From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pl1-f198.google.com (mail-pl1-f198.google.com [209.85.214.198]) by kanga.kvack.org (Postfix) with ESMTP id 934888E0004 for ; Sat, 8 Dec 2018 00:18:15 -0500 (EST) Received: by mail-pl1-f198.google.com with SMTP id g7so4226477plp.10 for ; Fri, 07 Dec 2018 21:18:15 -0800 (PST) Received: from bombadil.infradead.org (bombadil.infradead.org. [2607:7c80:54:e::133]) by mx.google.com with ESMTPS id i6si4511957plt.290.2018.12.07.21.18.13 for (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Fri, 07 Dec 2018 21:18:14 -0800 (PST) Date: Fri, 7 Dec 2018 21:18:10 -0800 From: Matthew Wilcox Subject: Re: [PATCH 1/2] mm: introduce put_user_page*(), placeholder versions Message-ID: <20181208051810.GA24118@bombadil.infradead.org> 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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <3c4d46c0-aced-f96f-1bf3-725d02f11b60@nvidia.com> Sender: owner-linux-mm@kvack.org List-ID: To: John Hubbard Cc: Jerome Glisse , Dan Williams , John Hubbard , Andrew Morton , Linux MM , Jan Kara , tom@talpey.com, Al Viro , benve@cisco.com, Christoph Hellwig , Christopher Lameter , "Dalessandro, Dennis" , Doug Ledford , Jason Gunthorpe , Michal Hocko , mike.marciniszyn@intel.com, rcampbell@nvidia.com, Linux Kernel Mailing List , linux-fsdevel 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. 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. 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. > diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h > index 5ed8f6292a53..1c789e324da8 100644 > --- a/include/linux/mm_types.h > +++ b/include/linux/mm_types.h > @@ -182,6 +182,9 @@ struct page { > /* Usage count. *DO NOT USE DIRECTLY*. See page_ref.h */ > atomic_t _refcount; > > + /* DMA usage count. See get_user_pages*(), put_user_page*(). */ > + atomic_t _dma_pinned_count; > + > #ifdef CONFIG_MEMCG > struct mem_cgroup *mem_cgroup; > #endif > > > ...because after all, the reason this is so difficult is that this fix has to work > in pretty much every configuration. get_user_pages() use is widespread, it's a very > general facility, and...it needs fixing. And we're out of space. > > I'm going to send out an updated RFC that shows the latest, and I think it's going > to include the above. > > -- > thanks, > John Hubbard > NVIDIA >