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Mike Rapoport Aug. 15, 2018, 6:34 a.m. UTC
As Vlastimil mentioned at [1], it would be nice to have some guide about
memory allocation. I've drafted an initial version that tries to summarize
"best practices" for allocation functions and GFP usage.

[1] https://www.spinics.net/lists/netfilter-devel/msg55542.html

From 8027c0d4b750b8dbd687234feda63305d0d5a057 Mon Sep 17 00:00:00 2001
From: Mike Rapoport <rppt@linux.vnet.ibm.com>
Date: Wed, 15 Aug 2018 09:10:06 +0300
Subject: [RFC PATCH] docs/core-api: add memory allocation guide

Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
---
 Documentation/core-api/gfp_mask-from-fs-io.rst |   2 +
 Documentation/core-api/index.rst               |   1 +
 Documentation/core-api/memory-allocation.rst   | 117 +++++++++++++++++++++++++
 Documentation/core-api/mm-api.rst              |   2 +
 4 files changed, 122 insertions(+)
 create mode 100644 Documentation/core-api/memory-allocation.rst

Comments

Mike Rapoport Aug. 15, 2018, 6:36 a.m. UTC | #1
(this time with the subject, sorry for the noise)

On Wed, Aug 15, 2018 at 09:34:47AM +0300, Mike Rapoport wrote:
> As Vlastimil mentioned at [1], it would be nice to have some guide about
> memory allocation. I've drafted an initial version that tries to summarize
> "best practices" for allocation functions and GFP usage.
> 
> [1] https://www.spinics.net/lists/netfilter-devel/msg55542.html
> 
> From 8027c0d4b750b8dbd687234feda63305d0d5a057 Mon Sep 17 00:00:00 2001
> From: Mike Rapoport <rppt@linux.vnet.ibm.com>
> Date: Wed, 15 Aug 2018 09:10:06 +0300
> Subject: [RFC PATCH] docs/core-api: add memory allocation guide
> 
> Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
> ---
>  Documentation/core-api/gfp_mask-from-fs-io.rst |   2 +
>  Documentation/core-api/index.rst               |   1 +
>  Documentation/core-api/memory-allocation.rst   | 117 +++++++++++++++++++++++++
>  Documentation/core-api/mm-api.rst              |   2 +
>  4 files changed, 122 insertions(+)
>  create mode 100644 Documentation/core-api/memory-allocation.rst
> 
> diff --git a/Documentation/core-api/gfp_mask-from-fs-io.rst b/Documentation/core-api/gfp_mask-from-fs-io.rst
> index e0df8f4..e7c32a8 100644
> --- a/Documentation/core-api/gfp_mask-from-fs-io.rst
> +++ b/Documentation/core-api/gfp_mask-from-fs-io.rst
> @@ -1,3 +1,5 @@
> +.. _gfp_mask_from_fs_io:
> +
>  =================================
>  GFP masks used from FS/IO context
>  =================================
> diff --git a/Documentation/core-api/index.rst b/Documentation/core-api/index.rst
> index cdc2020..8afc0da 100644
> --- a/Documentation/core-api/index.rst
> +++ b/Documentation/core-api/index.rst
> @@ -27,6 +27,7 @@ Core utilities
>     errseq
>     printk-formats
>     circular-buffers
> +   memory-allocation
>     mm-api
>     gfp_mask-from-fs-io
>     timekeeping
> diff --git a/Documentation/core-api/memory-allocation.rst b/Documentation/core-api/memory-allocation.rst
> new file mode 100644
> index 0000000..b1f2ad5
> --- /dev/null
> +++ b/Documentation/core-api/memory-allocation.rst
> @@ -0,0 +1,117 @@
> +=======================
> +Memory Allocation Guide
> +=======================
> +
> +Linux supplies variety of APIs for memory allocation. You can allocate
> +small chunks using `kmalloc` or `kmem_cache_alloc` families, large
> +virtually contiguous areas using `vmalloc` and it's derivatives, or
> +you can directly request pages from the page allocator with
> +`__get_free_pages`. It is also possible to use more specialized
> +allocators, for instance `cma_alloc` or `zs_malloc`.
> +
> +Most of the memory allocations APIs use GFP flags to express how that
> +memory should be allocated. The GFP acronym stands for "get free
> +pages", the underlying memory allocation function.
> +
> +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(<size>, GFP_KERNEL);
> +
> +Of course there are cases when other allocation APIs and different GFP
> +flags must be used.
> +
> +Get Free Page flags
> +===================
> +
> +The GFP flags control the allocators behavior. They tell what memory
> +zones can be used, how hard the allocator should try to find a free
> +memory, whether the memory can be accessed by the userspace etc. The
> +:ref:`Documentation/core-api/mm-api.rst <mm-api-gfp-flags>` provides
> +reference documentation for the GFP flags and their combinations and
> +here we briefly outline their recommended usage:
> +
> +  * Most of the times ``GFP_KERNEL`` is what you need. Memory for the
> +    kernel data structures, DMAable memory, inode cache, all these and
> +    many other allocations types can use ``GFP_KERNEL``. Note, that
> +    using ``GFP_KERNEL`` implies ``GFP_RECLAIM``, which means that
> +    direct reclaim may be triggered under memory pressure; the calling
> +    context must be allowed to sleep.
> +  * If the allocation is performed from an atomic context, e.g
> +    interrupt handler, use ``GFP_ATOMIC``.
> +  * Untrusted allocations triggered from userspace should be a subject
> +    of kmem accounting and must have ``__GFP_ACCOUNT`` bit set. There
> +    is handy ``GFP_KERNEL_ACCOUNT`` shortcut for ``GFP_KERNEL``
> +    allocations that should be accounted.
> +  * Userspace allocations should use either of the ``GFP_USER``,
> +    ``GFP_HIGHUSER`` and ``GFP_HIGHUSER_MOVABLE`` flags. The longer
> +    the flag name the less restrictive it is.
> +
> +    The ``GFP_HIGHUSER_MOVABLE`` does not require that allocated
> +    memory will be directly accessible by the kernel or the hardware
> +    and implies that the data may move.
> +
> +    The ``GFP_HIGHUSER`` means that the allocated memory is not
> +    movable, but it is not required to be directly accessible by the
