Re: [RFC 0/4] Transparent on-demand struct page initialization embedded in the buddy allocator

Robin Holt
2013-07-15T15:00:41Z

On Thu, Jul 11, 2013 at 09:03:51PM -0500, Robin Holt wrote:
> We have been working on this since we returned from shutdown and have
> something to discuss now. We restricted ourselves to 2MiB initialization
> to keep the patch set a little smaller and more clear.
>
> First, I think I want to propose getting rid of the page flag. If I knew
> of a concrete way to determine that the page has not been initialized,
> this patch series would look different. If there is no definitive
> way to determine that the struct page has been initialized aside from
> checking the entire page struct is zero, then I think I would suggest
> we change the page flag to indicate the page has been initialized.

Ingo or HPA,

Did I implement this wrong or is there a way to get rid of the page flag
which is not going to impact normal operation? I don't want to put too
much more effort into this until I know we are stuck going this direction.
Currently, the expand() function has a relatively expensive checked
against the 2MiB aligned pfn's struct page. I do not know of a way to
eliminate that check against the other page as the first reference we
see for a page is in the middle of that 2MiB aligned range.

To identify this as an area of concern, we had booted with a simulator,
setting watch points on the struct page array region once the
Uninitialized flag was set and maintaining that until it was cleared.

Thanks,
Robin