linux-kernel - Re: [PATCH] mm, slab: extend kmalloc() alignment for non power-of-two sizes

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Message-ID: <c8061293-8457-4d2b-85c2-7c631c00245a@suse.cz>
Date: Tue, 2 Jul 2024 23:18:36 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: Alice Ryhl <aliceryhl@...gle.com>
Cc: linux-mm@...ck.org, David Rientjes <rientjes@...gle.com>,
 Christoph Lameter <cl@...ux.com>, Hyeonggon Yoo <42.hyeyoo@...il.com>,
 Roman Gushchin <roman.gushchin@...ux.dev>, Kees Cook
 <keescook@...omium.org>, Boqun Feng <boqun.feng@...il.com>,
 rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org,
 patches@...ts.linux.dev
Subject: Re: [PATCH] mm, slab: extend kmalloc() alignment for non power-of-two
 sizes

On 7/2/24 6:40 PM, Alice Ryhl wrote:
> On Tue, Jul 2, 2024 at 5:58 PM Vlastimil Babka <vbabka@...e.cz> wrote:
> Thanks!
> 
> Since this change is motivated by Rust, why not also include the Rust
> change in this patch? You would need to remove the if inside
> krealloc_aligned in rust/kernel/alloc/allocator.rs and update the
> comments.

Right, thanks. Does this look ok? (not tested as I don't have the
environment working)

diff --git a/rust/kernel/alloc/allocator.rs b/rust/kernel/alloc/allocator.rs
index 229642960cd1..c619acb8b285 100644
--- a/rust/kernel/alloc/allocator.rs
+++ b/rust/kernel/alloc/allocator.rs
@@ -18,23 +18,16 @@ pub(crate) unsafe fn krealloc_aligned(ptr: *mut u8, new_layout: Layout, flags: F
     // Customized layouts from `Layout::from_size_align()` can have size < align, so pad first.
     let layout = new_layout.pad_to_align();
 
+    // Note that `layout.size()` (after padding) is guaranteed to be a multiple of `layout.align()`
+    // which together with the slab guarantee means the `krealloc` will return a properly aligned
+    // object (see comments in `kmalloc()` for more information).
     let mut size = layout.size();
 
-    if layout.align() > bindings::ARCH_SLAB_MINALIGN {
-        // The alignment requirement exceeds the slab guarantee, thus try to enlarge the size
-        // to use the "power-of-two" size/alignment guarantee (see comments in `kmalloc()` for
-        // more information).
-        //
-        // Note that `layout.size()` (after padding) is guaranteed to be a multiple of
-        // `layout.align()`, so `next_power_of_two` gives enough alignment guarantee.
-        size = size.next_power_of_two();
-    }
-
     // SAFETY:
     // - `ptr` is either null or a pointer returned from a previous `k{re}alloc()` by the
     //   function safety requirement.
     // - `size` is greater than 0 since it's either a `layout.size()` (which cannot be zero
-    //   according to the function safety requirement) or a result from `next_power_of_two()`.
+    //   according to the function safety requirement)
     unsafe { bindings::krealloc(ptr as *const core::ffi::c_void, size, flags.0) as *mut u8 }
 }