From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Subject: Re: [PATCH 1/2] mm: introduce put_user_page*(), placeholder versions To: Matthew Wilcox CC: Jerome Glisse , Dan Williams , John Hubbard , Andrew Morton , Linux MM , Jan Kara , , Al Viro , , Christoph Hellwig , Christopher Lameter , "Dalessandro, Dennis" , Doug Ledford , Jason Gunthorpe , Michal Hocko , , , Linux Kernel Mailing List , linux-fsdevel References: <3c91d335-921c-4704-d159-2975ff3a5f20@nvidia.com> <20181205011519.GV10377@bombadil.infradead.org> <20181205014441.GA3045@redhat.com> <59ca5c4b-fd5b-1fc6-f891-c7986d91908e@nvidia.com> <7b4733be-13d3-c790-ff1b-ac51b505e9a6@nvidia.com> <20181207191620.GD3293@redhat.com> <3c4d46c0-aced-f96f-1bf3-725d02f11b60@nvidia.com> <20181208051810.GA24118@bombadil.infradead.org> From: John Hubbard Message-ID: <7491557a-4fe9-71fd-e329-e84e9e830929@nvidia.com> Date: Wed, 12 Dec 2018 11:13:26 -0800 MIME-Version: 1.0 In-Reply-To: <20181208051810.GA24118@bombadil.infradead.org> Content-Type: text/plain; charset="utf-8" Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: On 12/7/18 9:18 PM, Matthew Wilcox wrote: > On Fri, Dec 07, 2018 at 04:52:42PM -0800, John Hubbard wrote: >> I see. OK, HMM has done an efficient job of mopping up unused fields, and now we are >> completely out of space. At this point, after thinking about it carefully, it seems clear >> that it's time for a single, new field: > > Sorry for not replying earlier; I'm travelling and have had trouble > keeping on top of my mail. Hi Matthew, > > Adding this field will grow struct page by 4-8 bytes, so it will no > longer be 64 bytes. This isn't an acceptable answer. I had to ask, though, just in case the historical rules might no longer be ask pressing. But OK. > > We have a few options for bits. One is that we have (iirc) two > bits available in page->flags on 32-bit. That'll force a few more > configurations into using _last_cpupid and/or page_ext. I'm not a huge > fan of this approach. > > The second is to use page->lru.next bit 1. This requires some care > because m68k allows misaligned pointers. If the list_head that it's > joined to is misaligned, we'll be in trouble. This can get tricky because > some pages are attached to list_heads which are on the stack ... and I > don't think gcc guarantees __aligned attributes work for stack variables. > > The third is to use page->lru.prev bit 0. We'd want to switch pgmap > and hmm_data around to make this work, and we'd want to record this > in mm_types.h so nobody tries to use a field which aliases with > page->lru.prev and has bit 0 set on a page which can be mapped to > userspace (which I currently believe to be true). > > The fourth is to use a bit in page->flags for 64-bit and a bit in > page_ext->flags for 32-bit. Or we could get rid of page_ext and grow > struct page with a ->flags2 on 32-bit. > > Fifth, it isn't clear to me how many bits might be left in ->_last_cpupid > at this point, and perhaps there's scope for using a bit in there. > Thanks for taking the time to collect and explain all of this, I'm stashing it away as I'm sure it will come up again. The latest approach to the gup/dma problem here might, or might not, actually need a single page bit. I'll know in a day or two. -- thanks, John Hubbard NVIDIA