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[2003:cb:c738:3100:8133:26cf:7877:94aa]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-38a8e38c6dbsm15361588f8f.55.2025.01.14.06.51.09 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 14 Jan 2025 06:51:11 -0800 (PST) Message-ID: <9be14739-82cc-42fb-bb38-a868231cdda6@redhat.com> Date: Tue, 14 Jan 2025 15:51:09 +0100 Precedence: bulk X-Mailing-List: linux-doc@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v6 10/26] mm/mm_init: Move p2pdma page refcount initialisation to p2pdma To: Alistair Popple , akpm@linux-foundation.org, dan.j.williams@intel.com, linux-mm@kvack.org Cc: alison.schofield@intel.com, lina@asahilina.net, zhang.lyra@gmail.com, gerald.schaefer@linux.ibm.com, vishal.l.verma@intel.com, dave.jiang@intel.com, logang@deltatee.com, bhelgaas@google.com, jack@suse.cz, jgg@ziepe.ca, catalin.marinas@arm.com, will@kernel.org, mpe@ellerman.id.au, npiggin@gmail.com, dave.hansen@linux.intel.com, ira.weiny@intel.com, willy@infradead.org, djwong@kernel.org, tytso@mit.edu, linmiaohe@huawei.com, peterx@redhat.com, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linuxppc-dev@lists.ozlabs.org, nvdimm@lists.linux.dev, linux-cxl@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-ext4@vger.kernel.org, linux-xfs@vger.kernel.org, jhubbard@nvidia.com, hch@lst.de, david@fromorbit.com, chenhuacai@kernel.org, kernel@xen0n.name, loongarch@lists.linux.dev References: <4f3fa1bb9cf4402d6131d0902472d8e9bae52f88.1736488799.git-series.apopple@nvidia.com> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 10.01.25 07:00, Alistair Popple wrote: > Currently ZONE_DEVICE page reference counts are initialised by core > memory management code in __init_zone_device_page() as part of the > memremap() call which driver modules make to obtain ZONE_DEVICE > pages. This initialises page refcounts to 1 before returning them to > the driver. > > This was presumably done because it drivers had a reference of sorts > on the page. It also ensured the page could always be mapped with > vm_insert_page() for example and would never get freed (ie. have a > zero refcount), freeing drivers of manipulating page reference counts. > > However it complicates figuring out whether or not a page is free from > the mm perspective because it is no longer possible to just look at > the refcount. Instead the page type must be known and if GUP is used a > secondary pgmap reference is also sometimes needed. > > To simplify this it is desirable to remove the page reference count > for the driver, so core mm can just use the refcount without having to > account for page type or do other types of tracking. This is possible > because drivers can always assume the page is valid as core kernel > will never offline or remove the struct page. > > This means it is now up to drivers to initialise the page refcount as > required. P2PDMA uses vm_insert_page() to map the page, and that > requires a non-zero reference count when initialising the page so set > that when the page is first mapped. > > Signed-off-by: Alistair Popple > Reviewed-by: Dan Williams > LGTM Acked-by: David Hildenbrand -- Cheers, David / dhildenb