* Re: [PATCH 1/2] docs/core-api: memory-allocation: add k[mz]alloc_obj() and clarify kmalloc
2026-08-31 15:13 ` [PATCH 1/2] " Mike Rapoport (Microsoft)
@ 2026-08-31 15:26 ` Suren Baghdasaryan
2026-08-31 17:20 ` Randy Dunlap
` (2 subsequent siblings)
3 siblings, 0 replies; 12+ messages in thread
From: Suren Baghdasaryan @ 2026-08-31 15:26 UTC (permalink / raw)
To: Mike Rapoport (Microsoft)
Cc: Jonathan Corbet, Andrew Morton, David Hildenbrand,
Liam R. Howlett, Lorenzo Stoakes, Michal Hocko, Randy Dunlap,
Shuah Khan, Vlastimil Babka, linux-doc, linux-kernel, linux-mm
On Mon, Aug 31, 2026 at 8:13 AM Mike Rapoport (Microsoft)
<rppt@kernel.org> 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
s/hte/the
> 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@kernel.org>
Reviewed-by: Suren Baghdasaryan <surenb@google.com>
> ---
> Documentation/core-api/memory-allocation.rst | 30 +++++++++++++++++++++-------
> 1 file changed, 23 insertions(+), 7 deletions(-)
>
> diff --git a/Documentation/core-api/memory-allocation.rst b/Documentation/core-api/memory-allocation.rst
> index 0f19dd5243239..8ecfa4d35f4f5 100644
> --- a/Documentation/core-api/memory-allocation.rst
> +++ b/Documentation/core-api/memory-allocation.rst
> @@ -19,6 +19,12 @@ Diversity of the allocation APIs combined with the numerous GFP flags
> makes the question "How should I allocate memory?" not that easy to
> answer, although very likely you should use
>
> +::
> +
> + kzalloc_obj(<VAR_OR_TYPE>);
> +
> +or
> +
> ::
>
> kzalloc(<size>, GFP_KERNEL);
> @@ -139,10 +145,13 @@ allocate memory for an array, there are kmalloc_array() and kcalloc()
> helpers. The helpers struct_size(), array_size() and array3_size() can
> be used to safely calculate object sizes without overflowing.
>
> -The maximal size of a chunk that can be allocated with `kmalloc` is
> -limited. The actual limit depends on the hardware and the kernel
> -configuration, but it is a good practice to use `kmalloc` for objects
> -smaller than page size.
> +Since 7.0 there are type aware kmalloc-family helpers that let you safely and
> +conveniently allocate a single object or arrays of objects with kzalloc_obj()
> +and kmalloc_obj() and their array versions kzalloc_objs() and
> +kmalloc_objs(). These helpers only need the type of the object that should be
> +allocated and the count of elements in the array for the array versions.
> +
> +As of v7.2, vast majority of the memory allocations use kzalloc_obj().
>
> The address of a chunk allocated with `kmalloc` is aligned to at least
> ARCH_KMALLOC_MINALIGN bytes. For sizes which are a power of two, the
> @@ -154,9 +163,16 @@ Chunks allocated with kmalloc() can be resized with krealloc(). Similarly
> to kmalloc_array(): a helper for resizing arrays is provided in the form of
> krealloc_array().
>
> -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.