> +    kernel or the hardware. An example may be a hardware allocation
> +    that maps data directly into userspace but has no addressing
> +    limitations.
> +
> +    The ``GFP_USER`` means that the allocated memory is not movable
> +    and it must be directly accessible by the kernel or the
> +    hardware. It is typically used by hardware for buffers that are
> +    mapped to userspace (e.g. graphics) that hardware still must DMA
> +    to.
> +
> +You may notice that quite a few allocations in the existing code
> +specify ``GFP_NOIO`` and ``GFP_NOFS``. Historically, they were used to
> +prevent recursion deadlocks caused by direct memory reclaim calling
> +back into the FS or IO paths and blocking on already held
> +resources. Since 4.12 the preferred way to address this issue is to
> +use new scope APIs described in
> +:ref:`Documentation/core-api/gfp_mask-from-fs-io.rst <gfp_mask_from_fs_io>`.
> +
> +Another legacy GFP flags are ``GFP_DMA`` and ``GFP_DMA32``. They are
> +used to ensure that the allocated memory is accessible by hardware
> +with limited addressing capabilities. So unless you are writing a
> +driver for a device with such restrictions, avoid using these flags.
> +
> +Selecting memory allocator
> +==========================
> +
> +The most straightforward way to allocate memory is to use a function
> +from the `kmalloc` family. And, to be on the safe size it's best to
> +use routines that set memory to zero, like `kzalloc`. If you need to
> +allocate memory for an array, there are `kmalloc_array` and `kcalloc`
> +helpers.
> +
> +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.
> +
> +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.
> +
> +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 try to allocate memory with `kmalloc` and if the allocation fails
> +it will be retried with `vmalloc`. There are restrictions on which GFP
> +flags can be used with `kvmalloc`, please see :c:func:`kvmalloc_node`
> +reference documentation. Note, that `kvmalloc` may return memory that
> +is not physically contiguous.
> +
> +If you need to allocate many identical objects you can use slab cache
> +allocator. The cache should be set up with `kmem_cache_create` before
> +it can be used. Afterwards `kmem_cache_alloc` and its convenience
> +wrappers can allocate memory from that cache.
> +
> +When the allocated memory is no longer needed it must be freed. You
> +can use `kvfree` for the memory allocated with `kmalloc`, `vmalloc`
> +and `kvmalloc`. The slab caches should be freed with
> +`kmem_cache_free`. And don't forget to destroy the cache with
> +`kmem_cache_destroy`.
> diff --git a/Documentation/core-api/mm-api.rst b/Documentation/core-api/mm-api.rst
> index 46ae353..5ce1ec1 100644
> --- a/Documentation/core-api/mm-api.rst
> +++ b/Documentation/core-api/mm-api.rst
> @@ -14,6 +14,8 @@ User Space Memory Access
>  .. kernel-doc:: mm/util.c
>     :functions: get_user_pages_fast
> 
> +.. _mm-api-gfp-flags:
> +
>  Memory Allocation Controls
>  ==========================
> 
> -- 
> 2.7.4
>
Michal Hocko Aug. 15, 2018, 8:15 a.m. UTC | #2
On Wed 15-08-18 09:36:49, Mike Rapoport wrote:
> (this time with the subject, sorry for the noise)
> 
> On Wed, Aug 15, 2018 at 09:34:47AM +0300, Mike Rapoport wrote:
> > As Vlastimil mentioned at [1], it would be nice to have some guide about
> > memory allocation. I've drafted an initial version that tries to summarize
> > "best practices" for allocation functions and GFP usage.
> > 
> > [1] https://www.spinics.net/lists/netfilter-devel/msg55542.html
> > 
> > From 8027c0d4b750b8dbd687234feda63305d0d5a057 Mon Sep 17 00:00:00 2001
> > From: Mike Rapoport <rppt@linux.vnet.ibm.com>
> > Date: Wed, 15 Aug 2018 09:10:06 +0300
> > Subject: [RFC PATCH] docs/core-api: add memory allocation guide
> > 
> > Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
> > ---
> >  Documentation/core-api/gfp_mask-from-fs-io.rst |   2 +
> >  Documentation/core-api/index.rst               |   1 +
> >  Documentation/core-api/memory-allocation.rst   | 117 +++++++++++++++++++++++++
> >  Documentation/core-api/mm-api.rst              |   2 +
> >  4 files changed, 122 insertions(+)
> >  create mode 100644 Documentation/core-api/memory-allocation.rst
> > 
> > diff --git a/Documentation/core-api/gfp_mask-from-fs-io.rst b/Documentation/core-api/gfp_mask-from-fs-io.rst
> > index e0df8f4..e7c32a8 100644
> > --- a/Documentation/core-api/gfp_mask-from-fs-io.rst
> > +++ b/Documentation/core-api/gfp_mask-from-fs-io.rst
> > @@ -1,3 +1,5 @@
> > +.. _gfp_mask_from_fs_io:
> > +
> >  =================================
> >  GFP masks used from FS/IO context
> >  =================================
> > diff --git a/Documentation/core-api/index.rst b/Documentation/core-api/index.rst
> > index cdc2020..8afc0da 100644
> > --- a/Documentation/core-api/index.rst
> > +++ b/Documentation/core-api/index.rst
> > @@ -27,6 +27,7 @@ Core utilities
> >     errseq
> >     printk-formats
> >     circular-buffers
> > +   memory-allocation
> >     mm-api
> >     gfp_mask-from-fs-io
> >     timekeeping
> > diff --git a/Documentation/core-api/memory-allocation.rst b/Documentation/core-api/memory-allocation.rst
> > new file mode 100644
> > index 0000000..b1f2ad5
> > --- /dev/null
> > +++ b/Documentation/core-api/memory-allocation.rst
> > @@ -0,0 +1,117 @@
> > +=======================
> > +Memory Allocation Guide
> > +=======================
> > +
> > +Linux supplies variety of APIs for memory allocation. You can allocate
> > +small chunks using `kmalloc` or `kmem_cache_alloc` families, large
> > +virtually contiguous areas using `vmalloc` and it's derivatives, or
> > +you can directly request pages from the page allocator with
> > +`__get_free_pages`. It is also possible to use more specialized

