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[80.230.48.7]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-4548ec6cd75sm26961587f8f.16.2026.05.11.14.59.25 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 11 May 2026 14:59:29 -0700 (PDT) Date: Mon, 11 May 2026 17:59:23 -0400 From: "Michael S. Tsirkin" To: Gregory Price Cc: linux-kernel@vger.kernel.org, "David Hildenbrand (Arm)" , Jason Wang , Xuan Zhuo , Eugenio =?iso-8859-1?Q?P=E9rez?= , Muchun Song , Oscar Salvador , Andrew Morton , Lorenzo Stoakes , "Liam R. 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Howlett" , Harry Yoo , Hao Li Subject: Re: [PATCH resend v6 03/30] mm: thread user_addr through page allocator for cache-friendly zeroing Message-ID: <20260511175032-mutt-send-email-mst@kernel.org> References: <9b53972f4854c1064b92cefc464f51949afeb83f.1778489843.git.mst@redhat.com> <20260511114853-mutt-send-email-mst@kernel.org> MIME-Version: 1.0 In-Reply-To: X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: NB2oKUWVMa53OkxW2SS6SduyTt82YOrtLrjbU32zNGs_1778536771 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=us-ascii Content-Disposition: inline X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: D66DF40013 X-Rspam-User: X-Stat-Signature: c6a9i97mm98y8sr9tgm43o4x4669uz5e X-HE-Tag: 1778536773-770011 X-HE-Meta: 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 TURmQLhj c69EHgQqdcHPZ1JmWq/nRDpfADBCex3hM9XI171VZdIDJfIj4XSczUOaAwtB7saRQC3nZOBmbOQMVYqOQEvJCcRXmAFT28lx/eU2brWer03IW7o8tSjyS7xU8qoObiPHIvQA2/YcYXBubwtzVDmdO0LdeLAhYaKriQ8VRhwToC7zLeDvU4tNeKmC2gck7c/Mxw/T9tODiwVy4hHTg+W2hHOA7qE7c9Z4AS0x3+3w1DH3UNoQn92X4XC6PmMcbrHzDrAu8NCH7da+z+AUj22Mbd5QcqGZ2TdlI2qZgYMKoabWzkufewLwx2Ddv01JOeOIc6FKvpdTeJBg+m8WTgWzjVxhj96zLTKfsrzTcVrGYFig5Rn/DxmljonZa3XoRi3GCv2h5BQhFg4/cAJgUjRBaVeX0fSp1mfMiEXCW Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Mon, May 11, 2026 at 12:52:10PM -0400, Gregory Price wrote: > On Mon, May 11, 2026 at 11:55:40AM -0400, Michael S. Tsirkin wrote: > > On Mon, May 11, 2026 at 11:37:37AM -0400, Gregory Price wrote: > > > On Mon, May 11, 2026 at 05:01:55AM -0400, Michael S. Tsirkin wrote: > > > > > > > +/* > > > > + * Sentinel for user_addr: indicates a non-user allocation. > > > > + * Cannot use 0 because address 0 is a valid userspace mapping. > > > > + */ > > > > +#define USER_ADDR_NONE ((unsigned long)-1) > > > > > > Ehm, hm. Does -1 hold as a non-user address across all architectures? > > > > > > What about in linear addressing / no VM mode? > > > > this is used on a fault. I don't think there are any faults then? > > But maybe FAULT_ADDR_NONE would be clearer. > > > > Meh, naming here is less relevant than the sentinel correctness. > > My only concern is really whether -1 could end up being a valid address > in some horrid future timeline and this all going belly up. > > Is why I asked about whether this is correct on all architectures. I think the answer is yes: on all architectures Linux supports, the last page of the address space is never a valid user mapping. The kernel enforces this -- mmap will not create a mapping whose end wraps past -1. if (addr > TASK_SIZE - len) return -ENOMEM; So the maximum vm_end = addr + len <= TASK_SIZE. On every architecture TASK_SIZE <= (unsigned long)-1, so vm_end can never wrap around to 0 and address -1 can never be within any VMA. > > > So the trade off is: > > > a) churn the current interface for everyone > > > b) add a user_ variant and know people will just get it wrong > > > > I was also explicitly asked not to proliferate too many new APIs. > > > > Yeah simply spelling it out, not asking for a change. Probably no > better way to go about it. > > ~Gregory