> +
> +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
>
> --
> 2.53.0
>
^ permalink raw reply [flat|nested] 12+ messages in thread* Re: [PATCH 1/2] docs/core-api: memory-allocation: add k[mz]alloc_obj() and clarify kmalloc
2026-08-31 15:13 ` [PATCH 1/2] " Mike Rapoport (Microsoft)
2026-08-31 15:26 ` Suren Baghdasaryan
@ 2026-08-31 17:20 ` Randy Dunlap
2026-09-01 3:39 ` SJ Park
2026-09-01 10:17 ` Vlastimil Babka (SUSE)
3 siblings, 0 replies; 12+ messages in thread
From: Randy Dunlap @ 2026-08-31 17:20 UTC (permalink / raw)
To: Mike Rapoport (Microsoft), Jonathan Corbet, Andrew Morton,
David Hildenbrand
Cc: Liam R. Howlett, Lorenzo Stoakes, Michal Hocko, Shuah Khan,
Suren Baghdasaryan, Vlastimil Babka, linux-doc, linux-kernel,
linux-mm
On 8/31/26 8:13 AM, 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@kernel.org>
> ---
> Documentation/core-api/memory-allocation.rst | 30 +++++++++++++++++++++-------
> 1 file changed, 23 insertions(+), 7 deletions(-)
>
> diff --git a/Documentation/core-api/memory-allocation.rst b/Documentation/core-api/memory-allocation.rst
> index 0f19dd5243239..8ecfa4d35f4f5 100644
> --- a/Documentation/core-api/memory-allocation.rst
> +++ b/Documentation/core-api/memory-allocation.rst
> @@ -19,6 +19,12 @@ Diversity of the allocation APIs combined with the numerous GFP flags
> makes the question "How should I allocate memory?" not that easy to
> answer, although very likely you should use
>
> +::
> +
> + kzalloc_obj(<VAR_OR_TYPE>);
> +
> +or
> +
> ::
>
> kzalloc(<size>, GFP_KERNEL);
> @@ -139,10 +145,13 @@ allocate memory for an array, there are kmalloc_array() and kcalloc()
> helpers. The helpers struct_size(), array_size() and array3_size() can
> be used to safely calculate object sizes without overflowing.
>
> -The maximal size of a chunk that can be allocated with `kmalloc` is
> -limited. The actual limit depends on the hardware and the kernel
> -configuration, but it is a good practice to use `kmalloc` for objects
> -smaller than page size.
> +Since 7.0 there are type aware kmalloc-family helpers that let you safely and
> +conveniently allocate a single object or arrays of objects with kzalloc_obj()
> +and kmalloc_obj() and their array versions kzalloc_objs() and
> +kmalloc_objs(). These helpers only need the type of the object that should be
> +allocated and the count of elements in the array for the array versions.
> +
> +As of v7.2, vast majority of the memory allocations use kzalloc_obj().
>
> The address of a chunk allocated with `kmalloc` is aligned to at least
> ARCH_KMALLOC_MINALIGN bytes. For sizes which are a power of two, the
> @@ -154,9 +163,16 @@ Chunks allocated with kmalloc() can be resized with krealloc(). Similarly
> to kmalloc_array(): a helper for resizing arrays is provided in the form of
> krealloc_array().
>
> -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
`MAX_ORDER`; the
> +same limit also applies to the page allocator.
> +
> +Internally, slab allocator differentiates allocations of different orders and
the slab allocator
?
> +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
>
--
~Randy
^ permalink raw reply [flat|nested] 12+ messages in thread* Re: [PATCH 1/2] docs/core-api: memory-allocation: add k[mz]alloc_obj() and clarify kmalloc
2026-08-31 15:13 ` [PATCH 1/2] " Mike Rapoport (Microsoft)
2026-08-31 15:26 ` Suren Baghdasaryan
2026-08-31 17:20 ` Randy Dunlap
@ 2026-09-01 3:39 ` SJ Park
2026-09-01 10:17 ` Vlastimil Babka (SUSE)
3 siblings, 0 replies; 12+ messages in thread
From: SJ Park @ 2026-09-01 3:39 UTC (permalink / raw)
To: Mike Rapoport (Microsoft)
Cc: SJ Park, Jonathan Corbet, Andrew Morton, David Hildenbrand,
Liam R. Howlett, Lorenzo Stoakes, Michal Hocko, Randy Dunlap,
Shuah Khan, Suren Baghdasaryan, Vlastimil Babka, linux-doc,
linux-kernel, linux-mm
On Mon, 31 Aug 2026 18:13:01 +0300 "Mike Rapoport (Microsoft)" <rppt@kernel.org> 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
s/hte/the/
> and make kzalloc_obj() the first answer to "How should I allocate
> memory?" question.
>
> While on it, clarify description of kmalloc() size limitations.
Looks good improvements to me.
