mmu-notifiers: core

With KVM/GFP/XPMEM there isn't just the primary CPU MMU pointing to pages.
 There are secondary MMUs (with secondary sptes and secondary tlbs) too.
sptes in the kvm case are shadow pagetables, but when I say spte in
mmu-notifier context, I mean "secondary pte".  In GRU case there's no
actual secondary pte and there's only a secondary tlb because the GRU
secondary MMU has no knowledge about sptes and every secondary tlb miss
event in the MMU always generates a page fault that has to be resolved by
the CPU (this is not the case of KVM where the a secondary tlb miss will
walk sptes in hardware and it will refill the secondary tlb transparently
to software if the corresponding spte is present).  The same way
zap_page_range has to invalidate the pte before freeing the page, the spte
(and secondary tlb) must also be invalidated before any page is freed and
reused.

Currently we take a page_count pin on every page mapped by sptes, but that
means the pages can't be swapped whenever they're mapped by any spte
because they're part of the guest working set.  Furthermore a spte unmap
event can immediately lead to a page to be freed when the pin is released
(so requiring the same complex and relatively slow tlb_gather smp safe
logic we have in zap_page_range and that can be avoided completely if the
spte unmap event doesn't require an unpin of the page previously mapped in
the secondary MMU).

The mmu notifiers allow kvm/GRU/XPMEM to attach to the tsk->mm and know
when the VM is swapping or freeing or doing anything on the primary MMU so
that the secondary MMU code can drop sptes before the pages are freed,
avoiding all page pinning and allowing 100% reliable swapping of guest
physical address space.  Furthermore it avoids the code that teardown the
mappings of the secondary MMU, to implement a logic like tlb_gather in
zap_page_range that would require many IPI to flush other cpu tlbs, for
each fixed number of spte unmapped.

To make an example: if what happens on the primary MMU is a protection
downgrade (from writeable to wrprotect) the secondary MMU mappings will be
invalidated, and the next secondary-mmu-page-fault will call
get_user_pages and trigger a do_wp_page through get_user_pages if it
called get_user_pages with write=1, and it'll re-establishing an updated
spte or secondary-tlb-mapping on the copied page.  Or it will setup a
readonly spte or readonly tlb mapping if it's a guest-read, if it calls
get_user_pages with write=0.  This is just an example.

This allows to map any page pointed by any pte (and in turn visible in the
primary CPU MMU), into a secondary MMU (be it a pure tlb like GRU, or an
full MMU with both sptes and secondary-tlb like the shadow-pagetable layer
with kvm), or a remote DMA in software like XPMEM (hence needing of
schedule in XPMEM code to send the invalidate to the remote node, while no
need to schedule in kvm/gru as it's an immediate event like invalidating
primary-mmu pte).

At least for KVM without this patch it's impossible to swap guests
reliably.  And having this feature and removing the page pin allows
several other optimizations that simplify life considerably.

Dependencies:

1) mm_take_all_locks() to register the mmu notifier when the whole VM
   isn't doing anything with "mm".  This allows mmu notifier users to keep
   track if the VM is in the middle of the invalidate_range_begin/end
   critical section with an atomic counter incraese in range_begin and
   decreased in range_end.  No secondary MMU page fault is allowed to map
   any spte or secondary tlb reference, while the VM is in the middle of
   range_begin/end as any page returned by get_user_pages in that critical
   section could later immediately be freed without any further
   ->invalidate_page notification (invalidate_range_begin/end works on
   ranges and ->invalidate_page isn't called immediately before freeing
   the page).  To stop all page freeing and pagetable overwrites the
   mmap_sem must be taken in write mode and all other anon_vma/i_mmap
   locks must be taken too.

2) It'd be a waste to add branches in the VM if nobody could possibly
   run KVM/GRU/XPMEM on the kernel, so mmu notifiers will only enabled if
   CONFIG_KVM=m/y.  In the current kernel kvm won't yet take advantage of
   mmu notifiers, but this already allows to compile a KVM external module
   against a kernel with mmu notifiers enabled and from the next pull from
   kvm.git we'll start using them.  And GRU/XPMEM will also be able to
   continue the development by enabling KVM=m in their config, until they
   submit all GRU/XPMEM GPLv2 code to the mainline kernel.  Then they can
   also enable MMU_NOTIFIERS in the same way KVM does it (even if KVM=n).
   This guarantees nobody selects MMU_NOTIFIER=y if KVM and GRU and XPMEM
   are all =n.

