On Fri, Aug 30, 2024 at 02:45:35PM +0000, Benno Lossin wrote: > On 30.08.24 00:04, Danilo Krummrich wrote: > > On Thu, Aug 29, 2024 at 06:32:42PM +0000, Benno Lossin wrote: > >> On 16.08.24 02:10, Danilo Krummrich wrote: > >>> Implement `Allocator` for `Kmalloc`, the kernel's default allocator, > >>> typically used for objects smaller than page size. > >>> > >>> All memory allocations made with `Kmalloc` end up in `krealloc()`. > >>> > >>> It serves as allocator for the subsequently introduced types `KBox` and > >>> `KVec`. > >>> > >>> Signed-off-by: Danilo Krummrich <dakr@xxxxxxxxxx> > >>> --- > >>> rust/kernel/alloc.rs | 2 +- > >>> rust/kernel/alloc/allocator.rs | 72 +++++++++++++++++++++++++++++++++- > >>> 2 files changed, 72 insertions(+), 2 deletions(-) > >>> > >>> diff --git a/rust/kernel/alloc.rs b/rust/kernel/alloc.rs > >>> index 9932f21b0539..477dbe3c5a2f 100644 > >>> --- a/rust/kernel/alloc.rs > >>> +++ b/rust/kernel/alloc.rs > >>> @@ -4,7 +4,7 @@ > >>> > >>> #[cfg(not(test))] > >>> #[cfg(not(testlib))] > >>> -mod allocator; > >>> +pub mod allocator; > >>> pub mod box_ext; > >>> pub mod vec_ext; > >>> > >>> diff --git a/rust/kernel/alloc/allocator.rs b/rust/kernel/alloc/allocator.rs > >>> index e32182f91167..78e7d5488843 100644 > >>> --- a/rust/kernel/alloc/allocator.rs > >>> +++ b/rust/kernel/alloc/allocator.rs > >>> @@ -1,12 +1,28 @@ > >>> // SPDX-License-Identifier: GPL-2.0 > >>> > >>> //! Allocator support. > >>> +//! > >>> +//! Documentation for the kernel's memory allocators can found in the "Memory Allocation Guide" > >>> +//! linked below. For instance, this includes the concept of "get free page" (GFP) flags and the > >>> +//! typical application of the different kernel allocators. > >>> +//! > >>> +//! Reference: <https://docs.kernel.org/core-api/memory-allocation.html> > >> > >> Thanks, this nice. > >> > >>> > >>> use super::{flags::*, Flags}; > >>> use core::alloc::{GlobalAlloc, Layout}; > >>> use core::ptr; > >>> +use core::ptr::NonNull; > >>> > >>> -struct Kmalloc; > >>> +use crate::alloc::{AllocError, Allocator}; > >>> +use crate::bindings; > >>> + > >>> +/// The contiguous kernel allocator. > >>> +/// > >>> +/// `Kmalloc` is typically used for physically contiguous allocations up to page size, but also > >>> +/// supports larger allocations up to `bindings::KMALLOC_MAX_SIZE`, which is hardware specific. > >> > >> Does putting a link here work? (I guess we don't yet export the bindings > >> documentation, so it will probably fail... When we decide to enable it, > >> we should create an issue to add missing links) > >> > >>> +/// > >>> +/// For more details see [self]. > >>> +pub struct Kmalloc; > >>> > >>> /// Returns a proper size to alloc a new object aligned to `new_layout`'s alignment. > >>> fn aligned_size(new_layout: Layout) -> usize { > >>> @@ -36,6 +52,60 @@ pub(crate) unsafe fn krealloc_aligned(ptr: *mut u8, new_layout: Layout, flags: F > >>> unsafe { bindings::krealloc(ptr as *const core::ffi::c_void, size, flags.0) as *mut u8 } > >>> } > >>> > >>> +/// # Invariants > >>> +/// > >>> +/// One of the following `krealloc`, `vrealloc`, `kvrealloc`. > >>> +struct ReallocFunc( > >>> + unsafe extern "C" fn(*const core::ffi::c_void, usize, u32) -> *mut core::ffi::c_void, > >>> +); > >>> + > >>> +impl ReallocFunc { > >>> + // INVARIANT: `krealloc` satisfies the type invariants. > >>> + const KREALLOC: Self = Self(bindings::krealloc); > >>> + > >>> + /// # Safety > >>> + /// > >>> + /// This method has the same safety requirements as [`Allocator::realloc`]. > >>> + unsafe fn call( > >>> + &self, > >>> + ptr: Option<NonNull<u8>>, > >>> + layout: Layout, > >>> + flags: Flags, > >>> + ) -> Result<NonNull<[u8]>, AllocError> { > >>> + let size = aligned_size(layout); > >>> + let ptr = match ptr { > >>> + Some(ptr) => ptr.as_ptr(), > >>> + None => ptr::null(), > >>> + }; > >>> + > >>> + // SAFETY: `ptr` is either NULL or valid by the safety requirements of this function. > >> > >> You need some justification as to why calling the three allowed > >> functions here. > > > > What kind of justification do I need? Can you please share some more details on > > what you think is missing here? > > So, you are calling a function pointer to an `unsafe` function. This > means that through some invariant you have to know what the safety > requirements are (otherwise how can you guarantee that this is OK?). You > have the invariant that the pointer points at one of the three functions > mentioned above. What are the safety requirements of those functions? I > would assume