On 06/20/2018 05:08 AM, Jan Kara wrote: > On Tue 19-06-18 11:11:48, John Hubbard wrote: >> On 06/19/2018 03:41 AM, Jan Kara wrote: >>> On Tue 19-06-18 02:02:55, Matthew Wilcox wrote: >>>> On Tue, Jun 19, 2018 at 10:29:49AM +0200, Jan Kara wrote: [...] >>> I'm also still pondering the idea of inserting a "virtual" VMA into vma >>> interval tree in the inode - as the GUP references are IMHO closest to an >>> mlocked mapping - and that would achieve all the functionality we need as >>> well. I just didn't have time to experiment with it. >> >> How would this work? Would it have the same virtual address range? And how >> does it avoid the problems we've been discussing? Sorry to be a bit slow >> here. :) > > The range covered by the virtual mapping would be the one sent to > get_user_pages() to get page references. And then we would need to teach > page_mkclean() to check for these virtual VMAs and block / skip / report > (different situations would need different behavior) such page. But this > second part is the same regardless how we identify a page that is pinned by > get_user_pages(). OK. That neatly avoids the need a new page flag, I think. But of course it is somewhat more extensive to implement. Sounds like something to keep in mind, in case it has better tradeoffs than the direction I'm heading so far. >>> And then there's the aspect that both these approaches are a bit too >>> heavyweight for some get_user_pages_fast() users (e.g. direct IO) - Al Viro >>> had an idea to use page lock for that path but e.g. fs/direct-io.c would have >>> problems due to lock ordering constraints (filesystem ->get_block would >>> suddently get called with the page lock held). But we can probably leave >>> performance optimizations for phase two. >> >> >> So I assume that phase one would be to apply this approach only to >> get_user_pages_longterm. (Please let me know if that's wrong.) > > No, I meant phase 1 would be to apply this to all get_user_pages() flavors. > Then phase 2 is to try to find a way to make get_user_pages_fast() fast > again. And then in parallel to that, we also need to find a way for > get_user_pages_longterm() to signal to the user pinned pages must be > released soon. Because after phase 1 pinned pages will block page > writeback and such system won't oops but will become unusable > sooner rather than later. And again this problem needs to be solved > regardless of a mechanism of identifying pinned pages. > OK, thanks, that does help. I had the priorities of these get_user_pages*() changes all scrambled, but between your and Dan's explanation, I finally understand the preferred ordering of this work.