The mmu_notifier_register call can fail because mm_take_all_locks may be
interrupted by a signal and return -EINTR.  Because mmu_notifier_reigster
is used when a driver startup, a failure can be gracefully handled.  Here
an example of the change applied to kvm to register the mmu notifiers.
Usually when a driver startups other allocations are required anyway and
-ENOMEM failure paths exists already.

 struct  kvm *kvm_arch_create_vm(void)
 {
        struct kvm *kvm = kzalloc(sizeof(struct kvm), GFP_KERNEL);
+       int err;

        if (!kvm)
                return ERR_PTR(-ENOMEM);

        INIT_LIST_HEAD(&kvm->arch.active_mmu_pages);

+       kvm->arch.mmu_notifier.ops = &kvm_mmu_notifier_ops;
+       err = mmu_notifier_register(&kvm->arch.mmu_notifier, current->mm);
+       if (err) {
+               kfree(kvm);
+               return ERR_PTR(err);
+       }
+
        return kvm;
 }

mmu_notifier_unregister returns void and it's reliable.

The patch also adds a few needed but missing includes that would prevent
kernel to compile after these changes on non-x86 archs (x86 didn't need
them by luck).

[akpm@linux-foundation.org: coding-style fixes]
[akpm@linux-foundation.org: fix mm/filemap_xip.c build]
[akpm@linux-foundation.org: fix mm/mmu_notifier.c build]
Signed-off-by: Andrea Arcangeli <andrea@qumranet.com>
Signed-off-by: Nick Piggin <npiggin@suse.de>
Signed-off-by: Christoph Lameter <cl@linux-foundation.org>
Cc: Jack Steiner <steiner@sgi.com>
Cc: Robin Holt <holt@sgi.com>
Cc: Nick Piggin <npiggin@suse.de>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Kanoj Sarcar <kanojsarcar@yahoo.com>
Cc: Roland Dreier <rdreier@cisco.com>
Cc: Steve Wise <swise@opengridcomputing.com>
Cc: Avi Kivity <avi@qumranet.com>
Cc: Hugh Dickins <hugh@veritas.com>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: Anthony Liguori <aliguori@us.ibm.com>
Cc: Chris Wright <chrisw@redhat.com>
Cc: Marcelo Tosatti <marcelo@kvack.org>
Cc: Eric Dumazet <dada1@cosmosbay.com>
Cc: "Paul E. McKenney" <paulmck@us.ibm.com>
Cc: Izik Eidus <izike@qumranet.com>
Cc: Anthony Liguori <aliguori@us.ibm.com>
Cc: Rik van Riel <riel@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 277 insertions, 0 deletions

