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[2003:cb:c738:3100:8133:26cf:7877:94aa]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-38a8e38428bsm15089893f8f.37.2025.01.14.06.59.31 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 14 Jan 2025 06:59:33 -0800 (PST) Message-ID: <927f9cef-3f97-4bef-b6d8-53e6ef1b78a8@redhat.com> Date: Tue, 14 Jan 2025 15:59:31 +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 11/26] mm: Allow compound zone device pages 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, Jason Gunthorpe References: <9210f90866fef17b54884130fb3e55ab410dd015.1736488799.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: > Zone device pages are used to represent various type of device memory > managed by device drivers. Currently compound zone device pages are > not supported. This is because MEMORY_DEVICE_FS_DAX pages are the only > user of higher order zone device pages and have their own page > reference counting. > > A future change will unify FS DAX reference counting with normal page > reference counting rules and remove the special FS DAX reference > counting. Supporting that requires compound zone device pages. > > Supporting compound zone device pages requires compound_head() to > distinguish between head and tail pages whilst still preserving the > special struct page fields that are specific to zone device pages. > > A tail page is distinguished by having bit zero being set in > page->compound_head, with the remaining bits pointing to the head > page. For zone device pages page->compound_head is shared with > page->pgmap. > > The page->pgmap field is common to all pages within a memory section. > Therefore pgmap is the same for both head and tail pages and can be > moved into the folio and we can use the standard scheme to find > compound_head from a tail page. The more relevant thing is that the pgmap field must be common to all pages in a folio, even if a folio exceeds memory sections (e.g., 128 MiB on x86_64 where we have 1 GiB folios). > > Signed-off-by: Alistair Popple > Reviewed-by: Jason Gunthorpe > Reviewed-by: Dan Williams > > --- > > Changes for v4: > - Fix build breakages reported by kernel test robot > > Changes since v2: > > - Indentation fix > - Rename page_dev_pagemap() to page_pgmap() > - Rename folio _unused field to _unused_pgmap_compound_head > - s/WARN_ON/VM_WARN_ON_ONCE_PAGE/ > > Changes since v1: > > - Move pgmap to the folio as suggested by Matthew Wilcox > --- [...] > static inline bool folio_is_device_coherent(const struct folio *folio) > diff --git a/include/linux/migrate.h b/include/linux/migrate.h > index 29919fa..61899ec 100644 > --- a/include/linux/migrate.h > +++ b/include/linux/migrate.h > @@ -205,8 +205,8 @@ struct migrate_vma { > unsigned long end; > > /* > - * Set to the owner value also stored in page->pgmap->owner for > - * migrating out of device private memory. The flags also need to > + * Set to the owner value also stored in page_pgmap(page)->owner > + * for migrating out of device private memory. The flags also need to > * be set to MIGRATE_VMA_SELECT_DEVICE_PRIVATE. > * The caller should always set this field when using mmu notifier > * callbacks to avoid device MMU invalidations for device private > diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h > index df8f515..54b59b8 100644 > --- a/include/linux/mm_types.h > +++ b/include/linux/mm_types.h > @@ -129,8 +129,11 @@ struct page { > unsigned long compound_head; /* Bit zero is set */ > }; > struct { /* ZONE_DEVICE pages */ > - /** @pgmap: Points to the hosting device page map. */ > - struct dev_pagemap *pgmap; > + /* > + * The first word is used for compound_head or folio > + * pgmap > + */ > + void *_unused_pgmap_compound_head; > void *zone_device_data; > /* > * ZONE_DEVICE private pages are counted as being > @@ -299,6 +302,7 @@ typedef struct { > * @_refcount: Do not access this member directly. Use folio_ref_count() > * to find how many references there are to this folio. > * @memcg_data: Memory Control Group data. > + * @pgmap: Metadata for ZONE_DEVICE mappings > * @virtual: Virtual address in the kernel direct map. > * @_last_cpupid: IDs of last CPU and last process that accessed the folio. > * @_entire_mapcount: Do not use directly, call folio_entire_mapcount(). > @@ -337,6 +341,7 @@ struct folio { > /* private: */ > }; > /* public: */ > + struct dev_pagemap *pgmap; Agreed, that should work. Acked-by: David Hildenbrand -- Cheers, David / dhildenb