From mboxrd@z Thu Jan 1 00:00:00 1970 From: John Hubbard Subject: Re: [PATCH 2/2] mm: set PG_dma_pinned on get_user_pages*() Date: Tue, 19 Jun 2018 19:03:52 -0700 Message-ID: References: <311eba48-60f1-b6cc-d001-5cc3ed4d76a9@nvidia.com> <20180618081258.GB16991@lst.de> <3898ef6b-2fa0-e852-a9ac-d904b47320d5@nvidia.com> <0e6053b3-b78c-c8be-4fab-e8555810c732@nvidia.com> <20180619082949.wzoe42wpxsahuitu@quack2.suse.cz> <20180619090255.GA25522@bombadil.infradead.org> <20180619104142.lpilc6esz7w3a54i@quack2.suse.cz> <70001987-3938-d33e-11e0-de5b19ca3bdf@nvidia.com> Mime-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: Content-Language: en-US Sender: linux-kernel-owner@vger.kernel.org To: Dan Williams Cc: Jan Kara , Matthew Wilcox , Christoph Hellwig , Jason Gunthorpe , John Hubbard , Michal Hocko , Christopher Lameter , Linux MM , LKML , linux-rdma List-Id: linux-rdma@vger.kernel.org On 06/19/2018 06:57 PM, Dan Williams wrote: > On Tue, Jun 19, 2018 at 6:34 PM, John Hubbard wrote: >> On 06/19/2018 06:24 PM, Dan Williams wrote: >>> On Tue, Jun 19, 2018 at 11:11 AM, 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: >>> [..] >>>>> 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.) >>> >>> I think that's wrong, because get_user_pages_longterm() is only a >>> filesystem-dax avoidance mechanism, it's not trying to address all the >>> problems that Jan is talking about. I don't see any viable half-step >>> solutions. >>> >> >> OK, but in that case, I'm slightly confused by Jan's comment above, about leaving >> performance optimizations until phase two. Because that *is* a half-step approach: >> phase one, phase two. > > No, sorry, I might be confusing things. The half step is leaving > truncate broken, or my strawman that only addressed unmap. > >> Are you disagreeing with Jan, or are you suggesting "fix get_user_pages first, and >> leave get_user_pages_fast alone for now?" > > I'm agreeing with Jan, we need to fix page_mkclean() and > try_to_unmap() without regressing truncate behavior. > OK, perfect, thanks for clarifying. It all sounds consistent now. :)