linux-kernel - Re: [PATCH v3 06/25] rust: alloc: implement `Vmalloc` allocator

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Message-ID: <Zq-bWJFwtWG23O-L@pollux>
Date: Sun, 4 Aug 2024 17:16:40 +0200
From: Danilo Krummrich <dakr@...nel.org>
To: Boqun Feng <boqun.feng@...il.com>
Cc: ojeda@...nel.org, alex.gaynor@...il.com, wedsonaf@...il.com,
	gary@...yguo.net, bjorn3_gh@...tonmail.com, benno.lossin@...ton.me,
	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, Vlastimil Babka <vbabka@...e.cz>
Subject: Re: [PATCH v3 06/25] rust: alloc: implement `Vmalloc` allocator

On Sun, Aug 04, 2024 at 02:41:26PM +0200, Danilo Krummrich wrote:
> On Sat, Aug 03, 2024 at 11:44:58PM -0700, Boqun Feng wrote:
> > On Thu, Aug 01, 2024 at 02:02:05AM +0200, 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()`.
> > > 
> > > Signed-off-by: Danilo Krummrich <dakr@...nel.org>
> > > ---
> > >  rust/helpers.c                      |  8 ++++++++
> > >  rust/kernel/alloc/allocator.rs      | 24 ++++++++++++++++++++++++
> > >  rust/kernel/alloc/allocator_test.rs |  1 +
> > >  3 files changed, 33 insertions(+)
> > > 
> > > diff --git a/rust/helpers.c b/rust/helpers.c
> > > index 92d3c03ae1bd..4c628986f0c9 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>
> > >  
> > > @@ -200,6 +201,13 @@ rust_helper_krealloc(const void *objp, size_t new_size, gfp_t flags)
> > >  }
> > >  EXPORT_SYMBOL_GPL(rust_helper_krealloc);
> > >  
> > > +void * __must_check __realloc_size(2)
> > > +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 397ae5bcc043..e9a3d0694f41 100644
> > > --- a/rust/kernel/alloc/allocator.rs
> > > +++ b/rust/kernel/alloc/allocator.rs
> > > @@ -16,6 +16,12 @@
> > >  /// `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;
> > > +
> > >  /// Returns a proper size to alloc a new object aligned to `new_layout`'s alignment.
> > >  fn aligned_size(new_layout: Layout) -> usize {
> > >      // Customized layouts from `Layout::from_size_align()` can have size < align, so pad first.
> > > @@ -58,6 +64,10 @@ fn krealloc() -> Self {
> > >          Self(bindings::krealloc)
> > >      }
> > >  
> > > +    fn vrealloc() -> Self {
> > > +        Self(bindings::vrealloc)
> > > +    }
> > > +
> > >      // SAFETY: `call` has the exact same safety requirements as `Allocator::realloc`.
> > >      unsafe fn call(
> > >          &self,
> > > @@ -136,6 +146,20 @@ 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();
> > > +
> > 
> > IIUC, vrealloc() calls __vmalloc_noprof() in allocation case, that is
> > calling __vmalloc_node_noprof() with align=1. In such a case, how would
> > vmalloc() guarantee the allocated memory is aligned to layout.align()?
> 
> True, good catch. I thought of this a while ago and then forgot to fix it.

Just for clarification, we're always PAGE_SIZE aligned (guaranteed by
__alloc_vmap_area()), which probably would always be sufficient. That's why I
didn't gave it too much attention in the first place and then forgot about it.

However, we indeed do not honor layout.align() if it's larger than PAGE_SIZE.

> 
> > 
> > [Cc Vlastimil]
> > 
> > Regards,
> > Boqun
> > 
> > > +        // 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) }
> > > +    }
> > > +}
> > > +
> > >  #[global_allocator]
> > >  static ALLOCATOR: Kmalloc = Kmalloc;
> > >  
> > > diff --git a/rust/kernel/alloc/allocator_test.rs b/rust/kernel/alloc/allocator_test.rs
> > > index 4785efc474a7..e7bf2982f68f 100644
> > > --- a/rust/kernel/alloc/allocator_test.rs
> > > +++ b/rust/kernel/alloc/allocator_test.rs
> > > @@ -7,6 +7,7 @@
> > >  use core::ptr::NonNull;
> > >  
> > >  pub struct Kmalloc;
> > > +pub type Vmalloc = Kmalloc;
> > >  
> > >  unsafe impl Allocator for Kmalloc {
> > >      unsafe fn realloc(
> > > -- 
> > > 2.45.2
> > > 
> >