linux-kernel - Re: [RFC v2 2/3] atomic: Specify alignment for atomic_t and atomic64

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Message-ID: <3b55179e-b290-bbf5-fdf2-bb7834884017@linux-m68k.org>
Date: Tue, 23 Sep 2025 18:05:51 +1000 (AEST)
From: Finn Thain <fthain@...ux-m68k.org>
To: Arnd Bergmann <arnd@...db.de>
cc: Geert Uytterhoeven <geert@...ux-m68k.org>, 
    Peter Zijlstra <peterz@...radead.org>, Will Deacon <will@...nel.org>, 
    Andrew Morton <akpm@...ux-foundation.org>, 
    Boqun Feng <boqun.feng@...il.com>, Jonathan Corbet <corbet@....net>, 
    Mark Rutland <mark.rutland@....com>, linux-kernel@...r.kernel.org, 
    Linux-Arch <linux-arch@...r.kernel.org>, linux-m68k@...r.kernel.org, 
    Lance Yang <lance.yang@...ux.dev>
Subject: Re: [RFC v2 2/3] atomic: Specify alignment for atomic_t and
 atomic64_t


On Tue, 23 Sep 2025, Arnd Bergmann wrote:

> On Tue, Sep 23, 2025, at 08:28, Finn Thain wrote:
> > On Mon, 22 Sep 2025, Arnd Bergmann wrote:
> >> On Mon, Sep 22, 2025, at 10:16, Finn Thain wrote:
> > @@ -8,7 +8,7 @@ struct vfsmount;
> >  struct path {
> >         struct vfsmount *mnt;
> >         struct dentry *dentry;
> > -} __randomize_layout;
> > +} __aligned(sizeof(long)) __randomize_layout;
> >
> > There's no need: struct path contains a struct dentry, which contains a 
> > seqcount_spinlock_t, which contains a spinlock_t which contains an 
> > atomic_t member, which is explicitly aligned.
> >
> > Despite that, there's still some kmem cache or other allocator somewhere 
> > that has produced some misaligned path and dentry structures. So we get 
> > misaligned atomics somewhere in the VFS and TTY layers. I was unable to 
> > find those allocations.
> 
> Ok, I see. Those would certainly be good to find. I would assume that 
> all kmem caches have a sensible alignment on each architecture, but I 
> think the definition in linux/slab.h actually ends up setting the 
> minimum to 2 here:
> 
> #ifndef ARCH_KMALLOC_MINALIGN
> #define ARCH_KMALLOC_MINALIGN __alignof__(unsigned long long)
> #elif ARCH_KMALLOC_MINALIGN > 8
> #define KMALLOC_MIN_SIZE ARCH_KMALLOC_MINALIGN
> #define KMALLOC_SHIFT_LOW ilog2(KMALLOC_MIN_SIZE)
> #endif
> 
> #ifndef ARCH_SLAB_MINALIGN
> #define ARCH_SLAB_MINALIGN __alignof__(unsigned long long)
> #endif
> 
> Maybe we should just change __alignof__(unsigned long long) to a plain 
> '8' here and make that the minimum alignment everywhere, same as the 
> atomic64_t alignment change.
> 

It would be better (less wasteful of memory) to specify the alignment 
parameter to kmem_cache_create() only at those call sites where it 
matters.

> Alternatively, we can keep the __alignof__ here in order to reduce 
> padding on architectures with a default 4-byte alignment for __u64, but 
> then override ARCH_SLAB_MINALIGN and ARCH_KMALLOC_MINALIGN on m68k to be 
> '4' instead of '2'.
> 

Raising that to 4 would probably have little or no effect (for m68k or any 
other arch). Back when I prepared the RFC patch series, I instrumented 
create_cache() in mm/slab_common.c, and those caches that were allocated 
at boot (for my usual minimal m68k .config) were already aligned to 4 
bytes or 16.

Also, increasing ARCH_SLAB_MINALIGN to 8 didn't solve the VFS/TTY layer 
problem I have with CONFIG_DEBUG_ATOMIC on m68k. So the culprit is not the 
obvious suspect (a kmem cache of objects with atomic64_t members). There's 
some other allocator at work and it's aligning objects to 2 bytes not 4.