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Howlett" , Miguel Ojeda , Boqun Feng , =?iso-8859-1?Q?Bj=F6rn?= 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 Message-ID: <20260218100915.GB1395266@noisy.programming.kicks-ass.net> References: <20260217102557.GX1395266@noisy.programming.kicks-ass.net> <20260217110911.GY1395266@noisy.programming.kicks-ass.net> <20260217120920.GZ1395266@noisy.programming.kicks-ass.net> <20260217154800.GY2995752@noisy.programming.kicks-ass.net> <20260218083754.GB2995752@noisy.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspamd-Server: rspam09 X-Stat-Signature: hu5e9frfbre5goy98jipqmaikd5sqk48 X-Rspamd-Queue-Id: B0AB440008 X-Rspam-User: X-HE-Tag: 1771409882-550781 X-HE-Meta: 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 GtCtX8k3 S1M/CjpTgtHrEkC/8T5ZlGD4aAkCfZfrHL3dtq6hqEIABwLXT2YJ3tOZwoK2loe8uKergBdQvR19bvM9hXkx9MU/i8qHOcgMvSyWt60vnWrbg91SZDkJyECju2y4AAfZHtjSUUuGek8hA/CqvwPidwtaJEAvf75V6e3ypEsOvKwKFC0xMw9mx7VxHoVuyHrW1UhlYVUNHmb9pizuUQOvUo1caOfIMvPkSvJ4zLiiUAKHv1CDG9OnsTvhX46EFilJDi8fj+TAakPgyUlWgh3foRzmKMtFnYG9aIMBV 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 Wed, Feb 18, 2026 at 09:31:13AM +0000, Alice Ryhl wrote: > > Must we really go write things like: > > > > struct foo val, *ptr; > > > > ptr = kmap_local_page(page); > > memcpy(ptr, val, sizeof(val)); > > kunmap_local(ptr); > > > > ptr = RELOC_HIDE(&val, 0); > > > > if (ptr->field) { > > ... > > } > > > > That seems 'unfortunate'. It basically means we must never use local > > stack for copies or somesuch. > > No I don't think RELOC_HIDE is what you want to be using here. > > The way to stop the compiler from doing this is to ensure that, in > LLVM's eyes, the memcpy is either a volatile memcpy, an atomic memcpy, > or an opaque function call. According to Gary's reply to my email on V3, > it sounds like an explicit call to memcpy like this apparently falls into > opaque function call, so it should be okay. Ah, so we should write: struct foo val, *ptr; ptr = kmap_local_page(page); __memcpy(ptr, val, sizeof(val)); kunmap_local(ptr); if (val.field) { ... } That forces things to be a function call since it elides the C memcpy intrinsic for not having the right name. But that also elides KASAN and such that want to hijack the various mem functions for instrumentation purposes.