From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 9771972633; Tue, 26 Aug 2025 23:38:18 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1756251498; cv=none; b=PH/nD9wsZq8Gdkp4evXhkl+/x4NesssR0yaJUYlDbRZOl99RL+3Id4O2p5g3tQkLExbHdgNL3SeHDBzFAvvJgDyMLlAAeurKSiesmBXZ2Gfz5+GVAlV3NRb2/QkdfbY+fVNYK+a0d0lju9PjeR+Glim2yYeKLE9bTDxdoEvLVbw= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1756251498; c=relaxed/simple; bh=kn6ieDwDxZsDgSRi1qDw+jZs8ann+wvttrNfWt18VCY=; h=Mime-Version:Content-Type:Date:Message-Id:Subject:Cc:To:From: References:In-Reply-To; b=Wmv5rAfZZP1RSZTI5AnG5++iVBTjpMiLkHsUibm2jEoD0eGHzc3J7vH8idCIwgi+7kMOoSWoFVG7Mv+NZvgEoUU/lIPdEjLsEf4dD43yQsOxh8h/dggFWa16Wd9FUUqMwKQVESpUsWb3WLQyWum7SwW4TEOU9T/mXbnusKu7Zos= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=E6QWL+9O; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="E6QWL+9O" Received: by smtp.kernel.org (Postfix) with ESMTPSA id E396BC4CEF1; Tue, 26 Aug 2025 23:38:14 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1756251498; bh=kn6ieDwDxZsDgSRi1qDw+jZs8ann+wvttrNfWt18VCY=; h=Date:Subject:Cc:To:From:References:In-Reply-To:From; b=E6QWL+9ORdmTU28VciO8c6Xlxg9A2oRj98hu0YZ11ppE8hBl8vCjRXMtn61rGYr1v MAd025GNlEZ7Tjhd0BLlW7UEf1vZzhPE8Rsg/UP+zN8NVtpw+bpkvnqK1rvFtO17OW 3JhSpJ2I0RCZtjh2xZFiW/9B/ropEwEGNuJ0amT65DewCcETzHVXyQIe/1APInhXtT MLamOYSmkUezjsOJsCTB68qOqFORdSUmVPq60vwp9N3bl2HEcG/wlRpqi4FKUFDvAt X3fYFyXyzMsw/VWe7OJzM39kqj74TVSwfOdOm2yh0oh/KYwmME9UQha7J9TSt6d44a aKSYwScs8XCdQ== Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset=UTF-8 Date: Wed, 27 Aug 2025 01:38:13 +0200 Message-Id: Subject: Re: [PATCH v3 3/5] rust: scatterlist: Add abstraction for sg_table Cc: , , , , , , , , , , , , , , , , To: "Alexandre Courbot" From: "Danilo Krummrich" References: <20250825132539.122412-1-dakr@kernel.org> <20250825132539.122412-4-dakr@kernel.org> In-Reply-To: On Tue Aug 26, 2025 at 5:18 PM CEST, Danilo Krummrich wrote: > On Tue Aug 26, 2025 at 4:36 PM CEST, Alexandre Courbot wrote: >> Even if this is for internal use, I think a short comment explaining >> what this is for, and why it needs to be pinned (pointed to by devres) > > That's not the reason this structure needs to be pinned. This is the reas= on for > Devres itself needs to be pinned. > > In fact, I think RawSGTable by itself does not need to be pinned. Just to expand on this a bit: Eventually it does need to be pinned, because DmaMappedSgt keeps a pointer = of the underlying struct sg_table. But, this happens in Owned: Ok(try_pin_init!(&this in Self { // SAFETY: // - `page_vec` is a `KVec` of valid `struct page *` obtained from `pa= ges`. // - The pages contained in `pages` remain valid for the entire lifeti= me of the // `RawSGTable`. sgt: unsafe { RawSGTable::new(&mut page_vec, size, max_segment, flags)= }?, dma <- { // SAFETY: `this` is a valid pointer to uninitialized memory. let sgt =3D unsafe { &raw mut (*this.as_ptr()).sgt }.cast(); =09 // SAFETY: `sgt` is guaranteed to be non-null. let sgt =3D unsafe { NonNull::new_unchecked(sgt) }; =09 // SAFETY: // - It is guaranteed that the object returned by `DmaMappedSgt::n= ew` won't out-live // `sgt`. // - `sgt` is never DMA unmapped manually. Devres::new(dev, unsafe { DmaMappedSgt::new(sgt, dev, dir) }) }, _pages: pages, })) So, it's fine to move RawSGTable around, *until* we obtain the address for DmaMappedSgt (within Owned, Devres is dropped before the RawSGTable). However, this is an implementation detail of Owned and has not= hing to do with RawSGTable by itself. Hence, we could also nuke #[pin_data] for RawSGTable and implement it as: /// A transparent wrapper around a `struct sg_table`. /// /// While we could also create the `struct sg_table` in the constructor of= [`Owned`], we can't tear /// down the `struct sg_table` in [`Owned::drop`]; the drop order in [`Own= ed`] matters. #[repr(transparent)] struct RawSGTable(Opaque); impl RawSGTable { /// # Safety /// /// - `pages` must be a slice of valid `struct page *`. /// - The pages pointed to by `pages` must remain valid for the entire= lifetime of the returned /// [`RawSGTable`]. unsafe fn new( pages: &mut [*mut bindings::page], size: usize, max_segment: u32, flags: alloc::Flags, ) -> Result { // `sg_alloc_table_from_pages_segment()` expects at least one page= , otherwise it // produces a NPE. if pages.is_empty() { return Err(EINVAL); } =09 let sgt =3D Opaque::zeroed(); =09 // SAFETY: // - `sgt.get()` is a valid pointer to uninitialized memory. // - As by the check above, `pages` is not empty. error::to_result(unsafe { bindings::sg_alloc_table_from_pages_segment( sgt.get(), pages.as_mut_ptr(), pages.len().try_into()?, 0, size, max_segment, flags.as_raw(), ) })?; =09 Ok(Self(sgt)) } =09 #[inline] fn as_raw(&self) -> *mut bindings::sg_table { self.0.get() } } =09 impl Drop for RawSGTable { #[inline] fn drop(&mut self) { // SAFETY: `sgt` is a valid and initialized `struct sg_table`. unsafe { bindings::sg_free_table(self.0.get()) }; } }