Re: [PATCH 0/2] s390: Remove uses of page->index

[Matthew Wilcox <willy@infradead.org>]

On Thu, Dec 19, 2024 at 05:33:32PM +0100, David Hildenbrand wrote:
> On 19.12.24 17:22, Matthew Wilcox (Oracle) wrote:
> > These two patches compile ... I can promise nothing more than that.
> > 
> > David suggested to me that the gmap code really should be using ptdesc,
> > and I think I agree with him.  The vsie code looks quite different
> > and probably shouldn't be using a ptdesc, but we can use page->private
> > instead of page->index.  It's not yet clear to me if we'll ever manage
> > to get rid of page->private.
> 
> Just curious, does that mean that memdesc would always contain these
> additional 8 bytes?

Eventually we'll have a choice to make.

1. Shrink struct page to 8 bytes, but we'll need to allocate a 32 byte
   memdesc for all the current users of page->private
2. Shrink struct page to 16 bytes

I genuinely don't know which will be better for the whole system.

If you're asking because we can defer some of the mapcount
work by using the 8 bytes of page->private to store mapcount, then
yes, let's do that.