Hi,
As Vlastimil mentioned at [1], it would be nice to have some guide about
memory allocation. This set adds such guide that summarizes the "best
practices".
The changes from the RFC include additions and corrections from Michal and
Randy. I've also added markup to cross-reference the kernel-doc
documentation.
I've split the patch into three to separate labels addition to the exiting
files from the new contents.
Note that the second patch depends on the mm docs update [2] that Andrew
took to the -mm tree.
[1] https://www.spinics.net/lists/netfilter-devel/msg55542.html
[2] https://lkml.org/lkml/2018/7/26/684
Mike Rapoport (3):
docs: core-api/gfp_mask-from-fs-io: add a label for cross-referencing
docs: core-api/mm-api: add a lable for GFP flags section
docs: core-api: add memory allocation guide
Documentation/core-api/gfp_mask-from-fs-io.rst | 2 +
Documentation/core-api/index.rst | 1 +
Documentation/core-api/memory-allocation.rst | 124 +++++++++++++++++++++++++
Documentation/core-api/mm-api.rst | 2 +
4 files changed, 129 insertions(+)
create mode 100644 Documentation/core-api/memory-allocation.rst
--
2.7.4
Signed-off-by: Mike Rapoport <[email protected]>
---
Documentation/core-api/gfp_mask-from-fs-io.rst | 2 ++
1 file changed, 2 insertions(+)
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
=================================
--
2.7.4
Signed-off-by: Mike Rapoport <[email protected]>
---
Documentation/core-api/mm-api.rst | 2 ++
1 file changed, 2 insertions(+)
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
Signed-off-by: Mike Rapoport <[email protected]>
Acked-by: Michal Hocko <[email protected]>
---
Documentation/core-api/index.rst | 1 +
Documentation/core-api/memory-allocation.rst | 124 +++++++++++++++++++++++++++
2 files changed, 125 insertions(+)
create mode 100644 Documentation/core-api/memory-allocation.rst
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..b9b0823
--- /dev/null
+++ b/Documentation/core-api/memory-allocation.rst
@@ -0,0 +1,124 @@
+=======================
+Memory Allocation Guide
+=======================
+
+Linux provides a 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 its derivatives,
+or you can directly request pages from the page allocator with
+`alloc_pages`. It is also possible to use more specialized allocators,
+for instance `cma_alloc` or `zs_malloc`.
+
+Most of the memory allocation 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 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 time ``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_NOWAIT``. This flag prevents direct reclaim and
+ IO or filesystem operations. Consequently, under memory pressure
+ ``GFP_NOWAIT`` allocation is likely to fail. Allocations which
+ have a reasonable fallback should be using ``GFP_NOWARN``.
+ * 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 the handy ``GFP_KERNEL_ACCOUNT`` shortcut for ``GFP_KERNEL``
+ allocations that should be accounted.
+ * Userspace allocations should use either of the ``GFP_USER``,
+ ``GFP_HIGHUSER`` or ``GFP_HIGHUSER_MOVABLE`` flags. The longer
+ the flag name the less restrictive it is.
+
+ ``GFP_HIGHUSER_MOVABLE`` does not require that allocated memory
+ will be directly accessible by the kernel or the hardware and
+ implies that the data is movable.
+
+ ``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.
+
+ ``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`` or ``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>`.
+
+Other 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.
+
+Selecting memory allocator
+==========================
+
+The most straightforward way to allocate memory is to use a function
+from the :c:func:`kmalloc` family. And, to be on the safe size it's
+best to use routines that set memory to zero, like
+:c:func:`kzalloc`. If you need to allocate memory for an array, there
+are :c:func:`kmalloc_array` and :c:func:`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 :c:func:`vmalloc` and
+:c:func:`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 :c:func:`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 the slab
+cache allocator. The cache should be set up with
+:c:func:`kmem_cache_create` before it can be used. Afterwards
+:c:func:`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 :c:func:`kvfree` for the memory allocated with `kmalloc`,
+`vmalloc` and `kvmalloc`. The slab caches should be freed with
+:c:func:`kmem_cache_free`. And don't forget to destroy the cache with
+:c:func:`kmem_cache_destroy`.
--
2.7.4
On 08/16/2018 06:03 AM, Mike Rapoport wrote:
> Signed-off-by: Mike Rapoport <[email protected]>
> Acked-by: Michal Hocko <[email protected]>
> ---
> Documentation/core-api/index.rst | 1 +
> Documentation/core-api/memory-allocation.rst | 124 +++++++++++++++++++++++++++
> 2 files changed, 125 insertions(+)
> create mode 100644 Documentation/core-api/memory-allocation.rst
> diff --git a/Documentation/core-api/memory-allocation.rst b/Documentation/core-api/memory-allocation.rst
> new file mode 100644
> index 0000000..b9b0823
> --- /dev/null
> +++ b/Documentation/core-api/memory-allocation.rst
> @@ -0,0 +1,124 @@
> +=======================
> +Memory Allocation Guide
> +=======================
> +
[snip]
> +
> +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 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 time ``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_NOWAIT``. This flag prevents direct reclaim and
> + IO or filesystem operations. Consequently, under memory pressure
> + ``GFP_NOWAIT`` allocation is likely to fail. Allocations which
> + have a reasonable fallback should be using ``GFP_NOWARN``.
> + * 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 the handy ``GFP_KERNEL_ACCOUNT`` shortcut for ``GFP_KERNEL``
> + allocations that should be accounted.
> + * Userspace allocations should use either of the ``GFP_USER``,
> + ``GFP_HIGHUSER`` or ``GFP_HIGHUSER_MOVABLE`` flags. The longer
> + the flag name the less restrictive it is.
> +
> + ``GFP_HIGHUSER_MOVABLE`` does not require that allocated memory
> + will be directly accessible by the kernel or the hardware and
> + implies that the data is movable.
> +
> + ``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.
> +
> + ``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`` or ``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>`.
> +
> +Other 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
please s/HW/hardware/
> +`dma_alloc*` APIs.
> +
> +Selecting memory allocator
> +==========================
and then you can add
Acked-by: Randy Dunlap <[email protected]>
Thanks.
--
~Randy