diff --git a/mm/mmu_notifier.c b/mm/mmu_notifier.c
new file mode 100644
index 000000000000..5f4ef0250bee
--- /dev/null
+++ b/mm/mmu_notifier.c
@@ -0,0 +1,277 @@
+/*
+ *  linux/mm/mmu_notifier.c
+ *
+ *  Copyright (C) 2008  Qumranet, Inc.
+ *  Copyright (C) 2008  SGI
+ *             Christoph Lameter <clameter@sgi.com>
+ *
+ *  This work is licensed under the terms of the GNU GPL, version 2. See
+ *  the COPYING file in the top-level directory.
+ */
+
+#include <linux/rculist.h>
+#include <linux/mmu_notifier.h>
+#include <linux/module.h>
+#include <linux/mm.h>
+#include <linux/err.h>
+#include <linux/rcupdate.h>
+#include <linux/sched.h>
+
+/*
+ * This function can't run concurrently against mmu_notifier_register
+ * because mm->mm_users > 0 during mmu_notifier_register and exit_mmap
+ * runs with mm_users == 0. Other tasks may still invoke mmu notifiers
+ * in parallel despite there being no task using this mm any more,
+ * through the vmas outside of the exit_mmap context, such as with
+ * vmtruncate. This serializes against mmu_notifier_unregister with
+ * the mmu_notifier_mm->lock in addition to RCU and it serializes
+ * against the other mmu notifiers with RCU. struct mmu_notifier_mm
+ * can't go away from under us as exit_mmap holds an mm_count pin
+ * itself.
+ */
+void __mmu_notifier_release(struct mm_struct *mm)
+{
+        struct mmu_notifier *mn;
+
+        spin_lock(&mm->mmu_notifier_mm->lock);
+        while (unlikely(!hlist_empty(&mm->mmu_notifier_mm->list))) {
+                mn = hlist_entry(mm->mmu_notifier_mm->list.first,
+                                 struct mmu_notifier,
+                                 hlist);
+                /*
+                 * We arrived before mmu_notifier_unregister so
+                 * mmu_notifier_unregister will do nothing other than
+                 * to wait ->release to finish and
+                 * mmu_notifier_unregister to return.
+                 */
+                hlist_del_init_rcu(&mn->hlist);
+                /*
+                 * RCU here will block mmu_notifier_unregister until
+                 * ->release returns.
+                 */
+                rcu_read_lock();
+                spin_unlock(&mm->mmu_notifier_mm->lock);
+                /*
+                 * if ->release runs before mmu_notifier_unregister it
+                 * must be handled as it's the only way for the driver
+                 * to flush all existing sptes and stop the driver
+                 * from establishing any more sptes before all the
+                 * pages in the mm are freed.
+                 */
+                if (mn->ops->release)
+                        mn->ops->release(mn, mm);
+                rcu_read_unlock();
+                spin_lock(&mm->mmu_notifier_mm->lock);
+        }
+        spin_unlock(&mm->mmu_notifier_mm->lock);
+
+        /*
+         * synchronize_rcu here prevents mmu_notifier_release to
+         * return to exit_mmap (which would proceed freeing all pages
+         * in the mm) until the ->release method returns, if it was
+         * invoked by mmu_notifier_unregister.
+         *
+         * The mmu_notifier_mm can't go away from under us because one
+         * mm_count is hold by exit_mmap.
+         */
+        synchronize_rcu();
+}
+
+/*
+ * If no young bitflag is supported by the hardware, ->clear_flush_young can
+ * unmap the address and return 1 or 0 depending if the mapping previously
+ * existed or not.
+ */
+int __mmu_notifier_clear_flush_young(struct mm_struct *mm,
+                                        unsigned long address)
+{
+        struct mmu_notifier *mn;
+        struct hlist_node *n;
+        int young = 0;
+
+        rcu_read_lock();
+        hlist_for_each_entry_rcu(mn, n, &mm->mmu_notifier_mm->list, hlist) {
+                if (mn->ops->clear_flush_young)
+                        young |= mn->ops->clear_flush_young(mn, mm, address);
+        }
+        rcu_read_unlock();
+
+        return young;
+}
+
+void __mmu_notifier_invalidate_page(struct mm_struct *mm,
+                                          unsigned long address)
+{
+        struct mmu_notifier *mn;
+        struct hlist_node *n;
+
+        rcu_read_lock();
+        hlist_for_each_entry_rcu(mn, n, &mm->mmu_notifier_mm->list, hlist) {
+                if (mn->ops->invalidate_page)
+                        mn->ops->invalidate_page(mn, mm, address);
+        }
+        rcu_read_unlock();
+}
+
+void __mmu_notifier_invalidate_range_start(struct mm_struct *mm,
+                                  unsigned long start, unsigned long end)
+{
+        struct mmu_notifier *mn;
+        struct hlist_node *n;
+
+        rcu_read_lock();
+        hlist_for_each_entry_rcu(mn, n, &mm->mmu_notifier_mm->list, hlist) {
+                if (mn->ops->invalidate_range_start)
+                        mn->ops->invalidate_range_start(mn, mm, start, end);
+        }
+        rcu_read_unlock();
+}
+
+void __mmu_notifier_invalidate_range_end(struct mm_struct *mm,
+                                  unsigned long start, unsigned long end)
+{
+        struct mmu_notifier *mn;
+        struct hlist_node *n;
+
+        rcu_read_lock();
+        hlist_for_each_entry_rcu(mn, n, &mm->mmu_notifier_mm->list, hlist) {