I would rather not mention __get_free_pages. alloc_pages is a more
generic API and less subtle one. If you want to mention __get_free_pages
then please make sure to mention the subtlety (namely that is can
allocate only lowmem memory).

> > +allocators, for instance `cma_alloc` or `zs_malloc`.
> > +
> > +Most of the memory allocations APIs use GFP flags to express how that
> > +memory should be allocated. The GFP acronym stands for "get free
> > +pages", the underlying memory allocation function.
> > +
> > +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(<size>, GFP_KERNEL);
> > +
> > +Of course there are cases when other allocation APIs and different GFP
> > +flags must be used.
> > +
> > +Get Free Page flags
> > +===================
> > +
> > +The GFP flags control the allocators behavior. They tell what memory
> > +zones can be used, how hard the allocator should try to find a free
> > +memory, whether the memory can be accessed by the userspace etc. The
> > +:ref:`Documentation/core-api/mm-api.rst <mm-api-gfp-flags>` provides
> > +reference documentation for the GFP flags and their combinations and
> > +here we briefly outline their recommended usage:
> > +
> > +  * Most of the times ``GFP_KERNEL`` is what you need. Memory for the
> > +    kernel data structures, DMAable memory, inode cache, all these and
> > +    many other allocations types can use ``GFP_KERNEL``. Note, that
> > +    using ``GFP_KERNEL`` implies ``GFP_RECLAIM``, which means that
> > +    direct reclaim may be triggered under memory pressure; the calling
> > +    context must be allowed to sleep.
> > +  * If the allocation is performed from an atomic context, e.g
> > +    interrupt handler, use ``GFP_ATOMIC``.

GFP_NOWAIT please. GFP_ATOMIC should be only used if accessing memory
reserves is justified. E.g. fallback allocation would be too costly. It
should be also noted that these allocation are quite likely to fail
especially under memory pressure.

> > +  * Untrusted allocations triggered from userspace should be a subject
> > +    of kmem accounting and must have ``__GFP_ACCOUNT`` bit set. There
> > +    is handy ``GFP_KERNEL_ACCOUNT`` shortcut for ``GFP_KERNEL``
> > +    allocations that should be accounted.
> > +  * Userspace allocations should use either of the ``GFP_USER``,
> > +    ``GFP_HIGHUSER`` and ``GFP_HIGHUSER_MOVABLE`` flags. The longer
> > +    the flag name the less restrictive it is.
> > +
> > +    The ``GFP_HIGHUSER_MOVABLE`` does not require that allocated
> > +    memory will be directly accessible by the kernel or the hardware
> > +    and implies that the data may move.

