On 2/23/18 6:48 AM, Igor Stoppa wrote:
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 | 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
genalloc
errseq
printk-formats
+ pmalloc
Interfaces for kernel debugging
===============================
diff --git a/Documentation/core-api/pmalloc.rst b/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
+============================
+
+Purpose
+-------
+
+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.
+
+
+Concept
+-------
+
+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.
+
+
+Caveats
+-------
+
+- 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?
Thanks,
Jay
--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@xxxxxxxxx. For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
