linux-kernel - Re: [PATCH v12 1/4] mm/vmalloc: allow to set node and align in vrealloc

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Message-Id: <C240E394-C3F6-4A46-A9F3-E6D95A3F4DF3@konsulko.se>
Date: Thu, 10 Jul 2025 16:07:19 +0200
From: Vitaly Wool <vitaly.wool@...sulko.se>
To: "Liam R. Howlett" <Liam.Howlett@...cle.com>
Cc: 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>,
 Vlastimil Babka <vbabka@...e.cz>,
 rust-for-linux@...r.kernel.org,
 Lorenzo Stoakes <lorenzo.stoakes@...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 1/4] mm/vmalloc: allow to set node and align in
 vrealloc



> On Jul 9, 2025, at 9:01 PM, Liam R. Howlett <Liam.Howlett@...cle.com> wrote:
> 
> * Vitaly Wool <vitaly.wool@...sulko.se> [250709 13:24]:
>> Reimplement vrealloc() to be able to set node and alignment should
>> a user need to do so. Rename the function to vrealloc_node_align()
>> to better match what it actually does now and introduce macros for
>> vrealloc() and friends for backward compatibility.
>> 
>> With that change we also provide the ability for the Rust part of
>> the kernel to set node and alignment in its allocations.
>> 
>> Signed-off-by: Vitaly Wool <vitaly.wool@...sulko.se>
>> Reviewed-by: Uladzislau Rezki (Sony) <urezki@...il.com>
>> Reviewed-by: Vlastimil Babka <vbabka@...e.cz>
>> ---
>> include/linux/vmalloc.h | 12 +++++++++---
>> mm/nommu.c              |  3 ++-
>> mm/vmalloc.c            | 31 ++++++++++++++++++++++++++-----
>> 3 files changed, 37 insertions(+), 9 deletions(-)
>> 
> ...
> 
>> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
>> index 6dbcdceecae1..03dd06097b25 100644
>> --- a/mm/vmalloc.c
>> +++ b/mm/vmalloc.c
>> @@ -4089,19 +4089,31 @@ void *vzalloc_node_noprof(unsigned long size, int node)
>> EXPORT_SYMBOL(vzalloc_node_noprof);
>> 
>> /**
>> - * vrealloc - reallocate virtually contiguous memory; contents remain unchanged
>> + * vrealloc_node_align_noprof - reallocate virtually contiguous memory; contents
>> + * remain unchanged
>>  * @p: object to reallocate memory for
>>  * @size: the size to reallocate
>> + * @align: requested alignment
>>  * @flags: the flags for the page level allocator
>> + * @nid: node number of the target node
>> + *
>> + * If @p is %NULL, vrealloc_XXX() behaves exactly like vmalloc(). If @size is
>> + * 0 and @p is not a %NULL pointer, the object pointed to is freed.
>>  *
>> - * If @p is %NULL, vrealloc() behaves exactly like vmalloc(). If @size is 0 and
>> - * @p is not a %NULL pointer, the object pointed to is freed.
>> + * if @nid is not NUMA_NO_NODE, this function will try to allocate memory on
>> + * the given node. If reallocation is not necessary (e. g. the new size is less
>> + * than the current allocated size), the current allocation will be preserved
>> + * unless __GFP_THISNODE is set. In the latter case a new allocation on the
>> + * requested node will be attempted.
> 
> I am having a very hard time understanding what you mean here.  What is
> the latter case?
> 
> If @nis is !NUMA_NO_NODE, the allocation will be attempted on the given
> node.  Then things sort of get confusing.  What is the latter case?

The latter case is __GFP_THISNODE present in flags. That’s the latest if-clause in this paragraph.

> 
>>  *
>>  * If __GFP_ZERO logic is requested, callers must ensure that, starting with the
>>  * initial memory allocation, every subsequent call to this API for the same
>>  * memory allocation is flagged with __GFP_ZERO. Otherwise, it is possible that
>>  * __GFP_ZERO is not fully honored by this API.
>>  *
>> + * If the requested alignment is bigger than the one the *existing* allocation
>> + * has, this function will fail.
>> + *
> 
> It might be better to say something like:
> Requesting an alignment that is bigger than the alignment of the
> *existing* allocation will fail.
> 
The whole function description in fact consists of several if-clauses (some of which are nested) so I am just following the pattern here.

~Vitaly