@may move@is movable@

> > +    The ``GFP_HIGHUSER`` means that the allocated memory is not
> > +    movable, but it is not required to be directly accessible by the
> > +    kernel or the hardware. An example may be a hardware allocation
> > +    that maps data directly into userspace but has no addressing
> > +    limitations.
> > +
> > +    The ``GFP_USER`` means that the allocated memory is not movable
> > +    and it must be directly accessible by the kernel or the
> > +    hardware. It is typically used by hardware for buffers that are
> > +    mapped to userspace (e.g. graphics) that hardware still must DMA
> > +    to.
> > +
> > +You may notice that quite a few allocations in the existing code
> > +specify ``GFP_NOIO`` and ``GFP_NOFS``. Historically, they were used to
> > +prevent recursion deadlocks caused by direct memory reclaim calling
> > +back into the FS or IO paths and blocking on already held
> > +resources. Since 4.12 the preferred way to address this issue is to
> > +use new scope APIs described in
> > +:ref:`Documentation/core-api/gfp_mask-from-fs-io.rst <gfp_mask_from_fs_io>`.
> > +
> > +Another legacy GFP flags are ``GFP_DMA`` and ``GFP_DMA32``. They are
> > +used to ensure that the allocated memory is accessible by hardware
> > +with limited addressing capabilities. So unless you are writing a
> > +driver for a device with such restrictions, avoid using these flags.

And even with HW with restrictions it is preferable to use dma_alloc*
APIs

Looks nice otherwise. Thanks! With the above changes feel free to add
Acked-by: Michal Hocko <mhocko@suse.com>
Mike Rapoport Aug. 15, 2018, 9:04 a.m. UTC | #3
On Wed, Aug 15, 2018 at 10:15:39AM +0200, Michal Hocko wrote:
> On Wed 15-08-18 09:36:49, Mike Rapoport wrote:
> > (this time with the subject, sorry for the noise)
> > 
> > On Wed, Aug 15, 2018 at 09:34:47AM +0300, Mike Rapoport wrote:
> > > As Vlastimil mentioned at [1], it would be nice to have some guide about
> > > memory allocation. I've drafted an initial version that tries to summarize
> > > "best practices" for allocation functions and GFP usage.
> > > 
> > > [1] https://www.spinics.net/lists/netfilter-devel/msg55542.html
> > > 
> > > From 8027c0d4b750b8dbd687234feda63305d0d5a057 Mon Sep 17 00:00:00 2001
> > > From: Mike Rapoport <rppt@linux.vnet.ibm.com>
> > > Date: Wed, 15 Aug 2018 09:10:06 +0300
> > > Subject: [RFC PATCH] docs/core-api: add memory allocation guide
> > > 
> > > Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
> > > ---
> > >  Documentation/core-api/gfp_mask-from-fs-io.rst |   2 +
> > >  Documentation/core-api/index.rst               |   1 +
> > >  Documentation/core-api/memory-allocation.rst   | 117 +++++++++++++++++++++++++
> > >  Documentation/core-api/mm-api.rst              |   2 +
> > >  4 files changed, 122 insertions(+)
> > >  create mode 100644 Documentation/core-api/memory-allocation.rst
> > > 
> > > diff --git a/Documentation/core-api/gfp_mask-from-fs-io.rst b/Documentation/core-api/gfp_mask-from-fs-io.rst
> > > index e0df8f4..e7c32a8 100644
> > > --- a/Documentation/core-api/gfp_mask-from-fs-io.rst
> > > +++ b/Documentation/core-api/gfp_mask-from-fs-io.rst
> > > @@ -1,3 +1,5 @@
> > > +.. _gfp_mask_from_fs_io:
> > > +
> > >  =================================
> > >  GFP masks used from FS/IO context
> > >  =================================
> > > diff --git a/Documentation/core-api/index.rst b/Documentation/core-api/index.rst
> > > index cdc2020..8afc0da 100644
> > > --- a/Documentation/core-api/index.rst
> > > +++ b/Documentation/core-api/index.rst
> > > @@ -27,6 +27,7 @@ Core utilities
> > >     errseq
> > >     printk-formats
> > >     circular-buffers
> > > +   memory-allocation
> > >     mm-api
> > >     gfp_mask-from-fs-io
> > >     timekeeping
> > > diff --git a/Documentation/core-api/memory-allocation.rst b/Documentation/core-api/memory-allocation.rst
> > > new file mode 100644
> > > index 0000000..b1f2ad5
> > > --- /dev/null
> > > +++ b/Documentation/core-api/memory-allocation.rst
> > > @@ -0,0 +1,117 @@
> > > +=======================
> > > +Memory Allocation Guide
> > > +=======================
> > > +
> > > +Linux supplies variety of APIs for memory allocation. You can allocate
> > > +small chunks using `kmalloc` or `kmem_cache_alloc` families, large
> > > +virtually contiguous areas using `vmalloc` and it's derivatives, or
> > > +you can directly request pages from the page allocator with
> > > +`__get_free_pages`. It is also possible to use more specialized
> 
> I would rather not mention __get_free_pages. alloc_pages is a more
> generic API and less subtle one. If you want to mention __get_free_pages
> then please make sure to mention the subtlety (namely that is can
> allocate only lowmem memory).
> 
> > > +allocators, for instance `cma_alloc` or `zs_malloc`.
> > > +
> > > +Most of the memory allocations APIs use GFP flags to express how that
> > > +memory should be allocated. The GFP acronym stands for "get free
> > > +pages", the underlying memory allocation function.
> > > +
> > > +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(<size>, GFP_KERNEL);
> > > +
> > > +Of course there are cases when other allocation APIs and different GFP
> > > +flags must be used.
> > > +
> > > +Get Free Page flags
> > > +===================
> > > +
> > > +The GFP flags control the allocators behavior. They tell what memory
> > > +zones can be used, how hard the allocator should try to find a free
> > > +memory, whether the memory can be accessed by the userspace etc. The
> > > +:ref:`Documentation/core-api/mm-api.rst <mm-api-gfp-flags>` provides
> > > +reference documentation for the GFP flags and their combinations and
> > > +here we briefly outline their recommended usage:
> > > +
> > > +  * Most of the times ``GFP_KERNEL`` is what you need. Memory for the
> > > +    kernel data structures, DMAable memory, inode cache, all these and
> > > +    many other allocations types can use ``GFP_KERNEL``. Note, that
> > > +    using ``GFP_KERNEL`` implies ``GFP_RECLAIM``, which means that
> > > +    direct reclaim may be triggered under memory pressure; the calling
> > > +    context must be allowed to sleep.
> > > +  * If the allocation is performed from an atomic context, e.g
> > > +    interrupt handler, use ``GFP_ATOMIC``.
> 
> GFP_NOWAIT please. GFP_ATOMIC should be only used if accessing memory
> reserves is justified. E.g. fallback allocation would be too costly. It
> should be also noted that these allocation are quite likely to fail
> especially under memory pressure.
 
