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Howlett" , Miguel Ojeda , Boqun Feng , Gary Guo , =?utf-8?Q?Bj=C3=B6rn?= Roy Baron , Benno Lossin , Trevor Gross , Danilo Krummrich , Will Deacon , Mark Rutland , linux-mm@kvack.org, rust-for-linux@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2] rust: page: add byte-wise atomic memory copy methods In-Reply-To: <2026021343-germicide-baritone-efe8@gregkh> References: <20260212-page-volatile-io-v2-1-a36cb97d15c2@kernel.org> <20260213095557.GS2995752@noisy.programming.kicks-ass.net> <40xUh92AU5E9oFxQrdej-AXVg76jmaWGKXZMLoOHXe35Lw9x_eNEoLup9bB60LyGZ_0USPmoxr-9hE3ujA67cQ==@protonmail.internalid> <2026021343-germicide-baritone-efe8@gregkh> Date: Fri, 13 Feb 2026 13:58:16 +0100 Message-ID: <877bsgu7fb.fsf@kernel.org> MIME-Version: 1.0 Content-Type: text/plain X-Rspamd-Server: rspam03 X-Rspamd-Queue-Id: 82652160004 X-Stat-Signature: rizocneeb7sxg9q1fcbjsz7k4atk9rhj X-Rspam-User: X-HE-Tag: 1770987508-412014 X-HE-Meta: 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 ePFC6975 Jf849LarCo5+9HbhFk9wPvOy4P3Q25BFVzZEKM8JeOyTCpj6AG2mUPd/9Fk9A5I4yVP2wiW8L+ngyURYAw4MDHHe/TcOe6oL2ztx7CDu61jSYLtCYHvi1F7coGGbnMhr2kQrseuBPKsn2DUSuycUCPLGLIku2heT9pgwZ/lNrMSH3u4vFvfOMW6PwLS6y/z4XiumXuV3L4gSP37tBAxU+/Wz7qWpUbsYBVnjnHx2KD5bXJIrOlWR4pNrqW0LCOf+VkmlsSvT7rnLRJo25wRiIIxfEDup5o3tZ9OTuqAOOk+SSlWSi1slorbtZDMJnSQ9NcLjnVmGjaTWwe8DH/Pgla1LSsg== 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: "Greg KH" writes: > On Fri, Feb 13, 2026 at 10:55:57AM +0100, Peter Zijlstra wrote: >> On Thu, Feb 12, 2026 at 03:51:24PM +0100, Andreas Hindborg wrote: >> > When copying data from buffers that are mapped to user space, it is >> > impossible to guarantee absence of concurrent memory operations on those >> > buffers. Copying data to/from `Page` from/to these buffers would be >> > undefined behavior if no special considerations are made. >> > >> > Add methods on `Page` to read and write the contents using byte-wise atomic >> > operations. >> >> I'm completely failing to understand. What!? >> >> There is no such thing as an 'atomic' byte load, nor does it help one >> whit against concurrent modification of the memory you're copying. > > I too am totally confused when reading this patch, and the previous > ones. Shouldn't the "normal" copy_from_user() stuff be used here > correctly? Why is anything new needed? One use for this is copying data out of a `struct bio_vec`. As far as I know, there is no way to know where the pages backing a bio_vec are mapped. They could be mapped to user space. Best regards, Andreas Hindborg