[PATCH v4 0/3] docs/core-api: add memory allocation guide

[PATCH v4 0/3] docs/core-api: add memory allocation guide

[Mike Rapoport]

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.

v3 -> v4:
  * make GFP_*USER* description less confusing

v2 -> v3:
  * s/HW/hardware

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

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   | 122 +++++++++++++++++++++++++
 Documentation/core-api/mm-api.rst              |   2 +
 4 files changed, 127 insertions(+)
 create mode 100644 Documentation/core-api/memory-allocation.rst

-- 
2.7.4

[PATCH v4 1/3] docs: core-api/gfp_mask-from-fs-io: add a label for cross-referencing

[Mike Rapoport]

Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
---
 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

[PATCH v4 2/3] docs: core-api/mm-api: add a lable for GFP flags section

[Mike Rapoport]

Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
---
 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

[PATCH v4 3/3] docs: core-api: add memory allocation guide

[Mike Rapoport]

Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
---
 Documentation/core-api/index.rst             |   1 +
 Documentation/core-api/memory-allocation.rst | 122 +++++++++++++++++++++++++++
 2 files changed, 123 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 26b735c..165d7688 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..f8bb9aa
--- /dev/null
+++ b/Documentation/core-api/memory-allocation.rst
@@ -0,0 +1,122 @@
+=======================
+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 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. 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.
+
+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 hardware 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

Re: [PATCH v4 0/3] docs/core-api: add memory allocation guide

[Mike Rapoport]

Ping?

On Fri, Sep 14, 2018 at 12:27:55PM +0300, Mike Rapoport wrote:
> 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.
> 
> v3 -> v4:
>   * make GFP_*USER* description less confusing
> 
> v2 -> v3:
>   * s/HW/hardware
> 
> [1] https://www.spinics.net/lists/netfilter-devel/msg55542.html
> 
> 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   | 122 +++++++++++++++++++++++++
>  Documentation/core-api/mm-api.rst              |   2 +
>  4 files changed, 127 insertions(+)
>  create mode 100644 Documentation/core-api/memory-allocation.rst
> 
> -- 
> 2.7.4
> 

-- 
Sincerely yours,
Mike.

Re: [PATCH v4 0/3] docs/core-api: add memory allocation guide

[Jonathan Corbet]

On Thu, 20 Sep 2018 07:29:30 +0300
Mike Rapoport <rppt@linux.vnet.ibm.com> wrote:

> Ping?
> 
> On Fri, Sep 14, 2018 at 12:27:55PM +0300, Mike Rapoport wrote:
> > 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.
> > 
> > v3 -> v4:
> >   * make GFP_*USER* description less confusing

Sorry...but it's been less than a week.  And this week has been ...
busy ...

Anyway, I've applied the set now, thanks.

jon