Re: [PATCH 1/2] docs/core-api: memory-allocation: add k[mz]alloc_obj() and clarify kmalloc

[Mike Rapoport]

On Tue, Sep 01, 2026 at 12:17:02PM +0200, Vlastimil Babka (SUSE) wrote:
> On 8/31/26 17:13, Mike Rapoport (Microsoft) wrote:
> > Since v7.0 the most used memory allocation function is kzalloc_obj().
> > 
> > Update hte memory-allocation guide to describe k[mz]alloc_obj() family
> > and make kzalloc_obj() the first answer to "How should I allocate
> > memory?" question.
> > 
> > While on it, clarify description of kmalloc() size limitations.
> > 
> > Signed-off-by: Mike Rapoport (Microsoft) <rppt@xxxxxxxxxx>
> 
> Acked-by: Vlastimil Babka (SUSE) <vbabka@xxxxxxxxxx>

Thanks!
 
> Suggestion below:
> 
> > ---
> >  Documentation/core-api/memory-allocation.rst | 30 +++++++++++++++++++++-------
> >  1 file changed, 23 insertions(+), 7 deletions(-)
> > 
> > -For large allocations you can use vmalloc() and vzalloc(), or directly
> > -request pages from the page allocator. The memory allocated by `vmalloc`
> > -and related functions is not physically contiguous.
> > +`kmalloc` always allocates physically contiguous memory and the maximal size of
> > +a chunk that can be allocated with `kmalloc` is limited by `MAX_ORDER`, the
> > +same limit also applies to the page allocator.
> 
> Would it be better to say the maximal size is KMALLOC_MAX_SIZE which is
> derived from page allocator's MAX_PAGE_ORDER? 

My point was that whenever you allocate with kmalloc() the limit is
MAX_PAGE_ORDER, not necessarily for actual slab allocations.

> (Also `MAX_ORDER` doesn't exist anymore)

Will fix.
 
> > +Internally, slab allocator differentiates allocations of different orders and
> > +delegates larger allocations to the page allocator, but for the users of
> > +`kmalloc` family it is entirely transparent.
> > +
> > +For large allocations that do not require physically contiguous memory you can
> > +use vmalloc() and vzalloc() family.
> >  
> >  If you are not sure whether the allocation size is too large for
> >  `kmalloc`, it is possible to use kvmalloc() and its derivatives. It will
> > 
> 

-- 
Sincerely yours,
Mike.