Re: [RFC v2 01/13] mm: export zap_page_range() for driver use

On Wed, Dec 23, 2020 at 02:32:07PM +0800, Yongji Xie wrote:

Now I want to map/unmap some pages in an userland vma dynamically. The
vm_insert_page() is being used for mapping. In the unmapping case, it
looks like the zap_page_range() does what I want. So I export it.
Otherwise, we need some ways to notify userspace to trigger it with
madvise(MADV_DONTNEED), which might not be able to meet all our needs.
For example, unmapping some pages in a memory shrinker function.

So I'd like to know what's the limitation to use zap_page_range() in a
module. And if we can't use it in a module, is there any acceptable
way to achieve that?

I think the anser is: don't play funny games with unmapped outside of
munmap.  Especially as synchronization is very hard to get right.