How about:

* If the allocation is performed from an atomic context, e.g interrupt
  handler, use ``GFP_NOWARN``. This flag prevents direct reclaim and IO or
  filesystem operations. Consequently, under memory pressure ``GFP_NOWARN``
  allocation is likely to fail.
* If you think that accessing memory reserves is justified and the kernel
  will be stressed unless allocation succeeds, you may use ``GFP_ATOMIC``.

> > > +  * Untrusted allocations triggered from userspace should be a subject
> > > +    of kmem accounting and must have ``__GFP_ACCOUNT`` bit set. There
> > > +    is handy ``GFP_KERNEL_ACCOUNT`` shortcut for ``GFP_KERNEL``
> > > +    allocations that should be accounted.
> > > +  * Userspace allocations should use either of the ``GFP_USER``,
> > > +    ``GFP_HIGHUSER`` and ``GFP_HIGHUSER_MOVABLE`` flags. The longer
> > > +    the flag name the less restrictive it is.
> > > +
> > > +    The ``GFP_HIGHUSER_MOVABLE`` does not require that allocated
> > > +    memory will be directly accessible by the kernel or the hardware
> > > +    and implies that the data may move.
> 
> @may move@is movable@

Ok
 
> > > +    The ``GFP_HIGHUSER`` means that the allocated memory is not
> > > +    movable, but it is not required to be directly accessible by the
> > > +    kernel or the hardware. An example may be a hardware allocation
> > > +    that maps data directly into userspace but has no addressing
> > > +    limitations.
> > > +
> > > +    The ``GFP_USER`` means that the allocated memory is not movable
> > > +    and it must be directly accessible by the kernel or the
> > > +    hardware. It is typically used by hardware for buffers that are
> > > +    mapped to userspace (e.g. graphics) that hardware still must DMA
> > > +    to.
> > > +
> > > +You may notice that quite a few allocations in the existing code
> > > +specify ``GFP_NOIO`` and ``GFP_NOFS``. Historically, they were used to
> > > +prevent recursion deadlocks caused by direct memory reclaim calling
> > > +back into the FS or IO paths and blocking on already held
> > > +resources. Since 4.12 the preferred way to address this issue is to
> > > +use new scope APIs described in
> > > +:ref:`Documentation/core-api/gfp_mask-from-fs-io.rst <gfp_mask_from_fs_io>`.
> > > +
> > > +Another legacy GFP flags are ``GFP_DMA`` and ``GFP_DMA32``. They are
> > > +used to ensure that the allocated memory is accessible by hardware
> > > +with limited addressing capabilities. So unless you are writing a
> > > +driver for a device with such restrictions, avoid using these flags.
> 
> And even with HW with restrictions it is preferable to use dma_alloc*
> APIs

Will add.
 
