[PATCH 4/6] Documentation for Pmalloc
igor.stoppa at gmail.com
Fri Apr 13 13:41:29 UTC 2018
Detailed documentation about the protectable memory allocator.
Signed-off-by: Igor Stoppa <igor.stoppa at huawei.com>
Documentation/core-api/index.rst | 1 +
Documentation/core-api/pmalloc.rst | 107 +++++++++++++++++++++++++++++++++++++
2 files changed, 108 insertions(+)
create mode 100644 Documentation/core-api/pmalloc.rst
diff --git a/Documentation/core-api/index.rst b/Documentation/core-api/index.rst
index c670a8031786..8f5de42d6571 100644
@@ -25,6 +25,7 @@ Core utilities
Interfaces for kernel debugging
diff --git a/Documentation/core-api/pmalloc.rst b/Documentation/core-api/pmalloc.rst
new file mode 100644
@@ -0,0 +1,107 @@
+.. SPDX-License-Identifier: GPL-2.0
+Protectable memory allocator
+The pmalloc library is meant to provide read-only status to data that,
+for some reason, could neither be declared as constant, nor could it take
+advantage of the qualifier __ro_after_init, but is write-once and
+read-only in spirit. At least as long as it doesn't get teared down.
+It protects data from both accidental and malicious overwrites.
+Example: A policy that is loaded from userspace.
+The MMU available in the system can be used to write protect memory pages.
+Unfortunately this feature cannot be used as-it-is, to protect sensitive
+data, because this potentially read-only data is typically interleaved
+with other data, which must stay writeable.
+pmalloc introduces the concept of protectable memory pools.
+A pool contains a list of areas of virtually contiguous pages of
+memory. An area is the minimum amount of memory that pmalloc allows to
+protect, because the user might have allocated a memory range that
+crosses the boundary between pages.
+When an allocation is performed, if there is not enough memory already
+available in the pool, a new area of suitable size is grabbed.
+The size chosen is the largest between the roundup (to PAGE_SIZE) of
+the request from pmalloc and friends and the refill parameter specified
+when creating the pool.
+When a pool is created, it is possible to specify two parameters:
+- refill size: the minimum size of the memory area to allocate when needed
+- align_order: the default alignment to use when reserving memory
+To facilitate the conversion of existing code to pmalloc pools, several
+helper functions are provided, mirroring their k/vmalloc counterparts.
+However one is missing. There is no pfree() because the memory protected
+by a pool will be released exclusively when the pool is destroyed.
+- When a pool is protected, whatever memory would be still available in
+ the current vmap_area (from which allocations are performed) is
+- As already explained, freeing of memory is not supported. Pages will be
+ returned to the system upon destruction of the memory pool that they
+ belong to.
+- The address range available for vmalloc (and thus for pmalloc too) is
+ limited, on 32-bit systems. However it shouldn't be an issue, since not
+ much data is expected tobe dynamically allocated and turned into
+- Regarding SMP systems, the allocations are expected to happen mostly
+ during an initial transient, after which there should be no more need
+ to perform cross-processor synchronizations of page tables.
+ Loading of kernel modules is an exception to this, but it's not expected
+ to happen with such high frequency to become a problem.
+The typical sequence, when using pmalloc, is:
+#. create a pool
+#. issue one or more allocation requests to the pool
+#. initialize the memory obtained, with the desired values
+#. write-protect the memory so far allocated
+#. iterate over the last 3 points as needed
+#. [optional] destroy the pool
+.. kernel-doc:: include/linux/pmalloc.h
+.. kernel-doc:: mm/pmalloc.c
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