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

Vlastimil Babka
2025-07-11T15:43:22Z

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 {
>