On Mon, 2 Jul 2018, John Hubbard wrote: > > If you establish a reference to a page then increase the page count. If > > the reference is a dma pin action also then increase the pinned count. > > > > That way you know how many of the references to the page are dma > > pins and you can correctly manage the state of the page if the dma pins go > > away. > > > > I think this sounds like what this patch already does, right? See: > __put_page_for_pinned_dma(), __get_page_for_pinned_dma(), and > pin_page_for_dma(). The locking seems correct to me, but I suspect it's > too heavyweight for such a hot path. But without adding a new put_user_page() > call, that was the best I could come up with. When I saw the patch it looked like you were avoiding to increment the page->count field. > What I'm hearing now from Jan and Michal is that the desired end result is > a separate API call, put_user_pages(), so that we can explicitly manage > these pinned pages. Certainly a good approach.
