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b=GihAZWbbJipVAD+i/w4emBsVDe0mvCMTwIU86Gke8HcIua2sz3THdEHa71P6aTNkD 2LmtUaYC0Ba5fUtTchBO53dhdwHgFvKiS9VDJDn0xs53utt/8DMth9xPhjNzkOxK5A 6k+5/v3hMooERNVQIFiUzuUfY9S6vIvUrwwjVH/pLJBtvI/1o4AhTqOfUwSKvPpaQI 1Vf6GcJEc6DJ1d1cFxS0lRWkwqdph2LbwbPoJzhdH07+FF+ueqFbVwDmWTOofN47Np 618eQBbus35tAfTw4stIQjzrbqOCAVgMr1T5RAr99gjxv51VcS+vLsRemAMFYxAF7B fZ6YDO6oWjONQ== Date: Tue, 24 Sep 2024 15:31:47 +0200 From: Danilo Krummrich To: Gary Guo Cc: Alice Ryhl , ojeda@kernel.org, alex.gaynor@gmail.com, wedsonaf@gmail.com, boqun.feng@gmail.com, bjorn3_gh@protonmail.com, benno.lossin@proton.me, a.hindborg@samsung.com, akpm@linux-foundation.org, daniel.almeida@collabora.com, faith.ekstrand@collabora.com, boris.brezillon@collabora.com, lina@asahilina.net, mcanal@igalia.com, zhiw@nvidia.com, cjia@nvidia.com, jhubbard@nvidia.com, airlied@redhat.com, ajanulgu@redhat.com, lyude@redhat.com, linux-kernel@vger.kernel.org, rust-for-linux@vger.kernel.org, linux-mm@kvack.org Subject: Re: [RFC PATCH] rust: alloc: pass `old_layout` to `Allocator` Message-ID: References: <20240911225449.152928-2-dakr@kernel.org> <20240921153315.70860-1-dakr@kernel.org> <20240923171315.64327fe7.gary@garyguo.net> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20240923171315.64327fe7.gary@garyguo.net> X-Rspamd-Queue-Id: 2C8EB1C0020 X-Stat-Signature: mx9wu7jptmpjbrushtqnh4m8dnoit8kb X-Rspamd-Server: rspam09 X-Rspam-User: X-HE-Tag: 1727184716-268312 X-HE-Meta: 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 2Rhe8LtK 3RFvruZeTTm84OW8cXgdRfWgZbWmxLPANZd4l3gZzr2QNTBEVkzRkf3muA7KmOna7rRGv0d+aJ5YouDfmjCORN2xb7QPOLTrV7AHhqCXjlvWnCnwaBJKpZvw9uN1XE2GwTJcKOMFemcNcy+DiQ2LmOIfm/MPFF0FfJnXQcpNPSjL3xjdrXS35yOnGExXjHjNqF7w5mWm1gRrTmPixyJJBByL1zXgOJnxZBrPylzkhD+SuogVZZyUT1nBo7PV3eoG7yYnPYrMZ7AB22pclpaXg63S6bmQ9pjM4ZFn5JQVnb5iompCwiLNr54Zgigj6DFvENfN5iajF2CfVTIncfnnDWVBqCNbGGB83gdyZuYJ4dRR339tAAUToQxiYpkY5NbKFeIkLt5CJJHN/RJzj0ZogPUs8Y/RSQpe/Vh+uaQeTxHOwjXodHr6PngziYwHDIVNCJr9R X-Bogosity: Ham, tests=bogofilter, spamicity=0.000001, 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 Mon, Sep 23, 2024 at 05:13:15PM +0100, Gary Guo wrote: > On Mon, 23 Sep 2024 15:56:28 +0200 > Alice Ryhl wrote: > > > On Sat, Sep 21, 2024 at 5:33 PM Danilo Krummrich wrote: > > > @@ -84,11 +92,18 @@ unsafe fn call( > > > &self, > > > ptr: Option>, > > > layout: Layout, > > > + old_layout: Layout, > > > flags: Flags, > > > ) -> Result, AllocError> { > > > let size = aligned_size(layout); > > > let ptr = match ptr { > > > - Some(ptr) => ptr.as_ptr(), > > > + Some(ptr) => { > > > + if old_layout.size() == 0 { > > > + ptr::null() > > > + } else { > > > + ptr.as_ptr() > > > + } > > > + } > > > > This is making Allocator work with zero-sized types, which deviates > > from std. We should not do that without a reason. What is the reason? > > > > Alice > > As Benno said, this makes the API closer to Rust `allocator_api` > Allocator trait as opposed to deviation. > > There's one benefit of doing this (discussed with Danilo off-list), > which is it removes ZST special casing from caller. This RFC patch > simplifies `Box` handling, and if we add this line to the safety doc > > `ptr` does not need to be a pointer returned by this > allocator if the layout is zero-sized. > > then the `Vec` can also be simplified, removing all logics handling ZST > specially, except for `Vec::new()` which it forges a well-aligned > dangling pointer from nowhere. Partially, we still need the additional `Layout` for `Allocator::free`, which in `Vec::drop` and `IntoIter::drop` looks like this: `let layout = Layout::array::(self.cap).unwrap();` I really dislike that this can potentially transform into `BUG()`, but that's probably unrelated to this patch series. > > Best, > Gary >