidns) },
-            _not_send: NotThreadSafe,
+            // CAST: The layout of `struct task_struct` and `CurrentTask` is identical.
+            task: Task::current_raw().cast(),
         }
     }
 
@@ -328,6 +275,70 @@ impl Task {
     }
 }
 
+impl CurrentTask {
+    /// Access the address space of the current task.
+    ///
+    /// This function does not touch the refcount of the mm.
+    #[inline]
+    pub fn mm(&self) -> Option<&MmWithUser> {
+        // SAFETY: The `mm` field of `current` is not modified from other threads, so reading it is
+        // not a data race.
+        let mm = unsafe { (*self.as_ptr()).mm };
+
+        if mm.is_null() {
+            return None;
+        }
+
+        // SAFETY: If `current->mm` is non-null, then it references a valid mm with a non-zero
+        // value of `mm_users`. Furthermore, the returned `&MmWithUser` borrows from this
+        // `CurrentTask`, so it cannot escape the scope in which the current pointer was obtained.
+        //
+        // This is safe even if `kthread_use_mm()`/`kthread_unuse_mm()` are used. There are two
+        // relevant cases:
+        // * If the `&CurrentTask` was created before `kthread_use_mm()`, then it cannot be
+        //   accessed during the `kthread_use_mm()`/`kthread_unuse_mm()` scope due to the
+        //   `NotThreadSafe` field of `CurrentTask`.
+        // * If the `&CurrentTask` was created within a `kthread_use_mm()`/`kthread_unuse_mm()`
+        //   scope, then the `&CurrentTask` cannot escape that scope, so the returned `&MmWithUser`
+        //   also cannot escape that scope.
+        // In either case, it's not possible to read `current->mm` and keep using it after the
+        // scope is ended with `kthread_unuse_mm()`.
+        Some(unsafe { MmWithUser::from_raw(mm) })
+    }
+
+    /// Access the pid namespace of the current task.
+    ///
+    /// This function does not touch the refcount of the namespace or use RCU protection.
+    ///
+    /// To access the pid namespace of another task, see [`Task::get_pid_ns`].
+    #[doc(alias = "task_active_pid_ns")]
+    #[inline]
+    pub fn active_pid_ns(&self) -> Option<&PidNamespace> {
+        // SAFETY: It is safe to call `task_active_pid_ns` without RCU protection when calling it
+        // on the current task.
+        let active_ns = unsafe { bindings::task_active_pid_ns(self.as_ptr()) };
+
+        if active_ns.is_null() {
+            return None;
+        }
+
+        // The lifetime of `PidNamespace` is bound to `Task` and `struct pid`.
+        //
+        // The `PidNamespace` of a `Task` doesn't ever change once the `Task` is alive.
+        //
+        // From system call context retrieving the `PidNamespace` for the current task is always
+        // safe and requires neither RCU locking nor a reference count to be held. Retrieving the
+        // `PidNamespace` after `release_task()` for current will return `NULL` but no codepath
+        // like that is exposed to Rust.
+        //
+        // SAFETY: If `current`'s pid ns is non-null, then it references a valid pid ns.
+        // Furthermore, the returned `&PidNamespace` borrows from this `CurrentTask`, so it cannot
+        // escape the scope in which the current pointer was obtained, e.g. it cannot live past a
+        // `release_task()` call.
+        Some(unsafe { PidNamespace::from_ptr(active_ns) })
+    }
+}
+
 // SAFETY: The type invariants guarantee that `Task` is always refcounted.
 unsafe impl crate::types::AlwaysRefCounted for Task {
     fn inc_ref(&self) {