+                if (mn->ops->invalidate_range_end)
+                        mn->ops->invalidate_range_end(mn, mm, start, end);
+        }
+        rcu_read_unlock();
+}
+
+static int do_mmu_notifier_register(struct mmu_notifier *mn,
+                                    struct mm_struct *mm,
+                                    int take_mmap_sem)
+{
+        struct mmu_notifier_mm *mmu_notifier_mm;
+        int ret;
+
+        BUG_ON(atomic_read(&mm->mm_users) <= 0);
+
+        ret = -ENOMEM;
+        mmu_notifier_mm = kmalloc(sizeof(struct mmu_notifier_mm), GFP_KERNEL);
+        if (unlikely(!mmu_notifier_mm))
+                goto out;
+
+        if (take_mmap_sem)
+                down_write(&mm->mmap_sem);
+        ret = mm_take_all_locks(mm);
+        if (unlikely(ret))
+                goto out_cleanup;
+
+        if (!mm_has_notifiers(mm)) {
+                INIT_HLIST_HEAD(&mmu_notifier_mm->list);
+                spin_lock_init(&mmu_notifier_mm->lock);
+                mm->mmu_notifier_mm = mmu_notifier_mm;
+                mmu_notifier_mm = NULL;
+        }
+        atomic_inc(&mm->mm_count);
+
+        /*
+         * Serialize the update against mmu_notifier_unregister. A
+         * side note: mmu_notifier_release can't run concurrently with
+         * us because we hold the mm_users pin (either implicitly as
+         * current->mm or explicitly with get_task_mm() or similar).
+         * We can't race against any other mmu notifier method either
+         * thanks to mm_take_all_locks().
+         */
+        spin_lock(&mm->mmu_notifier_mm->lock);
+        hlist_add_head(&mn->hlist, &mm->mmu_notifier_mm->list);
+        spin_unlock(&mm->mmu_notifier_mm->lock);
+
+        mm_drop_all_locks(mm);
+out_cleanup:
+        if (take_mmap_sem)
+                up_write(&mm->mmap_sem);
+        /* kfree() does nothing if mmu_notifier_mm is NULL */
+        kfree(mmu_notifier_mm);
+out:
+        BUG_ON(atomic_read(&mm->mm_users) <= 0);
+        return ret;
+}
+
+/*
+ * Must not hold mmap_sem nor any other VM related lock when calling
+ * this registration function. Must also ensure mm_users can't go down
+ * to zero while this runs to avoid races with mmu_notifier_release,
+ * so mm has to be current->mm or the mm should be pinned safely such
+ * as with get_task_mm(). If the mm is not current->mm, the mm_users
+ * pin should be released by calling mmput after mmu_notifier_register
+ * returns. mmu_notifier_unregister must be always called to
+ * unregister the notifier. mm_count is automatically pinned to allow
+ * mmu_notifier_unregister to safely run at any time later, before or
+ * after exit_mmap. ->release will always be called before exit_mmap
+ * frees the pages.
+ */
+int mmu_notifier_register(struct mmu_notifier *mn, struct mm_struct *mm)
+{
+        return do_mmu_notifier_register(mn, mm, 1);
+}
+EXPORT_SYMBOL_GPL(mmu_notifier_register);
+
+/*
+ * Same as mmu_notifier_register but here the caller must hold the
+ * mmap_sem in write mode.
+ */
+int __mmu_notifier_register(struct mmu_notifier *mn, struct mm_struct *mm)
+{
+        return do_mmu_notifier_register(mn, mm, 0);
+}
+EXPORT_SYMBOL_GPL(__mmu_notifier_register);
+
+/* this is called after the last mmu_notifier_unregister() returned */
+void __mmu_notifier_mm_destroy(struct mm_struct *mm)
+{
+        BUG_ON(!hlist_empty(&mm->mmu_notifier_mm->list));
+        kfree(mm->mmu_notifier_mm);
+        mm->mmu_notifier_mm = LIST_POISON1; /* debug */
+}
+
+/*
+ * This releases the mm_count pin automatically and frees the mm
+ * structure if it was the last user of it. It serializes against
+ * running mmu notifiers with RCU and against mmu_notifier_unregister
+ * with the unregister lock + RCU. All sptes must be dropped before
+ * calling mmu_notifier_unregister. ->release or any other notifier
+ * method may be invoked concurrently with mmu_notifier_unregister,
+ * and only after mmu_notifier_unregister returned we're guaranteed
+ * that ->release or any other method can't run anymore.
+ */
+void mmu_notifier_unregister(struct mmu_notifier *mn, struct mm_struct *mm)
+{
+        BUG_ON(atomic_read(&mm->mm_count) <= 0);
+
+        spin_lock(&mm->mmu_notifier_mm->lock);
+        if (!hlist_unhashed(&mn->hlist)) {
+                hlist_del_rcu(&mn->hlist);
+
+                /*
+                 * RCU here will force exit_mmap to wait ->release to finish
+                 * before freeing the pages.
+                 */
+                rcu_read_lock();
+                spin_unlock(&mm->mmu_notifier_mm->lock);
+                /*
+                 * exit_mmap will block in mmu_notifier_release to
+                 * guarantee ->release is called before freeing the
+                 * pages.
+                 */
+                if (mn->ops->release)
+                        mn->ops->release(mn, mm);
+                rcu_read_unlock();
+        } else
+                spin_unlock(&mm->mmu_notifier_mm->lock);
+
+        /*
+         * Wait any running method to finish, of course including
+         * ->release if it was run by mmu_notifier_relase instead of us.
+         */
+        synchronize_rcu();
+
+        BUG_ON(atomic_read(&mm->mm_count) <= 0);
+
+        mmdrop(mm);
+}
+EXPORT_SYMBOL_GPL(mmu_notifier_unregister);