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[80.230.68.31]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-47aa0960816sm13749921f8f.29.2026.07.05.01.25.55 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sun, 05 Jul 2026 01:25:57 -0700 (PDT) Date: Sun, 5 Jul 2026 04:25:53 -0400 From: "Michael S. Tsirkin" To: Andrew Morton 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: <20260705042313-mutt-send-email-mst@kernel.org> References: <444c24cf39f3f3620fc90ef4695bd6b0979f4c4b.1783232420.git.mst@redhat.com> <20260705011635.9974cac7f0defec9961e85c4@linux-foundation.org> MIME-Version: 1.0 In-Reply-To: <20260705011635.9974cac7f0defec9961e85c4@linux-foundation.org> X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: HI5Y9yHygyo2T966Z_2ZxyQk4s1TjYLdVpROnuFxOwg_1783239958 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=us-ascii Content-Disposition: inline X-Rspam-User: X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: 76F71140007 X-Stat-Signature: kkwbfztht1posbqe6451ow6zms6smzqm X-HE-Tag: 1783239968-509610 X-HE-Meta: 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 YmO9tXVH NcQQL2pwq4KumYRwtqyz34zNuKKMXH+3KIOyDtZ1fgmUDRd3R/8LFJft9M3NcJn3ayN4UxnLxXRg+b1jEtDLNFndom8nMzoPAi3X4KOHhp9Ijs7CSInC2LsL12YobhyLqvWh3q0BBy6ZIZ16LoTf8Bu1rFMFj3D3cu9f2U7paQ4SuiZx9lCJ8XprDexX9krIkVZKD8DcUhmBoviJpOtMLZWNmuCl+/sUkEQWM8dAfpuNk6ZRHVgZBlYfQtgWQ200FD0ObegqYsvefU5q7XnyX5QikqyJXYb6Uffr+3NDntMRKUG7C3nozRsPvtl+dt8+DblppFY/qC836jQ0j3t2em74KgYV6w5Dh3AyDHBEhZJhHkGzvOApgIuB1DSXriL+T6/u0/zdNOMEyIbHYSuFtr6c25DroFpGS2JODJjHcDfSE3GY= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Sun, Jul 05, 2026 at 01:16:35AM -0700, Andrew Morton wrote: > 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? If possible, I'd like it in 7.2, have a setup like this (small queue is useful for perf testing). It's very early in the cycle and the code seems straight forward... I judge the chances of breaking anything is tiny because it just failed probe previously. > 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 > will look into this, thanks!