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 U9iKRaWm 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 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 Fri Jan 30, 2026 at 1:48 PM GMT, Andreas Hindborg wrote: > "Gary Guo" 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 `Pa= ge` >>> 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 invol= ved >>> buffers. >>> >>> Signed-off-by: Andreas Hindborg >>> --- >>> 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( >>> /// # Safety >>> /// >>> /// * Callers must ensure that `dst` is valid for writing `len` by= tes. >>> + /// * 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 writ= e to the same page that >>> /// overlaps with this read. >>> pub unsafe fn read_raw(&self, dst: *mut u8, offset: usize, len: us= ize) -> 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 regio= n 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, le= n: usize) -> Result { >>> + self.with_pointer_into_page(offset, len, move |src| { >>> + // SAFETY: If `with_pointer_into_page` calls into this clo= sure, then >>> + // it has performed a bounds check and guarantees that `sr= c` is >>> + // valid for `len` bytes. >>> + // >>> + // There caller guarantees that there is no data race at t= he source. >>> + unsafe { bindings::memcpy_toio(dst.cast::(), src.c= ast::(), 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 fr= om `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 =3D self.offset() % PAGE_SIZE; > debug_assert!(src_offset <=3D PAGE_SIZE); > let length =3D (PAGE_SIZE - dst_offset) > .min(self.len() as usize) > .min(PAGE_SIZE - src_offset); > let page_idx =3D 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 =3D 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 `lengt= h` 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/3e46e95f0707fa71259b1d241f689= 144ad61cc62/rust/kernel/block/bio/vec.rs#L142