On 2/23/18 6:48 AM, Igor Stoppa wrote:
Detailed documentation about the protectable memory allocator.

Signed-off-by: Igor Stoppa <igor.sto...@huawei.com>
  Documentation/core-api/index.rst   |   1 +
  Documentation/core-api/pmalloc.rst | 114 +++++++++++++++++++++++++++++++++++++
  2 files changed, 115 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
--- a/Documentation/core-api/index.rst
+++ b/Documentation/core-api/index.rst
@@ -25,6 +25,7 @@ Core utilities
+   pmalloc
Interfaces for kernel debugging
diff --git a/Documentation/core-api/pmalloc.rst 
new file mode 100644
index 000000000000..d9725870444e
--- /dev/null
+++ b/Documentation/core-api/pmalloc.rst
@@ -0,0 +1,114 @@
+.. SPDX-License-Identifier: GPL-2.0
+Protectable memory allocator
+The pmalloc library is meant to provide R/O 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 genalloc, using the same concept of memory pools.
+The value added by pmalloc is that now the memory contained in a pool can
+become R/O, for the rest of the life of the pool.
+Different kernel drivers and threads can use different pools, for finer
+control of what becomes R/O and when. And for improved lockless concurrency.
+- 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.
+- Before destroying a pool, all the memory allocated from it must be
+  released.

Is that true?  pmalloc_destroy_pool() has:

+    pmalloc_pool_set_protection(pool, false);
+    gen_pool_for_each_chunk(pool, pmalloc_chunk_free, NULL);
+    gen_pool_destroy(pool);
+    kfree(data);

which to me looks like is the opposite, the data (ie, "memory") is being released first, then the pool is destroyed.

+- pmalloc does not provide locking support with respect to allocating vs
+  protecting an individual pool, for performance reasons.

What is the recommendation to using locks then, as the computing real-world mainly operates in multi-threaded/process world?  Maybe show an example of an issue that occur if locks aren't used and give a coding example.

+  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.

Why is 32-bit systems mentioned and not 64-bit?  Is there a problem with 64-bit here?


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