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

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Message-Id: <C6331389-1008-4D8B-99AA-C2640671E9A2@konsulko.se>
Date: Fri, 27 Jun 2025 14:01:23 +0200
From: Vitaly Wool <vitaly.wool@...sulko.se>
To: Uladzislau Rezki <urezki@...il.com>
Cc: Danilo Krummrich <dakr@...nel.org>,
 linux-mm@...ck.org,
 akpm@...ux-foundation.org,
 linux-kernel@...r.kernel.org,
 Alice Ryhl <aliceryhl@...gle.com>,
 rust-for-linux@...r.kernel.org
Subject: Re: [PATCH 2/4] mm/slub: allow to set node and align in k[v]realloc



> On Jun 27, 2025, at 1:42 PM, Uladzislau Rezki <urezki@...il.com> wrote:
> 
> Hello, Vitaly, Danilo.
> 
>> On Fri, Jun 27, 2025 at 11:37:14AM +0200, Vitaly Wool wrote:
>>> Reimplement k[v]realloc_node() to be able to set node and
>>> alignment should a user need to do so. In order to do that while
>>> retaining the maximal backward compatibility, the following rules
>>> are honored:
>>> * kmalloc/kzalloc/krealloc remain unchanged
>>> * kvmalloc/kvrealloc/kvcalloc remain unchanged
>>> * kvrealloc remains unchanged
>>> * kvrealloc_node is implemented as a new function taking align and
>>>  NUMA id as extra parameters compared to kvrealloc.
>>> * krealloc_node is implemented as a new function taking NUMA id
>>>  as an extra parameter compared to krealloc
>>> * kvmalloc_node/kvzalloc_node/kvcalloc_node get an extra parameter
>>>  (alignment)
>> 
>> I see what you're doing here:
>> 
>> You created vrealloc_node_noprof() in the previous patch, taking the following
>> arguments:
>> 
>> vrealloc_node_noprof(const void *p, size_t size,
>>      unsigned long align,
>>      gfp_t flags, int nid)
>> 
>> And now you're aligning the newly introduced krealloc_node() and
>> kvrealloc_node() with that.
>> 
>> The idea for having an align argument on krealloc_node() simply is that it
>> fails if the alignment requirement can't be fulfilled by the corresponding
>> kmalloc bucket, such that we can fall back to vrealloc_node() in
>> kvrealloc_node().
>> 
>> Generally, this makes sense to me.
>> 
>> However, now you consequently have to add the align argument to kvmalloc_node(),
>> kvzalloc_node(), kvcalloc_node() as well.
>> 
>> This is what creates this huge diffstat changing all the users.
>> 
>> IMHO, the problem here was introduced already with vrealloc_node_noprof() taking
>> an align argument in your previous patch, since now you have to adjust
>> everything else to logically follow the same naming scheme.
>> 
>> Instead, I think you should introduce vrealloc_node_align(),
>> 
> I am probably missing something. Could you please clarify why do you
> need the vrealloc_node_align() and other friends? Do you have users
> which require vrealloc() or kvrealloc() to support nid and align from
> Rust API point of view? 
> 
> 

Alignment for Rust allocators should generally be supported, it’s been listed as TODO for a while.
Node awareness is very desirable for e.g. KVBox and for the coming zpool mapping.

~Vitaly