linux-kernel - Re: [PATCH v12 2/4] mm/slub: allow to set node and align in k[v]realloc

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Message-ID: <45e705ef-e40b-4dd3-a9b9-1a713df5d4e5@konsulko.se>
Date: Mon, 14 Jul 2025 17:27:59 +0200
From: Vitaly Wool <vitaly.wool@...sulko.se>
To: Vlastimil Babka <vbabka@...e.cz>
Cc: Harry Yoo <harry.yoo@...cle.com>, linux-mm@...ck.org,
 akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
 Uladzislau Rezki <urezki@...il.com>, Danilo Krummrich <dakr@...nel.org>,
 Alice Ryhl <aliceryhl@...gle.com>, rust-for-linux@...r.kernel.org,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 "Liam R . Howlett" <Liam.Howlett@...cle.com>,
 Kent Overstreet <kent.overstreet@...ux.dev>, linux-bcachefs@...r.kernel.org,
 bpf@...r.kernel.org, Herbert Xu <herbert@...dor.apana.org.au>,
 Jann Horn <jannh@...gle.com>, Pedro Falcato <pfalcato@...e.de>
Subject: Re: [PATCH v12 2/4] mm/slub: allow to set node and align in
 k[v]realloc



On 7/14/25 10:14, Vlastimil Babka wrote:
> On 7/12/25 14:43, Vitaly Wool wrote:
>>
>>
>>> On Jul 11, 2025, at 5:43 PM, Vlastimil Babka <vbabka@...e.cz> wrote:
>>>
>>> On 7/11/25 10:58, Harry Yoo wrote:
>>>> On Wed, Jul 09, 2025 at 07:24:41PM +0200, Vitaly Wool wrote:
>>>>> static __always_inline __realloc_size(2) void *
>>>>> -__do_krealloc(const void *p, size_t new_size, gfp_t flags)
>>>>> +__do_krealloc(const void *p, size_t new_size, unsigned long align, gfp_t flags, int nid)
>>>>> {
>>>>> void *ret;
>>>>> size_t ks = 0;
>>>>> @@ -4859,6 +4859,20 @@ __do_krealloc(const void *p, size_t new_size, gfp_t flags)
>>>>> if (!kasan_check_byte(p))
>>>>> return NULL;
>>>>>
>>>>> + /* refuse to proceed if alignment is bigger than what kmalloc() provides */
>>>>> + if (!IS_ALIGNED((unsigned long)p, align) || new_size < align)
>>>>> + return NULL;
>>>>
>>>> Hmm but what happens if `p` is aligned to `align`, but the new object is not?
>>>>
>>>> For example, what will happen if we  allocate object with size=64, align=64
>>>> and then do krealloc with size=96, align=64...
>>>>
>>>> Or am I missing something?
>>>
>>> Good point. We extended the alignment guarantees in commit ad59baa31695
>>> ("slab, rust: extend kmalloc() alignment guarantees to remove Rust padding")
>>> for rust in a way that size 96 gives you alignment of 32. It assumes that
>>> rust side will ask for alignments that are power-of-two and sizes that are
>>> multiples of alignment. I think if that assumption is still honored than
>>> this will keep working, but the check added above (is it just a sanity check
>>> or something the rust side relies on?) doesn't seem correct?
>>>
>>
>> It is a sanity check and it should have looked like this:
>>
>>          if (!IS_ALIGNED((unsigned long)p, align) && new_size <= ks)
>>                  return NULL;
>>
>> and the reasoning for this is the following: if we don’t intend to reallocate (new size is not bigger than the original size), but the user requests a larger alignment, it’s a miss. Does that sound reasonable?
> 
> So taking a step back indeed the align passed to krealloc is indeed used
> only for this check. If it's really just a sanity check, then I'd rather not
> add this parameter to krealloc functions at all - kmalloc() itself also
> doesn't have it, so it's inconsistent that krealloc() would have it - but
> only to return NULL and not e.g. try to reallocate for alignment.
> 
> If it's not just a sanity check, it means it's expected that for some
> sequence of valid kvrealloc_node_align() calls it can return NULL and then
> rely on the fallback to vmalloc. That would be rather wasteful for the cases
> like going from 64 to 96 bytes etc. So in that case it would be better if
> krealloc did the reallocation, same as in cases when size increases. Of
> course it would still have to rely on the documented alignment guarantees
> only and not provide anything arbitrary. aligned_size() in
> rust/kernel/alloc/allocator.rs is responsible for that, AFAIK.
> 
> And I think it's not a sanity check but the latter - if the following is a
> valid k(v)realloc sequence (from Rust POV). The individual size+align
> combinations AFAIK are, but if it's valid to make them follow one another
> them like this, I don't know.
> 
> krealloc(size=96, align=32) -> can give object with 32 alignment only
> krealloc(size=64, align=64) -> doesn't increase size but wants alignment 64
> 

We should be able to correctly process these. I agree that making such 
cases fall back to vrealloc is suboptimal but it's a technically correct 
behavior. I understand that you would rather have a reallocation on the 
slub side in these cases, so this will look as

           if (!IS_ALIGNED((unsigned long)p, align) && new_size <= ks)
                   goto alloc_new;

I'll modify/retest for the next patchset iteration.

~Vitaly