Re: [PATCH] mm: introduce reference pages

John Hubbard
2020-08-03T03:28:12Z

On 7/31/20 1:32 PM, Peter Collingbourne wrote:
...

Hi,

I can see why you want to do this. A few points to consider, below.

btw, the patch would *not* apply for me, via `git am`. I finally used
patch(1) and that worked. Probably good to mention which tree and branch
this applies to, as a first step to avoiding that, but I'm not quite sure
what else went wrong. Maybe use stock git, instead of
2.28.0.163.g6104cc2f0b6-goog? Just guessing.

> @@ -1684,9 +1695,33 @@ static inline int accountable_mapping(struct file *file, vm_flags_t vm_flags)
> return (vm_flags & (VM_NORESERVE | VM_SHARED | VM_WRITE)) == VM_WRITE;
> }
>
> +static vm_fault_t refpage_fault(struct vm_fault *vmf)
> +{
> + struct page *page;
> +
> + if (get_user_pages((unsigned long)vmf->vma->vm_private_data, 1, 0,
> + &page, 0) != 1)
> + return VM_FAULT_SIGSEGV;
> +

This will end up overflowing the page->_refcount in some situations.

Some thoughts:

In order to implement this feature, the reference pages need to be made
at least a little bit more special, and probably little bit more like
zero pages. At one extreme, for example, zero pages could be a special
case of reference pages, although I'm not sure of a clean way to
implement that.

The reason that more special-ness is required, is that things such as
reference counting and locking can be special-cased with zero pages.
Doing so allows avoiding page->_refcount overflows, for example. Your
patch here, however, allows normal pages to be treated *almost* like a
zero page, in that it's a page full of constant value data. But because
a refpage can be any page, not just a special one that is defined at a
single location, that leads to problems with refcounts.

> + vmf->page = page;
> + return VM_FAULT_LOCKED;

Is the page really locked, or is this a case of "the page is special and
we can safely claim it is locked"? Maybe I'm just confused about the use
of VM_FAULT_LOCKED: I thought you only should set it after locking the
page.

> +}
> +
> +static void refpage_close(struct vm_area_struct *vma)
> +{
> + /* This function exists only to prevent is_mergeable_vma from allowing a
> + * reference page mapping to be merged with an anonymous mapping.
> + */

While it is true that implementing a vma's .close() method will prevent
vma merging, this is an abuse of that function: it depends on how that
function is implemented. And given that refpages represent significant
new capability, I think they deserve their own "if" clause (and perhaps
a VMA flag) in is_mergeable_vma(), instead of this kind of minor hack.

thanks,
--
John Hubbard
NVIDIA