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
