[PATCH 8/8] Documentation for Pmalloc
- Date: Tue, 13 Mar 2018 23:45:54 +0200
- From: Igor Stoppa <igor.stoppa@xxxxxxxxxx>
- Subject: [PATCH 8/8] Documentation for Pmalloc
Detailed documentation about the protectable memory allocator.
Signed-off-by: Igor Stoppa <igor.stoppa@xxxxxxxxxx>
Documentation/core-api/index.rst | 1 +
Documentation/core-api/pmalloc.rst | 111 +++++++++++++++++++++++++++++++++++++
2 files changed, 112 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 +1,111 @@
+.. 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.
+It protects data from both accidental and malicious overwrites.
+Example: A policy that is loaded from userspace.
+pmalloc builds on top of :ref:`genalloc <genalloc>`, using the same
+concept of memory pools.
+The value added by pmalloc is that now the memory contained in a pool can
+become read-only, for the rest of the life of the pool.
+Different kernel drivers and threads can use different pools, for finer
+control of what becomes read_only and when.
+And for improved lockless concurrency.
+- To facilitate the conversion of existing code to pmalloc pools, several
+ helper functions are provided, mirroring their k/vmalloc counterparts.
+ In particular, pfree(), which is mostly meant for error paths, when one
+ or more previous allocations must be rolled back.
+- Memory freed while a pool is not yet protected will be reused.
+- Once a pool is protected, it's not possible to allocate any more memory
+ from it.
+- Memory "freed" from a protected pool indicates that such memory is not
+ in use anymore by the requester; however, it will not become available
+ for further use, until the pool is destroyed.
+- pmalloc does not provide locking support with respect to allocating vs
+ protecting an individual pool, for performance reasons.
+ It is recommended not to share the same pool between unrelated functions.
+ Should sharing be a necessity, the user of the shared pool is expected
+ to implement locking for that pool.
+- pmalloc uses genalloc to optimize the use of the space it allocates
+ through vmalloc. Some more TLB entries will be used, however less than
+ in the case of using vmalloc directly. The exact number depends on the
+ size of each allocation request and possible slack.
+- Considering that not much data is supposed to be dynamically allocated
+ and then marked as read-only, it shouldn't be an issue that the address
+ range for pmalloc is limited, on 32-bit systems.
+- 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.
+The typical sequence, when using pmalloc, is:
+#. create a pool
+#. [optional] pre-allocate some memory in the pool
+#. issue one or more allocation requests to the pool with locking as needed
+#. initialize the memory obtained with desired values
+#. [optional] iterate over points 3 & 4 as needed
+#. write-protect the pool
+#. use in read-only mode the handles obtained through the allocations
+#. [optional] release all the memory allocated
+#. [optional, but depends on point 8] destroy the pool
+.. kernel-doc:: include/linux/pmalloc.h
+.. kernel-doc:: mm/pmalloc.c