> Looks nice otherwise. Thanks! With the above changes feel free to add
> Acked-by: Michal Hocko <mhocko@suse.com>

Thanks!

> -- 
> Michal Hocko
> SUSE Labs
>
Michal Hocko Aug. 15, 2018, 9:30 a.m. UTC | #4
On Wed 15-08-18 12:04:29, Mike Rapoport wrote:
[...]
> How about:
> 
> * If the allocation is performed from an atomic context, e.g interrupt
>   handler, use ``GFP_NOWARN``. This flag prevents direct reclaim and IO or
>   filesystem operations. Consequently, under memory pressure ``GFP_NOWARN``
>   allocation is likely to fail.

s@NOWARN@NOWAIT@ I guess. Looks good otherwise. I would even go and
mention GFP_NOWARN once you brought it up. Allocations which have a
reasonable fallback should be using NOWARN.

> * If you think that accessing memory reserves is justified and the kernel
>   will be stressed unless allocation succeeds, you may use ``GFP_ATOMIC``.

OK otherwise.
Randy Dunlap Aug. 15, 2018, 5:14 p.m. UTC | #5
On 08/14/2018 11:34 PM, Mike Rapoport wrote:
> As Vlastimil mentioned at [1], it would be nice to have some guide about
> memory allocation. I've drafted an initial version that tries to summarize
> "best practices" for allocation functions and GFP usage.
> 
> [1] https://www.spinics.net/lists/netfilter-devel/msg55542.html
> 
> From 8027c0d4b750b8dbd687234feda63305d0d5a057 Mon Sep 17 00:00:00 2001
> From: Mike Rapoport <rppt@linux.vnet.ibm.com>
> Date: Wed, 15 Aug 2018 09:10:06 +0300
> Subject: [RFC PATCH] docs/core-api: add memory allocation guide
> 
> Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
> ---
>  Documentation/core-api/gfp_mask-from-fs-io.rst |   2 +
>  Documentation/core-api/index.rst               |   1 +
>  Documentation/core-api/memory-allocation.rst   | 117 +++++++++++++++++++++++++
>  Documentation/core-api/mm-api.rst              |   2 +
>  4 files changed, 122 insertions(+)
>  create mode 100644 Documentation/core-api/memory-allocation.rst
> 

Hi Mike,

I have some suggestions below.


> diff --git a/Documentation/core-api/memory-allocation.rst b/Documentation/core-api/memory-allocation.rst
> new file mode 100644
> index 0000000..b1f2ad5
> --- /dev/null
> +++ b/Documentation/core-api/memory-allocation.rst
> @@ -0,0 +1,117 @@
> +=======================
> +Memory Allocation Guide
> +=======================
> +
> +Linux supplies variety of APIs for memory allocation. You can allocate

         supplies a variety
or even
         provides a variety

> +small chunks using `kmalloc` or `kmem_cache_alloc` families, large
> +virtually contiguous areas using `vmalloc` and it's derivatives, or

                                                  its

> +you can directly request pages from the page allocator with
> +`__get_free_pages`. It is also possible to use more specialized
> +allocators, for instance `cma_alloc` or `zs_malloc`.
> +
> +Most of the memory allocations APIs use GFP flags to express how that

                      allocation APIs

> +memory should be allocated. The GFP acronym stands for "get free
> +pages", the underlying memory allocation function.
> +
> +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(<size>, GFP_KERNEL);
> +
> +Of course there are cases when other allocation APIs and different GFP
> +flags must be used.
> +
> +Get Free Page flags
> +===================
> +
> +The GFP flags control the allocators behavior. They tell what memory
> +zones can be used, how hard the allocator should try to find a free

                                                        to find free

> +memory, whether the memory can be accessed by the userspace etc. The
> +:ref:`Documentation/core-api/mm-api.rst <mm-api-gfp-flags>` provides
> +reference documentation for the GFP flags and their combinations and
> +here we briefly outline their recommended usage:
> +
> +  * Most of the times ``GFP_KERNEL`` is what you need. Memory for the

I would write:
       Most of the time

> +    kernel data structures, DMAable memory, inode cache, all these and
> +    many other allocations types can use ``GFP_KERNEL``. Note, that
> +    using ``GFP_KERNEL`` implies ``GFP_RECLAIM``, which means that
> +    direct reclaim may be triggered under memory pressure; the calling
> +    context must be allowed to sleep.
> +  * If the allocation is performed from an atomic context, e.g
> +    interrupt handler, use ``GFP_ATOMIC``.
> +  * Untrusted allocations triggered from userspace should be a subject
> +    of kmem accounting and must have ``__GFP_ACCOUNT`` bit set. There
> +    is handy ``GFP_KERNEL_ACCOUNT`` shortcut for ``GFP_KERNEL``

       is the handy

> +    allocations that should be accounted.
> +  * Userspace allocations should use either of the ``GFP_USER``,
> +    ``GFP_HIGHUSER`` and ``GFP_HIGHUSER_MOVABLE`` flags. The longer

