Re: [PATCH RFC 0/6] Allow file-backed or shared device private pages

[Matthew Wilcox]

On Sun, Mar 16, 2025 at 11:04:07PM -0700, Christoph Hellwig wrote:
> On Sun, Mar 16, 2025 at 03:29:23PM +1100, Alistair Popple wrote:
> > This series lifts that restriction by allowing ZONE_DEVICE private pages to
> > exist in the pagecache.
> 
> You'd better provide a really good argument for why we'd even want
> to do that.  So far this cover letter fails to do that.

Alistair and I discussed this during his session at LSFMM today.
Here's what I think we agreed to.

The use case is a file containing a potentially very large data set.
Some phases of processing that data set are best done on the GPU, other
phases on the CPU.  We agreed that shared writable mmap was not actually
needed (it might need to be supported for correctness, but it's not a
performance requirement).

So, there's no need to put DEVICE_PRIVATE pages in the page cache.
Instead the GPU will take a copy of the page(s).  We agreed that there
will have to be some indication (probably a folio flag?) that the GPU has
or may have a copy of (some of) the folio so that it can be invalidated
if the page is removed due to truncation / eviction.

Alistair, let me know if that's not what you think we agreed to ;-)