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

[Vitaly Wool <vitaly.wool@konsulko.se>]

> On Jul 10, 2025, at 5:19 PM, Lorenzo Stoakes <lorenzo.stoakes@oracle.com> wrote:
> 
> On Thu, Jul 10, 2025 at 08:21:19AM +0200, Vitaly Wool wrote:
>> 
>> 
>>> On Jul 9, 2025, at 9:01 PM, Liam R. Howlett <Liam.Howlett@oracle.com> wrote:
>>> 
>>> * Vitaly Wool <vitaly.wool@konsulko.se <mailto:vitaly.wool@konsulko.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@konsulko.se>
>>>> Reviewed-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
>>>> Reviewed-by: Vlastimil Babka <vbabka@suse.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.
> 
> Agreed with Liam, this is completely unreadable.
> 
> I think the numa node stuff is unnecesasry, that's pretty much inferred.
> 
> I'd just go with something like 'if the function can void having to reallocate
> then it does'.
> 
> Nice and simple :)

I think it is important to stress that the function is not always following the specified nid.
How about “If the caller wants the new memory to be on specific node *only*, __GFP_THISNODE flag should be set, otherwise the function will try to avoid reallocation and possibly disregard the specified @nid” ?

> 
>>> 
>>> 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.
> 
> Right, but in no sane world is essentially describing a series of if-clauses in
> a kerneldoc a thing.
> 
> Just it keep it simple, this is meant to be an overview, people can go read the
> code if they need details :)
> 
Alright, no strong feelings about it anyway. Will reword as you guys suggest.

Thanks,
Vitaly