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

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Message-ID: <nsacpwgldqdidsqkqalxdhwptikk7srnhjncmjaulnzcf6nsmu@fisb5w4aamhl>
Date: Wed, 9 Jul 2025 15:01:39 -0400
From: "Liam R. Howlett" <Liam.Howlett@...cle.com>
To: Vitaly Wool <vitaly.wool@...sulko.se>
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

* 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?

>   *
>   * 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.

...

Thanks,
Liam