Message-Id: <DG1ZJ1350EMS.1S1360PA496QG@garyguo.net>
Date: Fri, 30 Jan 2026 14:14:35 +0000
From: "Gary Guo" <gary@...yguo.net>
To: "Andreas Hindborg" <a.hindborg@...nel.org>, "Gary Guo"
 <gary@...yguo.net>, "Alice Ryhl" <aliceryhl@...gle.com>, "Lorenzo Stoakes"
 <lorenzo.stoakes@...cle.com>, "Liam R. Howlett" <Liam.Howlett@...cle.com>,
 "Miguel Ojeda" <ojeda@...nel.org>, "Boqun Feng" <boqun.feng@...il.com>,
 Björn Roy Baron <bjorn3_gh@...tonmail.com>, "Benno Lossin"
 <lossin@...nel.org>, "Trevor Gross" <tmgross@...ch.edu>, "Danilo Krummrich"
 <dakr@...nel.org>
Cc: <linux-mm@...ck.org>, <rust-for-linux@...r.kernel.org>,
 <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] rust: page: add volatile memory copy methods

On Fri Jan 30, 2026 at 1:48 PM GMT, Andreas Hindborg wrote:
> "Gary Guo" <gary@...yguo.net> writes:
>
>> On Fri Jan 30, 2026 at 12:33 PM GMT, Andreas Hindborg wrote:
>>> When copying data from buffers that are mapped to user space, or from
>>> buffers that are used for dma, 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 regular memcpy
>>> operations are used.
>>>
>>> The operation can be made well defined, if the buffers that potentially
>>> observe racy operations can be said to exist outside of any Rust
>>> allocation. For this to be true, the kernel must only interact with the
>>> buffers using raw volatile reads and writes.
>>>
>>> Add methods on `Page` to read and write the contents using volatile
>>> operations.
>>>
>>> Also improve clarity by specifying additional requirements on
>>> `read_raw`/`write_raw` methods regarding concurrent operations on involved
>>> buffers.
>>>
>>> Signed-off-by: Andreas Hindborg <a.hindborg@...nel.org>
>>> ---
>>>  rust/kernel/page.rs | 53 +++++++++++++++++++++++++++++++++++++++++++++++++++++
>>>  1 file changed, 53 insertions(+)
>>>
>>> diff --git a/rust/kernel/page.rs b/rust/kernel/page.rs
>>> index 432fc0297d4a8..6568a0d3b3baa 100644
>>> --- a/rust/kernel/page.rs
>>> +++ b/rust/kernel/page.rs
>>> @@ -7,6 +7,7 @@
>>>      bindings,
>>>      error::code::*,
>>>      error::Result,
>>> +    ffi::c_void,
>>>      uaccess::UserSliceReader,
>>>  };
>>>  use core::{
>>> @@ -260,6 +261,8 @@ fn with_pointer_into_page<T>(
>>>      /// # Safety
>>>      ///
>>>      /// * Callers must ensure that `dst` is valid for writing `len` bytes.
>>> +    /// * Callers must ensure that there are no other concurrent reads or writes to/from the
>>> +    ///   destination memory region.
>>>      /// * Callers must ensure that this call does not race with a write to the same page that
>>>      ///   overlaps with this read.
>>>      pub unsafe fn read_raw(&self, dst: *mut u8, offset: usize, len: usize) -> Result {
>>> @@ -274,6 +277,30 @@ pub unsafe fn read_raw(&self, dst: *mut u8, offset: usize, len: usize) -> Result
>>>          })
>>>      }
>>>
>>> +    /// Maps the page and reads from it into the given IO memory region using volatile memory
>>> +    /// operations.
>>> +    ///
>>> +    /// This method will perform bounds checks on the page offset. If `offset .. offset+len` goes
>>> +    /// outside of the page, then this call returns [`EINVAL`].
>>> +    ///
>>> +    /// # Safety
>>> +    /// Callers must ensure that:
>>> +    ///
>>> +    /// * The destination memory region is outside of any Rust memory allocation.
>>> +    /// * The destination memory region is writable.
>>> +    /// * This call does not race with a write to the same source page that overlaps with this read.
>>> +    pub unsafe fn read_raw_toio(&self, dst: *mut u8, offset: usize, len: usize) -> Result {
>>> +        self.with_pointer_into_page(offset, len, move |src| {
>>> +            // SAFETY: If `with_pointer_into_page` calls into this closure, then
>>> +            // it has performed a bounds check and guarantees that `src` is
>>> +            // valid for `len` bytes.
>>> +            //
>>> +            // There caller guarantees that there is no data race at the source.
>>> +            unsafe { bindings::memcpy_toio(dst.cast::<c_void>(), src.cast::<c_void>(), len) };
>>
>> I feel that this should be a generic utility that integrates with our IO infra
>> that allows you to copy/from IO to a slice.
>
> While that might also be useful, for my particular use case I am copying
> between two pages. One is mapped from user space, the other one is
> allocated by a driver. No slices involved. Pasting for reference [1]:

Then what you need is a byte-wise atomic memcpy, not memcpy_{from,to}io.

Best,
Gary

>
>
>     /// Copy data to the current page of this segment from `src_page`.
>     ///
>     /// Copies  `PAGE_SIZE - (self.offset() % PAGE_SIZE` bytes of data from `src_page` to this
>     /// segment starting at `self.offset()` from offset `self.offset() % PAGE_SIZE`. This call
>     /// will advance offset and reduce length of `self`.
>     ///
>     /// Returns the number of bytes copied.
>     pub fn copy_from_page(&mut self, src_page: &Page, src_offset: usize) -> usize {
>         let dst_offset = self.offset() % PAGE_SIZE;
>         debug_assert!(src_offset <= PAGE_SIZE);
>         let length = (PAGE_SIZE - dst_offset)
>             .min(self.len() as usize)
>             .min(PAGE_SIZE - src_offset);
>         let page_idx = self.offset() / PAGE_SIZE;
>
>         // SAFETY: self.bio_vec is valid and thus bv_page must be a valid
>         // pointer to a `struct page`.
>         let dst_page = unsafe { Page::from_raw(self.bio_vec.bv_page.add(page_idx)) };
>
>         dst_page
>             .with_pointer_into_page(dst_offset, length, |dst| {
>                 // SAFETY:
>                 // - If `with_pointer_into_page` calls this closure, then it has performed bounds
>                 //   checks and guarantees that `dst` is valid for `length` bytes.
>                 // - Since we have a shared reference to `src_page`, the read cannot race with any
>                 //   writes to `src_page`.
>                 unsafe { src_page.read_raw_toio(dst, src_offset, length) }
>             })
>             .expect("Assertion failure, bounds check failed.");
>
>         self.advance(length as u32)
>             .expect("Assertion failure, bounds check failed.");
>
>         length
>
>
>
> Best regards,
> Andreas Hindborg
>
> [1] https://github.com/metaspace/linux/blob/3e46e95f0707fa71259b1d241f689144ad61cc62/rust/kernel/block/bio/vec.rs#L142

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