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Fri, 16 Aug 2019 10:21:53 -0700 (PDT) MIME-Version: 1.0 References: <20190815180325.GA4920@redhat.com> <20190815194339.GC9253@redhat.com> <20190815203306.GB25517@redhat.com> <20190815204128.GI22970@mellanox.com> <20190816004053.GB9929@mellanox.com> <20190816122414.GC5412@mellanox.com> In-Reply-To: <20190816122414.GC5412@mellanox.com> From: Dan Williams Date: Fri, 16 Aug 2019 10:21:41 -0700 Message-ID: Subject: Re: [PATCH 04/15] mm: remove the pgmap field from struct hmm_vma_walk To: Jason Gunthorpe Cc: Jerome Glisse , Christoph Hellwig , Ben Skeggs , Felix Kuehling , Ralph Campbell , "linux-mm@kvack.org" , "nouveau@lists.freedesktop.org" , "dri-devel@lists.freedesktop.org" , "amd-gfx@lists.freedesktop.org" , "linux-kernel@vger.kernel.org" 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, Aug 16, 2019 at 5:24 AM Jason Gunthorpe wrote: > > On Thu, Aug 15, 2019 at 08:54:46PM -0700, Dan Williams wrote: > > > > However, this means we cannot do any processing of ZONE_DEVICE pages > > > outside the driver lock, so eg, doing any DMA map that might rely on > > > MEMORY_DEVICE_PCI_P2PDMA has to be done in the driver lock, which is > > > a bit unfortunate. > > > > Wouldn't P2PDMA use page pins? Not needing to hold a lock over > > ZONE_DEVICE page operations was one of the motivations for plumbing > > get_dev_pagemap() with a percpu-ref. > > hmm_range_fault() doesn't use page pins at all, so if a ZONE_DEVICE > page comes out of it then it needs to use another locking pattern. > > If I follow it all right: > > We can do a get_dev_pagemap inside the page_walk and touch the pgmap, > or we can do the 'device mutex && retry' pattern and touch the pgmap > in the driver, under that lock. > > However in all cases the current get_dev_pagemap()'s in the page walk > are not necessary, and we can delete them. Yes, as long as 'struct page' instances resulting from that lookup are not passed outside of that lock.