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    SubjectRe: [PATCH 7/7] Documentation for Pmalloc
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    On 2/23/18 6:48 AM, Igor Stoppa wrote:
    > Detailed documentation about the protectable memory allocator.
    >
    > Signed-off-by: Igor Stoppa <igor.stoppa@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
    > 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

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