                        or

> +    the flag name the less restrictive it is.
> +
> +    The ``GFP_HIGHUSER_MOVABLE`` does not require that allocated

s/The//

> +    memory will be directly accessible by the kernel or the hardware
> +    and implies that the data may move.
> +
> +    The ``GFP_HIGHUSER`` means that the allocated memory is not

s/The//

> +    movable, but it is not required to be directly accessible by the
> +    kernel or the hardware. An example may be a hardware allocation
> +    that maps data directly into userspace but has no addressing
> +    limitations.
> +
> +    The ``GFP_USER`` means that the allocated memory is not movable

s/The//

> +    and it must be directly accessible by the kernel or the
> +    hardware. It is typically used by hardware for buffers that are
> +    mapped to userspace (e.g. graphics) that hardware still must DMA
> +    to.
> +
> +You may notice that quite a few allocations in the existing code
> +specify ``GFP_NOIO`` and ``GFP_NOFS``. Historically, they were used to

                        or

> +prevent recursion deadlocks caused by direct memory reclaim calling
> +back into the FS or IO paths and blocking on already held
> +resources. Since 4.12 the preferred way to address this issue is to
> +use new scope APIs described in
> +:ref:`Documentation/core-api/gfp_mask-from-fs-io.rst <gfp_mask_from_fs_io>`.
> +
> +Another legacy GFP flags are ``GFP_DMA`` and ``GFP_DMA32``. They are

   Other

> +used to ensure that the allocated memory is accessible by hardware
> +with limited addressing capabilities. So unless you are writing a
> +driver for a device with such restrictions, avoid using these flags.
> +
> +Selecting memory allocator
> +==========================
> +
> +The most straightforward way to allocate memory is to use a function
> +from the `kmalloc` family. And, to be on the safe size it's best to
> +use routines that set memory to zero, like `kzalloc`. If you need to
> +allocate memory for an array, there are `kmalloc_array` and `kcalloc`
> +helpers.
> +
> +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.
> +
> +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.
> +
> +If you are not sure whether the allocation size is too large for
> +`kmalloc` it is possible to use `kvmalloc` and its derivatives. It

   `kmalloc`,

> +will try to allocate memory with `kmalloc` and if the allocation fails
> +it will be retried with `vmalloc`. There are restrictions on which GFP
> +flags can be used with `kvmalloc`, please see :c:func:`kvmalloc_node`

                                    ^ s/,/;/

> +reference documentation. Note, that `kvmalloc` may return memory that

                            Note that

> +is not physically contiguous.
> +
> +If you need to allocate many identical objects you can use slab cache

                                                      can use the slab cache

> +allocator. The cache should be set up with `kmem_cache_create` before
> +it can be used. Afterwards `kmem_cache_alloc` and its convenience
> +wrappers can allocate memory from that cache.
> +
> +When the allocated memory is no longer needed it must be freed. You
> +can use `kvfree` for the memory allocated with `kmalloc`, `vmalloc`
> +and `kvmalloc`. The slab caches should be freed with
> +`kmem_cache_free`. And don't forget to destroy the cache with
> +`kmem_cache_destroy`.

Thanks for the new documentation.

