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Wysocki" , Danilo Krummrich , Andrew Morton , David Hildenbrand , Lorenzo Stoakes , Keith Busch , Jens Axboe , Christoph Hellwig , Sagi Grimberg , houtao1@huawei.com Subject: Re: [PATCH 01/13] PCI/P2PDMA: Release the per-cpu ref of pgmap when vm_insert_page() fails Message-ID: <20260109150342.GA544448@bhelgaas> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspamd-Queue-Id: 30DDBA0008 X-Rspamd-Server: rspam06 X-Stat-Signature: 5nr4aqzwzefkp31tp8k458y8egpajrng X-Rspam-User: X-HE-Tag: 1767971025-840818 X-HE-Meta: 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 bEdf5jNp yDX2WohnWGLKjFCh1hlI4jsrXSxHQPvsdj2mqk16uz9LdliCuC90c+BYh3i8oYKkb0jzQGba080WNNpSMHo68mGmooHNmdACEmHLSFtFeCRgXn8sTcBrg+dZWxT1F1Opom58MCLZrYNw5ro9ouv3rnDNnkDZRbEBNS9VtyJKtbUFqGC88eoZ/1pHiAE8e6htg3/JtJAUOn9+gJtoxEl/sWluYvJfxPWSGWc3Fe4tYqp/o0pF8MtYQWCIC12ayRKsOPSgGUeRyBNXeDx5bbSL/2AUKgezbtg9P2pquDAw+LV3HqyOSvrHqehsHVTqC2MDSgSoUY2qkkn1oKe9H5CXZnMcXX4yYw5UdD9qJXohjfFDbW4GxVnrt7a+NxQ== 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 Fri, Jan 09, 2026 at 11:41:51AM +1100, Alistair Popple wrote: > On 2026-01-09 at 02:55 +1100, Bjorn Helgaas wrote... > > On Thu, Jan 08, 2026 at 02:23:16PM +1100, Alistair Popple wrote: > > > On 2025-12-20 at 15:04 +1100, Hou Tao wrote... > > > > From: Hou Tao > > > > > > > > When vm_insert_page() fails in p2pmem_alloc_mmap(), p2pmem_alloc_mmap() > > > > doesn't invoke percpu_ref_put() to free the per-cpu ref of pgmap > > > > acquired after gen_pool_alloc_owner(), and memunmap_pages() will hang > > > > forever when trying to remove the PCIe device. > > > > > > > > Fix it by adding the missed percpu_ref_put(). > ... > > Looking at this again, I'm confused about why in the normal, non-error > > case, we do the percpu_ref_tryget_live_rcu(ref), followed by another > > percpu_ref_get(ref) for each page, followed by just a single > > percpu_ref_put() at the exit. > > > > So we do ref_get() "1 + number of pages" times but we only do a single > > ref_put(). Is there a loop of ref_put() for each page elsewhere? > > Right, the per-page ref_put() happens when the page is freed (ie. the struct > page refcount drops to zero) - in this case free_zone_device_folio() will call > p2pdma_folio_free() which has the corresponding percpu_ref_put(). I don't see anything that looks like a loop to call ref_put() for each page in free_zone_device_folio() or in p2pdma_folio_free(), but this is all completely out of my range, so I'll take your word for it :) Bjorn