On Wed, Mar 26, 2025 at 02:14:59AM +0000, Matthew Wilcox wrote: > 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. Thanks for writing up this summary. > > 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). Right. I agree we don't currently have a good usecase for writeback so the next revision will definitely only support read-only access. > 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 ;-) That all looks about right. I think the flag/indication is a good idea and is probably the best solution, but I will need to write the code to truely convince myself of that :-)