>
> Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Acked-by: SJ Park <sj@kernel.org>
> ---
> Documentation/core-api/memory-allocation.rst | 30 +++++++++++++++++++++-------
> 1 file changed, 23 insertions(+), 7 deletions(-)
>
> diff --git a/Documentation/core-api/memory-allocation.rst b/Documentation/core-api/memory-allocation.rst
> index 0f19dd5243239..8ecfa4d35f4f5 100644
> --- a/Documentation/core-api/memory-allocation.rst
> +++ b/Documentation/core-api/memory-allocation.rst
> @@ -19,6 +19,12 @@ Diversity of the allocation APIs combined with the numerous GFP flags
> makes the question "How should I allocate memory?" not that easy to
> answer, although very likely you should use
>
> +::
> +
> + kzalloc_obj(<VAR_OR_TYPE>);
> +
> +or
> +
> ::
>
> kzalloc(<size>, GFP_KERNEL);
> @@ -139,10 +145,13 @@ allocate memory for an array, there are kmalloc_array() and kcalloc()
> helpers. The helpers struct_size(), array_size() and array3_size() can
> be used to safely calculate object sizes without overflowing.
>
> -The maximal size of a chunk that can be allocated with `kmalloc` is
> -limited. The actual limit depends on the hardware and the kernel
> -configuration, but it is a good practice to use `kmalloc` for objects
> -smaller than page size.
> +Since 7.0 there are type aware kmalloc-family helpers that let you safely and
> +conveniently allocate a single object or arrays of objects with kzalloc_obj()
> +and kmalloc_obj() and their array versions kzalloc_objs() and
> +kmalloc_objs(). These helpers only need the type of the object that should be
> +allocated and the count of elements in the array for the array versions.
> +
> +As of v7.2, vast majority of the memory allocations use kzalloc_obj().
As the commit message says "Since v7.0 the most used memory allocation function
is kzalloc_obj()", should we s/7.2/7.0/ ?
I also feel like the above sentence might not really needed, or can be
version-neutral, e.g., Since its introduction, vast majoritiy of ... switched
to use ... ? No strong opinion.
>
> The address of a chunk allocated with `kmalloc` is aligned to at least
> ARCH_KMALLOC_MINALIGN bytes. For sizes which are a power of two, the
> @@ -154,9 +163,16 @@ Chunks allocated with kmalloc() can be resized with krealloc(). Similarly
> to kmalloc_array(): a helper for resizing arrays is provided in the form of
> krealloc_array().
>
> -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.
I initially thought if it makes look more consistent by using kmalloc() instead
of `kmalloc`. But seems anyway we are mixing those here and there. So I have
no opinion.
> +
> +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
>
> --
> 2.53.0
Thanks,
SJ
^ permalink raw reply [flat|nested] 12+ messages in thread
* Re: [PATCH 1/2] docs/core-api: memory-allocation: add k[mz]alloc_obj() and clarify kmalloc
2026-08-31 15:13 ` [PATCH 1/2] " Mike Rapoport (Microsoft)
` (2 preceding siblings ...)
2026-09-01 3:39 ` SJ Park
@ 2026-09-01 10:17 ` Vlastimil Babka (SUSE)
2026-09-01 16:10 ` Mike Rapoport
3 siblings, 1 reply; 12+ messages in thread
From: Vlastimil Babka (SUSE) @ 2026-09-01 10:17 UTC (permalink / raw)
To: Mike Rapoport (Microsoft), Jonathan Corbet, Andrew Morton,
David Hildenbrand
Cc: Liam R. Howlett, Lorenzo Stoakes, Michal Hocko, Randy Dunlap,
Shuah Khan, Suren Baghdasaryan, linux-doc, linux-kernel, linux-mm
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@kernel.org>
Acked-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
Suggestion below:
> ---
> Documentation/core-api/memory-allocation.rst | 30 +++++++++++++++++++++-------
> 1 file changed, 23 insertions(+), 7 deletions(-)
>
> diff --git a/Documentation/core-api/memory-allocation.rst b/Documentation/core-api/memory-allocation.rst
> index 0f19dd5243239..8ecfa4d35f4f5 100644
> --- a/Documentation/core-api/memory-allocation.rst
> +++ b/Documentation/core-api/memory-allocation.rst
> @@ -19,6 +19,12 @@ Diversity of the allocation APIs combined with the numerous GFP flags
> makes the question "How should I allocate memory?" not that easy to
> answer, although very likely you should use
>
> +::
> +
> + kzalloc_obj(<VAR_OR_TYPE>);
> +
> +or
> +
> ::
>
> kzalloc(<size>, GFP_KERNEL);
> @@ -139,10 +145,13 @@ allocate memory for an array, there are kmalloc_array() and kcalloc()
> helpers. The helpers struct_size(), array_size() and array3_size() can
> be used to safely calculate object sizes without overflowing.