Patch
diff mbox series

diff --git a/Documentation/core-api/gfp_mask-from-fs-io.rst b/Documentation/core-api/gfp_mask-from-fs-io.rst
index e0df8f4..e7c32a8 100644
--- a/Documentation/core-api/gfp_mask-from-fs-io.rst
+++ b/Documentation/core-api/gfp_mask-from-fs-io.rst
@@ -1,3 +1,5 @@ 
+.. _gfp_mask_from_fs_io:
+
 =================================
 GFP masks used from FS/IO context
 =================================
diff --git a/Documentation/core-api/index.rst b/Documentation/core-api/index.rst
index cdc2020..8afc0da 100644
--- a/Documentation/core-api/index.rst
+++ b/Documentation/core-api/index.rst
@@ -27,6 +27,7 @@  Core utilities
    errseq
    printk-formats
    circular-buffers
+   memory-allocation
    mm-api
    gfp_mask-from-fs-io
    timekeeping
diff --git a/Documentation/core-api/memory-allocation.rst b/Documentation/core-api/memory-allocation.rst
new file mode 100644
index 0000000..b1f2ad5
--- /dev/null
+++ b/Documentation/core-api/memory-allocation.rst
@@ -0,0 +1,117 @@ 
+=======================
+Memory Allocation Guide
+=======================
+
+Linux supplies variety of APIs for memory allocation. You can allocate
+small chunks using `kmalloc` or `kmem_cache_alloc` families, large
+virtually contiguous areas using `vmalloc` and it's derivatives, or
+you can directly request pages from the page allocator with
+`__get_free_pages`. It is also possible to use more specialized
+allocators, for instance `cma_alloc` or `zs_malloc`.
+
+Most of the memory allocations APIs use GFP flags to express how that
+memory should be allocated. The GFP acronym stands for "get free
+pages", the underlying memory allocation function.
+
+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(<size>, GFP_KERNEL);
+
+Of course there are cases when other allocation APIs and different GFP
+flags must be used.
+
+Get Free Page flags
+===================
+
+The GFP flags control the allocators behavior. They tell what memory
+zones can be used, how hard the allocator should try to find a free
+memory, whether the memory can be accessed by the userspace etc. The
+:ref:`Documentation/core-api/mm-api.rst <mm-api-gfp-flags>` provides
+reference documentation for the GFP flags and their combinations and
+here we briefly outline their recommended usage:
+
+  * Most of the times ``GFP_KERNEL`` is what you need. Memory for the
+    kernel data structures, DMAable memory, inode cache, all these and
+    many other allocations types can use ``GFP_KERNEL``. Note, that
+    using ``GFP_KERNEL`` implies ``GFP_RECLAIM``, which means that
+    direct reclaim may be triggered under memory pressure; the calling
+    context must be allowed to sleep.
+  * If the allocation is performed from an atomic context, e.g
+    interrupt handler, use ``GFP_ATOMIC``.
+  * Untrusted allocations triggered from userspace should be a subject
+    of kmem accounting and must have ``__GFP_ACCOUNT`` bit set. There
+    is handy ``GFP_KERNEL_ACCOUNT`` shortcut for ``GFP_KERNEL``
+    allocations that should be accounted.
+  * Userspace allocations should use either of the ``GFP_USER``,
+    ``GFP_HIGHUSER`` and ``GFP_HIGHUSER_MOVABLE`` flags. The longer
+    the flag name the less restrictive it is.
+
+    The ``GFP_HIGHUSER_MOVABLE`` does not require that allocated
+    memory will be directly accessible by the kernel or the hardware
+    and implies that the data may move.
+
+    The ``GFP_HIGHUSER`` means that the allocated memory is not
+    movable, but it is not required to be directly accessible by the
+    kernel or the hardware. An example may be a hardware allocation
+    that maps data directly into userspace but has no addressing
+    limitations.
+
+    The ``GFP_USER`` means that the allocated memory is not movable
+    and it must be directly accessible by the kernel or the
+    hardware. It is typically used by hardware for buffers that are
+    mapped to userspace (e.g. graphics) that hardware still must DMA
+    to.
+
+You may notice that quite a few allocations in the existing code
+specify ``GFP_NOIO`` and ``GFP_NOFS``. Historically, they were used to
+prevent recursion deadlocks caused by direct memory reclaim calling
+back into the FS or IO paths and blocking on already held
+resources. Since 4.12 the preferred way to address this issue is to
+use new scope APIs described in
+:ref:`Documentation/core-api/gfp_mask-from-fs-io.rst <gfp_mask_from_fs_io>`.
+
+Another legacy GFP flags are ``GFP_DMA`` and ``GFP_DMA32``. They are
+used to ensure that the allocated memory is accessible by hardware
+with limited addressing capabilities. So unless you are writing a
+driver for a device with such restrictions, avoid using these flags.
+
+Selecting memory allocator
+==========================
+
+The most straightforward way to allocate memory is to use a function
+from the `kmalloc` family. And, to be on the safe size it's best to
+use routines that set memory to zero, like `kzalloc`. If you need to
+allocate memory for an array, there are `kmalloc_array` and `kcalloc`
+helpers.
+
+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.
+
+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.
+
+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 try to allocate memory with `kmalloc` and if the allocation fails
+it will be retried with `vmalloc`. There are restrictions on which GFP
+flags can be used with `kvmalloc`, please see :c:func:`kvmalloc_node`
+reference documentation. Note, that `kvmalloc` may return memory that
+is not physically contiguous.
+
+If you need to allocate many identical objects you can use slab cache
+allocator. The cache should be set up with `kmem_cache_create` before
+it can be used. Afterwards `kmem_cache_alloc` and its convenience
+wrappers can allocate memory from that cache.
+
+When the allocated memory is no longer needed it must be freed. You
+can use `kvfree` for the memory allocated with `kmalloc`, `vmalloc`
+and `kvmalloc`. The slab caches should be freed with
+`kmem_cache_free`. And don't forget to destroy the cache with
+`kmem_cache_destroy`.
diff --git a/Documentation/core-api/mm-api.rst b/Documentation/core-api/mm-api.rst
index 46ae353..5ce1ec1 100644
--- a/Documentation/core-api/mm-api.rst
+++ b/Documentation/core-api/mm-api.rst
@@ -14,6 +14,8 @@  User Space Memory Access
 .. kernel-doc:: mm/util.c
    :functions: get_user_pages_fast
 
+.. _mm-api-gfp-flags:
+
 Memory Allocation Controls
 ==========================