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[2003:cb:c73b:5600:c716:d8e0:609d:ae92]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-388c8046da0sm12634055f8f.74.2024.12.17.14.14.42 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 17 Dec 2024 14:14:44 -0800 (PST) Message-ID: <359a1cf2-c5b0-4682-ba3c-980d77c4cfdb@redhat.com> Date: Tue, 17 Dec 2024 23:14:42 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4 10/25] 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: 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 References: From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: 0040A160014 X-Stat-Signature: q3s1dhq81st66maije8xdx837rhk773s X-Rspam-User: X-HE-Tag: 1734473667-698755 X-HE-Meta: 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 I0Y/AYfu 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 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: List-Subscribe: List-Unsubscribe: On 17.12.24 06:12, 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. It probably dates back to copying that code from other zone-init code where we (a) Treat all available-at-boot memory as allocated before we release it to the buddy (b) Treat all hotplugged memory as allocated until we release it to the buddy As a side note, I'm working on converting (b) -- PageOffline pages -- to have a refcount of 0 ("frozen"). > > 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 > > --- > > Changes since v2: > > - Initialise the page refcount for all pages covered by the kaddr > --- > drivers/pci/p2pdma.c | 13 +++++++++++-- > mm/memremap.c | 17 +++++++++++++---- > mm/mm_init.c | 22 ++++++++++++++++++---- > 3 files changed, 42 insertions(+), 10 deletions(-) > > diff --git a/drivers/pci/p2pdma.c b/drivers/pci/p2pdma.c > index 0cb7e0a..04773a8 100644 > --- a/drivers/pci/p2pdma.c > +++ b/drivers/pci/p2pdma.c > @@ -140,13 +140,22 @@ static int p2pmem_alloc_mmap(struct file *filp, struct kobject *kobj, > rcu_read_unlock(); > > for (vaddr = vma->vm_start; vaddr < vma->vm_end; vaddr += PAGE_SIZE) { > - ret = vm_insert_page(vma, vaddr, virt_to_page(kaddr)); > + struct page *page = virt_to_page(kaddr); > + > + /* > + * Initialise the refcount for the freshly allocated page. As > + * we have just allocated the page no one else should be > + * using it. > + */ > + VM_WARN_ON_ONCE_PAGE(!page_ref_count(page), page); > + set_page_count(page, 1); > + ret = vm_insert_page(vma, vaddr, page); > if (ret) { > gen_pool_free(p2pdma->pool, (uintptr_t)kaddr, len); > return ret; > } > percpu_ref_get(ref); > - put_page(virt_to_page(kaddr)); > + put_page(page); > kaddr += PAGE_SIZE; > len -= PAGE_SIZE; > } > diff --git a/mm/memremap.c b/mm/memremap.c > index 40d4547..07bbe0e 100644 > --- a/mm/memremap.c > +++ b/mm/memremap.c > @@ -488,15 +488,24 @@ void free_zone_device_folio(struct folio *folio) > folio->mapping = NULL; > folio->page.pgmap->ops->page_free(folio_page(folio, 0)); > > - if (folio->page.pgmap->type != MEMORY_DEVICE_PRIVATE && > - folio->page.pgmap->type != MEMORY_DEVICE_COHERENT) > + switch (folio->page.pgmap->type) { > + case MEMORY_DEVICE_PRIVATE: > + case MEMORY_DEVICE_COHERENT: > + put_dev_pagemap(folio->page.pgmap); > + break; > + > + case MEMORY_DEVICE_FS_DAX: > + case MEMORY_DEVICE_GENERIC: > /* > * Reset the refcount to 1 to prepare for handing out the page > * again. > */ > folio_set_count(folio, 1); > - else > - put_dev_pagemap(folio->page.pgmap); > + break; > + > + case MEMORY_DEVICE_PCI_P2PDMA: > + break; > + } > } > > void zone_device_page_init(struct page *page) > diff --git a/mm/mm_init.c b/mm/mm_init.c > index 24b68b4..f021e63 100644 > --- a/mm/mm_init.c > +++ b/mm/mm_init.c > @@ -1017,12 +1017,26 @@ static void __ref __init_zone_device_page(struct page *page, unsigned long pfn, > } > > /* > - * ZONE_DEVICE pages are released directly to the driver page allocator > - * which will set the page count to 1 when allocating the page. > + * ZONE_DEVICE pages other than MEMORY_TYPE_GENERIC and > + * MEMORY_TYPE_FS_DAX pages are released directly to the driver page > + * allocator which will set the page count to 1 when allocating the > + * page. > + * > + * MEMORY_TYPE_GENERIC and MEMORY_TYPE_FS_DAX pages automatically have > + * their refcount reset to one whenever they are freed (ie. after > + * their refcount drops to 0). > */ > - if (pgmap->type == MEMORY_DEVICE_PRIVATE || > - pgmap->type == MEMORY_DEVICE_COHERENT) > + switch (pgmap->type) { > + case MEMORY_DEVICE_PRIVATE: > + case MEMORY_DEVICE_COHERENT: > + case MEMORY_DEVICE_PCI_P2PDMA: > set_page_count(page, 0); > + break; > + > + case MEMORY_DEVICE_FS_DAX: > + case MEMORY_DEVICE_GENERIC: > + break; > + } > } > > /* But that's a bit weird: we call __init_single_page()->init_page_count() to initialize it to 1, to then set it back to 0. Maybe we can just pass to __init_single_page() the refcount we want to have directly? Can be a patch on top of course. Apart from that Acked-by: David Hildenbrand -- Cheers, David / dhildenb