Re: [PATCH v3 1/3] mm: document zone device struct page field usage

[Christoph Hellwig]

On Wed, Jul 24, 2019 at 04:26:58PM -0700, Ralph Campbell wrote:
> Struct page for ZONE_DEVICE private pages uses the page->mapping and
> and page->index fields while the source anonymous pages are migrated to
> device private memory. This is so rmap_walk() can find the page when
> migrating the ZONE_DEVICE private page back to system memory.
> ZONE_DEVICE pmem backed fsdax pages also use the page->mapping and
> page->index fields when files are mapped into a process address space.
> 
> Add comments to struct page and remove the unused "_zd_pad_1" field
> to make this more clear.

I still think we should also fix up the layout, and I haven't seen
a reply from Matthew justifying his curses for your patch that makes
the struct page layout actually match how it is used.