>
> -The maximal size of a chunk that can be allocated with `kmalloc` is
> -limited. The actual limit depends on the hardware and the kernel
> -configuration, but it is a good practice to use `kmalloc` for objects
> -smaller than page size.
> +Since 7.0 there are type aware kmalloc-family helpers that let you safely and
> +conveniently allocate a single object or arrays of objects with kzalloc_obj()
> +and kmalloc_obj() and their array versions kzalloc_objs() and
> +kmalloc_objs(). These helpers only need the type of the object that should be
> +allocated and the count of elements in the array for the array versions.
> +
> +As of v7.2, vast majority of the memory allocations use kzalloc_obj().
>
> The address of a chunk allocated with `kmalloc` is aligned to at least
> ARCH_KMALLOC_MINALIGN bytes. For sizes which are a power of two, the
> @@ -154,9 +163,16 @@ Chunks allocated with kmalloc() can be resized with krealloc(). Similarly
> to kmalloc_array(): a helper for resizing arrays is provided in the form of
> krealloc_array().
>
> -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? (Also `MAX_ORDER` doesn't
exist anymore)
> +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
>
^ permalink raw reply [flat|nested] 12+ messages in thread* Re: [PATCH 1/2] docs/core-api: memory-allocation: add k[mz]alloc_obj() and clarify kmalloc
2026-09-01 10:17 ` Vlastimil Babka (SUSE)
@ 2026-09-01 16:10 ` Mike Rapoport
2026-09-02 7:50 ` Vlastimil Babka (SUSE)
0 siblings, 1 reply; 12+ messages in thread
From: Mike Rapoport @ 2026-09-01 16:10 UTC (permalink / raw)
To: Vlastimil Babka (SUSE)
Cc: Jonathan Corbet, Andrew Morton, David Hildenbrand,
Liam R. Howlett, Lorenzo Stoakes, Michal Hocko, Randy Dunlap,
Shuah Khan, Suren Baghdasaryan, linux-doc, linux-kernel, linux-mm
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@kernel.org>
>
> Acked-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
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.
^ permalink raw reply [flat|nested] 12+ messages in thread
* Re: [PATCH 1/2] docs/core-api: memory-allocation: add k[mz]alloc_obj() and clarify kmalloc
2026-09-01 16:10 ` Mike Rapoport
@ 2026-09-02 7:50 ` Vlastimil Babka (SUSE)
0 siblings, 0 replies; 12+ messages in thread
From: Vlastimil Babka (SUSE) @ 2026-09-02 7:50 UTC (permalink / raw)
To: Mike Rapoport
Cc: Jonathan Corbet, Andrew Morton, David Hildenbrand,
Liam R. Howlett, Lorenzo Stoakes, Michal Hocko, Randy Dunlap,
Shuah Khan, Suren Baghdasaryan, linux-doc, linux-kernel, linux-mm
On 9/1/26 18:10, Mike Rapoport wrote:
> 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@kernel.org>
>>
>> Acked-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
>
> 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.
The actual slab allocations limit is KMALLOC_MAX_CACHE_SIZE and that's
indeed not the point and hence I wouldn't mention it here. KMALLOC_MAX_SIZE
is literally a conversion of MAX_PAGE_ORDER to bytes, and since kmalloc()
takes bytes and not order as an argument, it seems more natural to me, e.g.:
`kmalloc` always allocates physically contiguous memory and the maximal size
of a chunk that can be allocated with `kmalloc` is limited by
`KMALLOC_MAX_SIZE`, which matches the page allocator's MAX_PAGE_ORDER limit.
>> (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
>> >
>>
>
^ permalink raw reply [flat|nested] 12+ messages in thread