linux-kernel - Re: [PATCH v4 06/28] rust: alloc: implement `Vmalloc` allocator

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Message-ID: <64498fd9-b3f1-46f4-aebe-e5ff65634a86@proton.me>
Date: Wed, 07 Aug 2024 07:23:48 +0000
From: Benno Lossin <benno.lossin@...ton.me>
To: Danilo Krummrich <dakr@...nel.org>
Cc: ojeda@...nel.org, alex.gaynor@...il.com, wedsonaf@...il.com, boqun.feng@...il.com, gary@...yguo.net, bjorn3_gh@...tonmail.com, a.hindborg@...sung.com, aliceryhl@...gle.com, akpm@...ux-foundation.org, daniel.almeida@...labora.com, faith.ekstrand@...labora.com, boris.brezillon@...labora.com, lina@...hilina.net, mcanal@...lia.com, zhiw@...dia.com, acurrid@...dia.com, cjia@...dia.com, jhubbard@...dia.com, airlied@...hat.com, ajanulgu@...hat.com, lyude@...hat.com, linux-kernel@...r.kernel.org, rust-for-linux@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v4 06/28] rust: alloc: implement `Vmalloc` allocator

On 06.08.24 21:01, Danilo Krummrich wrote:
> On Tue, Aug 06, 2024 at 05:00:24PM +0000, Benno Lossin wrote:
>> On 05.08.24 17:19, Danilo Krummrich wrote:
>>> Implement `Allocator` for `Vmalloc`, the kernel's virtually contiguous
>>> allocator, typically used for larger objects, (much) larger than page
>>> size.
>>>
>>> All memory allocations made with `Vmalloc` end up in `vrealloc()`.
>>>
>>> Reviewed-by: Alice Ryhl <aliceryhl@...gle.com>
>>> Signed-off-by: Danilo Krummrich <dakr@...nel.org>
>>> ---
>>>  rust/helpers.c                      |  7 +++++++
>>>  rust/kernel/alloc/allocator.rs      | 32 +++++++++++++++++++++++++++++
>>>  rust/kernel/alloc/allocator_test.rs |  1 +
>>>  3 files changed, 40 insertions(+)
>>>
>>> diff --git a/rust/helpers.c b/rust/helpers.c
>>> index 9f7275493365..7406943f887d 100644
>>> --- a/rust/helpers.c
>>> +++ b/rust/helpers.c
>>> @@ -33,6 +33,7 @@
>>>  #include <linux/sched/signal.h>
>>>  #include <linux/slab.h>
>>>  #include <linux/spinlock.h>
>>> +#include <linux/vmalloc.h>
>>>  #include <linux/wait.h>
>>>  #include <linux/workqueue.h>
>>>
>>> @@ -199,6 +200,12 @@ void *rust_helper_krealloc(const void *objp, size_t new_size, gfp_t flags)
>>>  }
>>>  EXPORT_SYMBOL_GPL(rust_helper_krealloc);
>>>
>>> +void *rust_helper_vrealloc(const void *p, size_t size, gfp_t flags)
>>> +{
>>> +	return vrealloc(p, size, flags);
>>> +}
>>> +EXPORT_SYMBOL_GPL(rust_helper_vrealloc);
>>> +
>>>  /*
>>>   * `bindgen` binds the C `size_t` type as the Rust `usize` type, so we can
>>>   * use it in contexts where Rust expects a `usize` like slice (array) indices.
>>> diff --git a/rust/kernel/alloc/allocator.rs b/rust/kernel/alloc/allocator.rs
>>> index c6ad1dd59dd0..bb55895cbd03 100644
>>> --- a/rust/kernel/alloc/allocator.rs
>>> +++ b/rust/kernel/alloc/allocator.rs
>>> @@ -9,6 +9,7 @@
>>>
>>>  use crate::alloc::{AllocError, Allocator};
>>>  use crate::bindings;
>>> +use crate::pr_warn;
>>>
>>>  /// The contiguous kernel allocator.
>>>  ///
>>> @@ -16,6 +17,12 @@
>>>  /// `bindings::krealloc`.
>>>  pub struct Kmalloc;
>>>
>>> +/// The virtually contiguous kernel allocator.
>>> +///
>>> +/// The vmalloc allocator allocates pages from the page level allocator and maps them into the
>>> +/// contiguous kernel virtual space.
>>> +pub struct Vmalloc;
>>
>> One thing that I should also have mentioned for `Kmalloc`, do we want
>> these types to also have values? I don't think that we need them to be,
>> so we could declare them as `pub enum Vmalloc {}`.
> 
> What the difference? Would `pub enum Vmalloc {}` be better for some reason?

It doesn't make a huge difference, it doesn't allow you to create a
value of type `Vmalloc` (as there are no values of that type). So
you can't accidentally use the type where it shouldn't be used.
If we use `pub struct Vmalloc;`, then you can do this:

    let a = Vmalloc;

you can't really do anything with it (as there are no methods on that
type), but it might be confusing for people.

>>> @@ -141,6 +153,26 @@ unsafe fn alloc_zeroed(&self, layout: Layout) -> *mut u8 {
>>>      }
>>>  }
>>>
>>> +unsafe impl Allocator for Vmalloc {
>>> +    unsafe fn realloc(
>>> +        ptr: Option<NonNull<u8>>,
>>> +        layout: Layout,
>>> +        flags: Flags,
>>> +    ) -> Result<NonNull<[u8]>, AllocError> {
>>> +        let realloc = ReallocFunc::vrealloc();
>>> +
>>> +        // TODO: Support alignments larger than PAGE_SIZE.
>>> +        if layout.align() > bindings::PAGE_SIZE {
>>> +            pr_warn!("Vmalloc does not support alignments larger than PAGE_SIZE yet.\n");
>>> +            return Err(AllocError);
>>> +        }
>>> +
>>> +        // SAFETY: If not `None`, `ptr` is guaranteed to point to valid memory, which was previously
>>> +        // allocated with this `Allocator`.
>>> +        unsafe { realloc.call(ptr, layout, flags) }
>>
>> I am a bit confused, for `Kmalloc`, you manually returned
>> `NonNull::dangling` when allocating a zero-sized allocation, but here
>> you don't?
>>
> 
> I do, it's the exact same implementation for krealloc(), vrealloc() and
> kvrealloc(). That why I added the `ReallocFunc` abstraction.

Oh yeah, my bad.

---
Cheers
Benno