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

[Ralph Campbell]

On 7/24/19 10:38 PM, Christoph Hellwig wrote:
> 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.

Well, I can kind of see this both ways since ZONE_DEVICE
MEMORY_DEVICE_DEVDAX and MEMORY_DEVICE_PCI_P2PDMA don't
seem to use the 3 words like MEMORY_DEVICE_PRIVATE and
MEMORY_DEVICE_FS_DAX.

I like v3 because not all of the ZONE_DEVICE types are handled
the same in regards to using the 3 words and there may be future
ZONE_DEVICE types that use the 3 words differently which might
require a union.

I agree, I would like to hear from Matthew on his thoughts.