On Fri, Mar 08, 2019 at 02:48:45PM -0500, Andrea Arcangeli wrote: > Hello Jeson, > > On Fri, Mar 08, 2019 at 04:50:36PM +0800, Jason Wang wrote: > > Just to make sure I understand here. For boosting through huge TLB, do > > you mean we can do that in the future (e.g by mapping more userspace > > pages to kenrel) or it can be done by this series (only about three 4K > > pages were vmapped per virtqueue)? > > When I answered about the advantages of mmu notifier and I mentioned > guaranteed 2m/gigapages where available, I overlooked the detail you > were using vmap instead of kmap. So with vmap you're actually doing > the opposite, it slows down the access because it will always use a 4k > TLB even if QEMU runs on THP or gigapages hugetlbfs. > > If there's just one page (or a few pages) in each vmap there's no need > of vmap, the linearity vmap provides doesn't pay off in such > case. > > So likely there's further room for improvement here that you can > achieve in the current series by just dropping vmap/vunmap. > > You can just use kmap (or kmap_atomic if you're in preemptible > section, should work from bh/irq). > > In short the mmu notifier to invalidate only sets a "struct page * > userringpage" pointer to NULL without calls to vunmap. > > In all cases immediately after gup_fast returns you can always call > put_page immediately (which explains why I'd like an option to drop > FOLL_GET from gup_fast to speed it up). By the way this is on my todo list, i want to merge HMM page snapshoting with gup code which means mostly allowing to gup_fast without taking a reference on the page (so without FOLL_GET). I hope to get to that some- time before summer. > > Then you can check the sequence_counter and inc/dec counter increased > by _start/_end. That will tell you if the page you got and you called > put_page to immediately unpin it or even to free it, cannot go away > under you until the invalidate is called. > > If sequence counters and counter tells that gup_fast raced with anyt > mmu notifier invalidate you can just repeat gup_fast. Otherwise you're > done, the page cannot go away under you, the host virtual to host > physical mapping cannot change either. And the page is not pinned > either. So you can just set the "struct page * userringpage = page" > where "page" was the one setup by gup_fast. > > When later the invalidate runs, you can just call set_page_dirty if > gup_fast was called with "write = 1" and then you clear the pointer > "userringpage = NULL". > > When you need to read/write to the memory > kmap/kmap_atomic(userringpage) should work. > > In short because there's no hardware involvement here, the established > mapping is just the pointer to the page, there is no need of setting > up any pagetables or to do any TLB flushes (except on 32bit archs if > the page is above the direct mapping but it never happens on 64bit > archs). Agree. The vmap is probably overkill if you only have a handfull of them kmap will be faster. Cheers, Jérôme
