Re: [Xen-devel] [PATCH v3] xen/grant-table: Avoid m2p_override duringmapping

From: Zoltan Kiss
Date: Tue Jan 21 2014 - 14:43:58 EST

On 21/01/14 14:22, Stefano Stabellini wrote:
On Tue, 21 Jan 2014, David Vrabel wrote:
On 21/01/14 12:26, Stefano Stabellini wrote:
On Mon, 20 Jan 2014, Zoltan Kiss wrote:

- ret = m2p_add_override(mfn, pages[i], kmap_ops ?
- &kmap_ops[i] : NULL);
+ if (m2p_override)
+ ret = m2p_add_override(mfn, pages[i], kmap_ops ?
+ &kmap_ops[i] : NULL);
+ else {
+ unsigned long pfn = page_to_pfn(pages[i]);
+ WARN_ON(PagePrivate(pages[i]));
+ SetPagePrivate(pages[i]);
+ set_page_private(pages[i], mfn);
+ pages[i]->index = pfn_to_mfn(pfn);
+ if (unlikely(!set_phys_to_machine(pfn, FOREIGN_FRAME(mfn))))
+ return -ENOMEM;

What happens if the page is PageHighMem?

This looks like a subset of m2p_add_override, but it is missing some
relevant bits, like the PageHighMem check, or the p2m(m2p(mfn)) == mfn
check. Maybe we can find a way to avoid duplicating the code.
We could split m2p_add_override in two functions or add yet another
parameter to it.

The PageHighMem() check isn't relevant as we're not mapping anything
here. Also, a page for a kernel grant mapping only cannot be highmem.

The check for a local mfn and the additional set_phys_to_machine() is
only necessary if something tries an mfn_to_pfn() on the local mfn. We
can only omit adding an m2p override if we know thing will try
mfn_to_pfn(), therefore the check and set_phys_to_machine() is unnecessary.

OK, you convinced me that the two checks are superfluous for this case.

Can we still avoid the code duplication by removing the corresponding
code from m2p_add_override and m2p_remove_override and doing the
set_page_private thing uniquely in grant-table.c?
Yes, I moved these parts out from the m2p* funcions to the gntmap functions. One change is that now we pass pfn/mfn to m2p* functions as they are changing right before the call. Also, to avoid racing I clear the page->private value before calling m2p_remove_override. I'll send in the patch soon.

To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at