On Fri, May 26, 2023 at 06:46:15PM +0200, David Hildenbrand wrote: > On 26.05.23 18:44, Matthew Wilcox wrote: > > On Fri, May 26, 2023 at 09:44:34AM -0600, Khalid Aziz wrote: > > > > Oh, I think I found it! pin_user_pages_remote() is called by > > > > vaddr_get_pfns(). If these are the pages you're concerned about, > > > > then the efficient way to do what you want is simply to call > > > > folio_maybe_dma_pinned(). Far more efficient than the current mess > > > > of total_mapcount(). > > > > > > vfio pinned pages triggered this change. Wouldn't checking refcounts against > > > mapcount provide a more generalized way of detecting non-migratable pages? > > > > Well, you changed the comment to say that we were concerned about > > long-term pins. If we are, than folio_maybe_dma_pinned() is how to test > > for long-term pins. If we want to skip pages which are short-term pinned, > > then we need to not change the comment, and keep using mapcount/refcount > > differences. > > > > folio_maybe_dma_pinned() is all about FOLL_PIN, not FOLL_LONGTERM. But according to our documentation, FOLL_LONGTERM implies FOLL_PIN. Anyway, right now, the code skips any pages which are merely FOLL_GET, so we'll skip fewer pages if we do only skip the FOLL_PIN ones, regardless if we'd prefer to only skip the FOLL_LONGTERM ones. > folio_maybe_dma_pinned() would skip migrating any page that has more than > 1024 references. (shared libraries?) True, but maybe we should be skipping any page with that many mappings, given how disruptive it is to the rest of the system to unmap a page from >1024 processes.
