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Fri, 06 Nov 2020 02:28:10 -0800 (PST) MIME-Version: 1.0 References: <20201104140023.GQ36674@ziepe.ca> <20201104162125.GA13007@infradead.org> <20201104163758.GA17425@infradead.org> <20201104164119.GA18218@infradead.org> <20201104181708.GU36674@ziepe.ca> <20201105092524.GQ401619@phenom.ffwll.local> <20201105124950.GZ36674@ziepe.ca> <7ae3486d-095e-cf4e-6b0f-339d99709996@nvidia.com> In-Reply-To: From: Daniel Vetter Date: Fri, 6 Nov 2020 11:27:59 +0100 Message-ID: Subject: Re: [PATCH v5 05/15] mm/frame-vector: Use FOLL_LONGTERM To: John Hubbard , Thomas Hellstrom Cc: Jason Gunthorpe , Christoph Hellwig , "J??r??me Glisse" , linux-samsung-soc , Jan Kara , Pawel Osciak , KVM list , Mauro Carvalho Chehab , LKML , DRI Development , Tomasz Figa , Linux MM , Kyungmin Park , Daniel Vetter , Andrew Morton , Marek Szyprowski , Dan Williams , Linux ARM , "open list:DMA BUFFER SHARING FRAMEWORK" Content-Type: text/plain; charset="UTF-8" X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Fri, Nov 6, 2020 at 11:01 AM Daniel Vetter wrote: > > On Fri, Nov 6, 2020 at 5:08 AM John Hubbard wrote: > > > > On 11/5/20 4:49 AM, Jason Gunthorpe wrote: > > > On Thu, Nov 05, 2020 at 10:25:24AM +0100, Daniel Vetter wrote: > > >>> /* > > >>> * If we can't determine whether or not a pte is special, then fail immediately > > >>> * for ptes. Note, we can still pin HugeTLB and THP as these are guaranteed not > > >>> * to be special. > > >>> * > > >>> * For a futex to be placed on a THP tail page, get_futex_key requires a > > >>> * get_user_pages_fast_only implementation that can pin pages. Thus it's still > > >>> * useful to have gup_huge_pmd even if we can't operate on ptes. > > >>> */ > > >> > > >> We support hugepage faults in gpu drivers since recently, and I'm not > > >> seeing a pud_mkhugespecial anywhere. So not sure this works, but probably > > >> just me missing something again. > > > > > > It means ioremap can't create an IO page PUD, it has to be broken up. > > > > > > Does ioremap even create anything larger than PTEs? > > gpu drivers also tend to use vmf_insert_pfn* directly, so we can do > on-demand paging and move buffers around. From what I glanced for > lowest level we to the pte_mkspecial correctly (I think I convinced > myself that vm_insert_pfn does that), but for pud/pmd levels it seems > just yolo. So I dug around a bit more and ttm sets PFN_DEV | PFN_MAP to get past the various pft_t_devmap checks (see e.g. vmf_insert_pfn_pmd_prot()). x86-64 has ARCH_HAS_PTE_DEVMAP, and gup.c seems to handle these specially, but frankly I got totally lost in what this does. The comment above the pfn_t_devmap check makes me wonder whether doing this is correct or not. Also adding Thomas Hellstrom, who implemented the huge map support in ttm. -Daniel > remap_pfn_range seems to indeed split down to pte level always. > > > From my reading, yes. See ioremap_try_huge_pmd(). > > The ioremap here shouldn't matter, since this is for kernel-internal > mappings. So that's all fine I think. > -Daniel > -- > Daniel Vetter > Software Engineer, Intel Corporation > http://blog.ffwll.ch -- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch