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Tsirkin" Cc: linux-kernel@vger.kernel.org, "David Hildenbrand (Arm)" , Gregory Price , Zi Yan , Pankaj Gupta , Jason Wang , Xuan Zhuo , Eugenio =?ISO-8859-1?Q?P=E9rez?= , Lorenzo Stoakes , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Brendan Jackman , Johannes Weiner , Alexander Duyck , virtualization@lists.linux.dev, linux-mm@kvack.org Subject: Re: [PATCH v3] mm: page_reporting: allow driver to set batch capacity Message-Id: <20260705011635.9974cac7f0defec9961e85c4@linux-foundation.org> In-Reply-To: <444c24cf39f3f3620fc90ef4695bd6b0979f4c4b.1783232420.git.mst@redhat.com> References: <444c24cf39f3f3620fc90ef4695bd6b0979f4c4b.1783232420.git.mst@redhat.com> X-Mailer: Sylpheed 3.8.0beta1 (GTK+ 2.24.33; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-Rspam-User: X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: 253E5100006 X-Stat-Signature: c15xtnjwfz4qishsps68w7c6c4j53rcw X-HE-Tag: 1783239397-67738 X-HE-Meta: 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 S6sytCLH H10/RyaylS2FQPCePIvfV9GmQxxjyihpqkCHqT8yM+KVRChpKG1xKQXvIEuleBztiX3p7oKhQeDQpFhno/75o9egKDx7rGWv/v5uYvtOiOdkr6fM7gUNEdKiWJZo4Lz7Bqx3kzLxLfRFjAeON0x0ZKgGkICr4lYItOoGHZS7xCQOunjkaifJ/Cc9BC1bFhLvVt44OIqxTwO5k22mCPJ8vgbhSmpQ9iX0ucDUgBToC41y1MSmAk8PxLqsbs+j62Ho8XKqHRjstUmVv4lsplo+fSyJLTve/ECMPsp9n723dU0LCbfg6Kgt7Eq4myBAT3OciAQ09Ukc2uno3DoTNzexgOtO97hk7f9zUl55UlF1KZ7zxWuVAUBHY27SEc+WUufAQicgA Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Sun, 5 Jul 2026 02:25:13 -0400 "Michael S. Tsirkin" wrote: > At the moment, if a virtio balloon device has a page reporting vq but > its size is < PAGE_REPORTING_CAPACITY (32), the balloon driver fails > probe. > > But, there's no way for host to know this value, so it can easily > create a smaller vq and suddenly adding the reporting capability > to the device makes all of the driver fail. Not pretty. > > Add a capacity field to page_reporting_dev_info so drivers can > control the maximum number of pages per report batch. > > In virtio-balloon, set the capacity to the reporting virtqueue size, > letting page_reporting adapt to whatever the device provides. > > Capacity need not be a power of two. Code previously called out > division by PAGE_REPORTING_CAPACITY as cheap since it was a power > of 2, but no performance difference was observed with non-power-of-2 > values. > > If capacity is 0 or exceeds PAGE_REPORTING_CAPACITY, it defaults > to PAGE_REPORTING_CAPACITY. The 0 check and the clamping is done in > page_reporting_register(), before the reporting work is scheduled, > so we never get division by 0. Thanks. What's the priority here? Should we fix 7.2? Earlier? It seems that Sashiko has found a pre-existing issue, a hard-to-hit error path thing: https://sashiko.dev/#/patchset/444c24cf39f3f3620fc90ef4695bd6b0979f4c4b.1783232420.git.mst@redhat.com