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mm/mmu_notifiers: do not speculatively allocate a mmu_notifier_mm

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A prior commit e0f3c3f78d ("mm/mmu_notifier: init notifier if necessary")
made an attempt at doing this, but had to be reverted as calling
the GFP_KERNEL allocator under the i_mmap_mutex causes deadlock, see
commit 35cfa2b0b4 ("mm/mmu_notifier: allocate mmu_notifier in advance").

However, we can avoid that problem by doing the allocation only under
the mmap_sem, which is already happening.

Since all writers to mm->mmu_notifier_mm hold the write side of the
mmap_sem reading it under that sem is deterministic and we can use that to
decide if the allocation path is required, without speculation.

The actual update to mmu_notifier_mm must still be done under the
mm_take_all_locks() to ensure read-side coherency.

Link: https://lore.kernel.org/r/20190806231548.25242-3-jgg@ziepe.ca
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Ralph Campbell <rcampbell@nvidia.com>
Tested-by: Ralph Campbell <rcampbell@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>
This commit is contained in:
Jason Gunthorpe 2019-08-06 20:15:39 -03:00
parent 56c57103db
commit 70df291bf8
1 changed files with 22 additions and 12 deletions
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34
mm/mmu_notifier.c
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@ -242,27 +242,32 @@ EXPORT_SYMBOL_GPL(__mmu_notifier_invalidate_range);
*/
int __mmu_notifier_register(struct mmu_notifier *mn, struct mm_struct *mm)
{
struct mmu_notifier_mm *mmu_notifier_mm;
struct mmu_notifier_mm *mmu_notifier_mm = NULL;
int ret;
lockdep_assert_held_write(&mm->mmap_sem);
BUG_ON(atomic_read(&mm->mm_users) <= 0);
mmu_notifier_mm = kmalloc(sizeof(struct mmu_notifier_mm), GFP_KERNEL);
if (unlikely(!mmu_notifier_mm))
return -ENOMEM;
if (!mm->mmu_notifier_mm) {
/*
* kmalloc cannot be called under mm_take_all_locks(), but we
* know that mm->mmu_notifier_mm can't change while we hold
* the write side of the mmap_sem.
*/
mmu_notifier_mm =
kmalloc(sizeof(struct mmu_notifier_mm), GFP_KERNEL);
if (!mmu_notifier_mm)
return -ENOMEM;
INIT_HLIST_HEAD(&mmu_notifier_mm->list);
spin_lock_init(&mmu_notifier_mm->lock);
}
ret = mm_take_all_locks(mm);
if (unlikely(ret))
goto out_clean;
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;
}
/* Pairs with the mmdrop in mmu_notifier_unregister_* */
mmgrab(mm);
/*
@ -273,14 +278,19 @@ int __mmu_notifier_register(struct mmu_notifier *mn, struct mm_struct *mm)
* We can't race against any other mmu notifier method either
* thanks to mm_take_all_locks().
*/
if (mmu_notifier_mm)
mm->mmu_notifier_mm = mmu_notifier_mm;
spin_lock(&mm->mmu_notifier_mm->lock);
hlist_add_head_rcu(&mn->hlist, &mm->mmu_notifier_mm->list);
spin_unlock(&mm->mmu_notifier_mm->lock);
mm_drop_all_locks(mm);
BUG_ON(atomic_read(&mm->mm_users) <= 0);
return 0;
out_clean:
kfree(mmu_notifier_mm);
BUG_ON(atomic_read(&mm->mm_users) <= 0);
return ret;
}
EXPORT_SYMBOL_GPL(__mmu_notifier_register);