Re: [PATCH 1 of 9] Lock the entire mm to prevent any mmu related operation to happen

Rusty Russell
2008-04-22T05:06:24Z

On Wednesday 09 April 2008 01:44:04 Andrea Arcangeli wrote:
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -1050,6 +1050,15 @@
> unsigned long addr, unsigned long len,
> unsigned long flags, struct page **pages);
>
> +struct mm_lock_data {
> + spinlock_t **i_mmap_locks;
> + spinlock_t **anon_vma_locks;
> + unsigned long nr_i_mmap_locks;
> + unsigned long nr_anon_vma_locks;
> +};
> +extern struct mm_lock_data *mm_lock(struct mm_struct * mm);
> +extern void mm_unlock(struct mm_struct *mm, struct mm_lock_data *data);

As far as I can tell you don't actually need to expose this struct at all?

> + data->i_mmap_locks = vmalloc(nr_i_mmap_locks *
> + sizeof(spinlock_t));

This is why non-typesafe allocators suck. You want 'sizeof(spinlock_t *)'
here.

> + data->anon_vma_locks = vmalloc(nr_anon_vma_locks *
> + sizeof(spinlock_t));

and here.

> + err = -EINTR;
> + i_mmap_lock_last = NULL;
> + nr_i_mmap_locks = 0;
> + for (;;) {
> + spinlock_t *i_mmap_lock = (spinlock_t *) -1UL;
> + for (vma = mm->mmap; vma; vma = vma->vm_next) {
...
> + data->i_mmap_locks[nr_i_mmap_locks++] = i_mmap_lock;
> + }
> + data->nr_i_mmap_locks = nr_i_mmap_locks;

How about you track your running counter in data->nr_i_mmap_locks, leave
nr_i_mmap_locks alone, and BUG_ON(data->nr_i_mmap_locks != nr_i_mmap_locks)?

Even nicer would be to wrap this in a "get_sorted_mmap_locks()" function.

Similarly for anon_vma locks.

Unfortunately, I just don't think we can fail locking like this. In your next
patch unregistering a notifier can fail because of it: that not usable.

I think it means you need to add a linked list element to the vma for the
CONFIG_MMU_NOTIFIER case. Or track the max number of vmas for any mm, and
keep a pool to handle mm_lock for this number (ie. if you can't enlarge the
pool, fail the vma allocation).

Both have their problems though...
Rusty.

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