On 07/01/2018 11:34 PM, Leon Romanovsky wrote: > On Sun, Jul 01, 2018 at 11:10:04PM -0700, John Hubbard wrote: >> On 07/01/2018 10:52 PM, Leon Romanovsky wrote: >>> On Thu, Jun 28, 2018 at 11:17:43AM +0200, Jan Kara wrote: >>>> On Wed 27-06-18 19:42:01, John Hubbard wrote: >>>>> On 06/27/2018 10:02 AM, Jan Kara wrote: >>>>>> On Wed 27-06-18 08:57:18, Jason Gunthorpe wrote: >>>>>>> On Wed, Jun 27, 2018 at 02:42:55PM +0200, Jan Kara wrote: >>>>>>>> On Wed 27-06-18 13:59:27, Michal Hocko wrote: >>>>>>>>> On Wed 27-06-18 13:53:49, Jan Kara wrote: >>>>>>>>>> On Wed 27-06-18 13:32:21, Michal Hocko wrote: >>>>>>>>> [...] >>>>> One question though: I'm still vague on the best actions to take in the >>>>> following functions: >>>>> >>>>> page_mkclean_one >>>>> try_to_unmap_one >>>>> >>>>> At the moment, they are both just doing an evil little early-out: >>>>> >>>>> if (PageDmaPinned(page)) >>>>> return false; >>>>> >>>>> ...but we talked about maybe waiting for the condition to clear, instead? >>>>> Thoughts? >>>> >>>> What needs to happen in page_mkclean() depends on the caller. Most of the >>>> callers really need to be sure the page is write-protected once >>>> page_mkclean() returns. Those are: >>>> >>>> pagecache_isize_extended() >>>> fb_deferred_io_work() >>>> clear_page_dirty_for_io() if called for data-integrity writeback - which >>>> is currently known only in its caller (e.g. write_cache_pages()) where >>>> it can be determined as wbc->sync_mode == WB_SYNC_ALL. Getting this >>>> information into page_mkclean() will require some plumbing and >>>> clear_page_dirty_for_io() has some 50 callers but it's doable. >>>> >>>> clear_page_dirty_for_io() for cleaning writeback (wbc->sync_mode != >>>> WB_SYNC_ALL) can just skip pinned pages and we probably need to do that as >>>> otherwise memory cleaning would get stuck on pinned pages until RDMA >>>> drivers release its pins. >>> >>> Sorry for naive question, but won't it create too much dirty pages >>> so writeback will be called "non-stop" to rebalance watermarks without >>> ability to progress? >>> >> >> That is an interesting point. >> >> Holding off page writeback of this region does seem like it could cause >> problems under memory pressure. Maybe adjusting the watermarks so that we >> tell the writeback system, "all is well, just ignore this region until >> we're done with it" might help? Any ideas here are welcome... > > AFAIR, it is per-zone, so the solution to count dirty-but-untouchable > number of pages to take them into account for accounting can work, but > it seems like an overkill. Can we create special ZONE for such gup > pages, or this is impossible too? > Let's see what Michal and others prefer. The zone idea intrigues me. thanks, -- John Hubbard NVIDIA
