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

[Vlastimil Babka <vbabka@suse.cz>]

On 7/11/25 10:58, Harry Yoo wrote:
> On Wed, Jul 09, 2025 at 07:24:41PM +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, add
>> k[v]realloc_node_align() functions and redefine the rest of API
>> using these new ones.
>> 
>> While doing that, we also keep the number of  _noprof variants to a
>> minimum, which implies some changes to the existing users of older
>> _noprof functions, that basically being bcachefs.
>> 
>> With that change we also provide the ability for the Rust part of
>> the kernel to set node and alignment in its K[v]xxx
>> [re]allocations.
>> 
>> Signed-off-by: Vitaly Wool <vitaly.wool@konsulko.se>
>> ---
>>  fs/bcachefs/darray.c   |  2 +-
>>  fs/bcachefs/util.h     |  2 +-
>>  include/linux/bpfptr.h |  2 +-
>>  include/linux/slab.h   | 38 +++++++++++++++----------
>>  lib/rhashtable.c       |  4 +--
>>  mm/slub.c              | 64 +++++++++++++++++++++++++++++-------------
>>  6 files changed, 72 insertions(+), 40 deletions(-)
>  
>> diff --git a/mm/slub.c b/mm/slub.c
>> index c4b64821e680..6fad4cdea6c4 100644
>> --- a/mm/slub.c
>> +++ b/mm/slub.c
>> @@ -4845,7 +4845,7 @@ void kfree(const void *object)
>>  EXPORT_SYMBOL(kfree);
>>  
>>  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?

>> +	/*
>> +	 * 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.
>> +	 */
>> +	if (unlikely(flags & __GFP_THISNODE) && nid != NUMA_NO_NODE &&
>> +		     nid != page_to_nid(virt_to_page(p)))
>> +		goto alloc_new;
>> +
>>  	if (is_kfence_address(p)) {
>>  		ks = orig_size = kfence_ksize(p);
>>  	} else {
>