Re: [PATCH 1/3] uprobes: install_breakpoint() should fail if is_swbp_insn() == T

[Oleg Nesterov <oleg@redhat.com>]

On 06/01, Srikar Dronamraju wrote:
>
> * Oleg Nesterov <oleg@redhat.com> [2012-06-01 17:53:12]:
>
> > But. Doesn't this mean we can greatly simplify register_for_each_vma()
> > and make it O(n) ?
> >
> > Unless I missed something, we can simply create the list of
> > mm/vaddr structures under ->i_mmap_mutex (vma_prio_tree_foreach), then
> > register_for_each_vma() can process the list and that is all.
>
>
> If I remember correctly, we cannot allocate the list elements under
> i_mmap_mutex. We dont know how many list elements to allocate.
>
> This is what Peter had to say : https://lkml.org/lkml/2011/6/27/72
>
> "Because we try to take i_mmap_mutex during reclaim, trying to unmap
> pages. So suppose we do an allocation while holding i_mmap_mutex, find
> there's no free memory, try and unmap a page in order to free it, and
> we're stuck."

Yes, try_to_unmap.

But. What do you think about the pseudo-code below? Only to illustrate
the approach, the code is not complete.

In the "likely" case we do vma_prio_tree_foreach() twice, this is
better than the current quadratic behaviour.

Oleg.

struct map_info {
	struct map_info *next;
	struct mm_struct *mm;
	loff_t vaddr;
};

static struct map_info *
build_map_info_list(struct address_space *mapping, loff_t offset,
							bool is_register)
{
	struct map_info *prev = NULL;
	struct map_info *curr;
	int more;

again:
	more = 0;
	curr = NULL;
	vma_prio_tree_foreach(vma, &iter, &mapping->i_mmap, pgoff, pgoff) {
		struct map_info *info = prev;

		if (!valid_vma(vma, is_register))
			continue;

		if (!info) {
			more++;
			continue;
		}

		if (!atomic_inc_not_zero(&vma->vm_mm->mm_users))
			contunue;

		prev = info->next;

		info->mm = vma->vm_mm;
		info->vaddr = vma_address(vma, offset);

		info->next = curr;
		curr = info;
	}

	if (!more) {
		while (prev) {
			map_info *tmp = prev;
			prev = prev->next;
			kfree(tmp);
		}

		return curr;
	}

	prev = curr;

	while (curr) {
		mmput(curr->mm);
		curr = curr->next;
	}

	while (more--) {
		struct map_info *info = kmalloc(...);
		if (!info)
			return ERR_PTR(-ENOMEM);	// MEMORY LEAK
		info->next = prev;
		prev = info;
	}

	goto again;
}