that the only one is that `ptr` is valid. So you can use: > > // SAFETY: > // - `self.0` is one of `krealloc`, `vrealloc`, `kvrealloc` and thus only requires that `ptr` is > // NULL or valid. I'm fine adding it, but I'd like to understand why you think it's required in the safety comment here? Isn't this implicit by being the type invariant? > // - `ptr` is either NULL or valid by the safety requirements of this function. This is the part I already have. > > >>> + let raw_ptr = unsafe { > >>> + // If `size == 0` and `ptr != NULL` the memory behind the pointer is freed. > >>> + self.0(ptr.cast(), size, flags.0).cast() > >>> + }; > >>> + > >>> + let ptr = if size == 0 { > >>> + NonNull::dangling() > >>> + } else { > >>> + NonNull::new(raw_ptr).ok_or(AllocError)? > >>> + }; > >>> + > >>> + Ok(NonNull::slice_from_raw_parts(ptr, size)) > >>> + } > >>> +} > >>> + > >>> +unsafe impl Allocator for Kmalloc { > >> > >> Missing SAFETY comment. > > > > Yeah, I think we came across this in an earlier version of the series. I asked > > you about the content and usefulness of a comment here, since I'd just end up > > re-iterating what the `Allocator` trait documentation says. > > > > IIRC, you replied that you want to think of something that'd make sense to add > > here. > > Oh yeah, sorry I forgot about that. > > > What do you think should be written here? > > I think the best way to do it, would be to push this question down into > `ReallocFunc::call`. So we would put this on the trait: > > // SAFETY: `realloc` delegates to `ReallocFunc::call`, which guarantees that > // - memory remains valid until it is explicitly freed, > // - passing a pointer to a vaild memory allocation is OK, > // - `realloc` satisfies the guarantees, since `ReallocFunc::call` has the same. So, we'd also need the same for: - `unsafe impl Allocator for Vmalloc` - `unsafe impl Allocator for KVmalloc` > > We then need to put this on `ReallocFunc::call`: > > /// # Guarantees > /// > /// This method has the same guarantees as `Allocator::realloc`. Additionally > /// - it accepts any pointer to a valid memory allocation allocated by this function. You propose this, since for `Allocator::realloc` memory allocated with `Allocator::alloc` would be fine too I guess. But if e.g. `Kmalloc` wouldn't use the default `Allocator::alloc`, this would be valid too. We could instead write something like: "it accepts any pointer to a valid memory allocation allocated with the same kernel allocator." > /// - memory allocated by this function remains valid until it is passed to this function. Same here, `Kmalloc` could implement its own `Allocator::free`. Maybe just "...until it is explicitly freed.". Anyway, I'm fine with both, since non of the kernel allocators uses anything else than `ReallocFunc::call` to allocate and free memory. > > Finally, we need a `GUARANTEE` comment (just above the return [^1] > value) that establishes these guarantees: > > // GUARANTEE: Since we called `self.0` with `size` above and by the type invariants of `Self`, > // `self.0` is one of `krealloc`, `vrealloc`, `kvrealloc`. Those functions provide the guarantees of > // this function. > > I am not really happy with the last sentence, but I also don't think > that there is value in listing out all the guarantees, only to then say > "all of this is guaranteed by us calling one of these three functions. > > > [^1]: I am not sure that there is the right place. If you have any > suggestions, feel free to share them. Either way, I'm fine with this proposal. > > > >>> + #[inline] > >>> + unsafe fn realloc( > >>> + ptr: Option<NonNull<u8>>, > >>> + layout: Layout, > >>> + flags: Flags, > >>> + ) -> Result<NonNull<[u8]>, AllocError> { > >>> + // SAFETY: `ReallocFunc::call` has the same safety requirements as `Allocator::realloc`. > >>> + unsafe { ReallocFunc::KREALLOC.call(ptr, layout, flags) } > >>> + } > >>> +} > > Oh one more thing, I know that you already have a lot of patches in this > series, but could you split this one into two? So the first one should > introduce `ReallocFunc` and the second one add the impl for `Kmalloc`? > I managed to confuse me twice because of that :) Generally, I'm fine with that, but I'm not sure if I can avoid an intermediate compiler warning about unused code doing that. > > --- > Cheers, > Benno > > >>> + > >>> unsafe impl GlobalAlloc for Kmalloc { > >>> unsafe fn alloc(&self, layout: Layout) -> *mut u8 { > >>> // SAFETY: `ptr::null_mut()` is null and `layout` has a non-zero size by the function safety > >>> -- > >>> 2.46.0 > >>> > >> >
