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

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

[Yongji Xie]

On Tue, Dec 22, 2020 at 11:44 PM Christoph Hellwig <hch@infradead.org> wrote:
>
> On Tue, Dec 22, 2020 at 10:52:09PM +0800, Xie Yongji wrote:
> > Export zap_page_range() for use in VDUSE.
>
> Err, no.  This has absolutely no business being used by drivers.

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?

Thanks.
Yongji

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

[Christoph Hellwig]

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.

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

[Yongji Xie]

On Wed, Dec 23, 2020 at 4:13 PM Christoph Hellwig <hch@infradead.org> wrote:
>
> 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.

OK, I will try to let userspace do this.

Thanks,
Yongji