![Vlastimil Babka](/assets/img/avatar_default.png)
In most configurations, kmalloc() happens to return naturally aligned (i.e. aligned to the block size itself) blocks for power of two sizes. That means some kmalloc() users might unknowingly rely on that alignment, until stuff breaks when the kernel is built with e.g. CONFIG_SLUB_DEBUG or CONFIG_SLOB, and blocks stop being aligned. Then developers have to devise workaround such as own kmem caches with specified alignment [1], which is not always practical, as recently evidenced in [2]. The topic has been discussed at LSF/MM 2019 [3]. Adding a 'kmalloc_aligned()' variant would not help with code unknowingly relying on the implicit alignment. For slab implementations it would either require creating more kmalloc caches, or allocate a larger size and only give back part of it. That would be wasteful, especially with a generic alignment parameter (in contrast with a fixed alignment to size). Ideally we should provide to mm users what they need without difficult workarounds or own reimplementations, so let's make the kmalloc() alignment to size explicitly guaranteed for power-of-two sizes under all configurations. What this means for the three available allocators? * SLAB object layout happens to be mostly unchanged by the patch. The implicitly provided alignment could be compromised with CONFIG_DEBUG_SLAB due to redzoning, however SLAB disables redzoning for caches with alignment larger than unsigned long long. Practically on at least x86 this includes kmalloc caches as they use cache line alignment, which is larger than that. Still, this patch ensures alignment on all arches and cache sizes. * SLUB layout is also unchanged unless redzoning is enabled through CONFIG_SLUB_DEBUG and boot parameter for the particular kmalloc cache. With this patch, explicit alignment is guaranteed with redzoning as well. This will result in more memory being wasted, but that should be acceptable in a debugging scenario. * SLOB has no implicit alignment so this patch adds it explicitly for kmalloc(). The potential downside is increased fragmentation. While pathological allocation scenarios are certainly possible, in my testing, after booting a x86_64 kernel+userspace with virtme, around 16MB memory was consumed by slab pages both before and after the patch, with difference in the noise. [1] https://lore.kernel.org/linux-btrfs/c3157c8e8e0e7588312b40c853f65c02fe6c957a.1566399731.git.christophe.leroy@c-s.fr/ [2] https://lore.kernel.org/linux-fsdevel/20190225040904.5557-1-ming.lei@redhat.com/ [3] https://lwn.net/Articles/787740/ [akpm@linux-foundation.org: documentation fixlet, per Matthew] Link: http://lkml.kernel.org/r/20190826111627.7505-3-vbabka@suse.cz Signed-off-by: Vlastimil Babka <vbabka@suse.cz> Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org> Acked-by: Michal Hocko <mhocko@suse.com> Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com> Acked-by: Christoph Hellwig <hch@lst.de> Cc: David Sterba <dsterba@suse.cz> Cc: Christoph Lameter <cl@linux.com> Cc: Pekka Enberg <penberg@kernel.org> Cc: David Rientjes <rientjes@google.com> Cc: Ming Lei <ming.lei@redhat.com> Cc: Dave Chinner <david@fromorbit.com> Cc: "Darrick J . Wong" <darrick.wong@oracle.com> Cc: Christoph Hellwig <hch@lst.de> Cc: James Bottomley <James.Bottomley@HansenPartnership.com> Cc: Vlastimil Babka <vbabka@suse.cz> Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
131 lines
6.1 KiB
ReStructuredText
131 lines
6.1 KiB
ReStructuredText
.. _memory_allocation:
|
|
|
|
=======================
|
|
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.
|
|
|
|
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
|
|
alignment is also guaranteed to be at least the respective 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` or :c:func:`kmem_cache_create_usercopy`
|
|
before it can be used. The second function should be used if a part of
|
|
the cache might be copied to the userspace. After the cache is
|
|
created :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`.
|