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Date: Mon, 26 Feb 2018 10:32:37 -0800
From: J Freyensee <why2jjj.linux@...il.com>
To: Igor Stoppa <igor.stoppa@...wei.com>, david@...morbit.com,
willy@...radead.org, keescook@...omium.org, mhocko@...nel.org
Cc: labbott@...hat.com, linux-security-module@...r.kernel.org,
linux-mm@...ck.org, linux-kernel@...r.kernel.org,
kernel-hardening@...ts.openwall.com
Subject: Re: [PATCH 7/7] Documentation for Pmalloc
[...]
On 2/26/18 7:39 AM, Igor Stoppa wrote:
>
> On 24/02/18 02:26, J Freyensee wrote:
>>
>> On 2/23/18 6:48 AM, Igor Stoppa wrote:
> [...]
>
>>> +- 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.
> well, this is embarrassing ... yes I had this prototype code, because I
> was wondering if it wouldn't make more sense to tear down the pool as
> fast as possible. It slipped in, apparently.
>
> I'm actually tempted to leave it in and fix the comment.
Sure, one or the other.
>
> [...]
>
>>> +
>>> +- 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?
> How common are multi-threaded allocations of write-once memory?
> Here we are talking exclusively about the part of the memory life-cycle
> where it is allocated (from pmalloc).
Yah, that's true, good point.
>
>> Maybe show
>> an example of an issue that occur if locks aren't used and give a coding
>> example.
> An example of how to use a mutex to access a shared resource? :-O
>
> This part below, under your question, was supposed to be the answer :-(
>
>>> + 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.
My bad, I was suggesting a code sample, if there was a simple code
sample to provide (like 5-10 lines?). If it's a lot of code to write,
no bother.
> [...]
>
>>> +- 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?
> Because, as written, on 32 bit system the vmalloc range is relatively
> small, so one might wonder if there are enough addresses.
>
>> Is there a problem with 64-bit here?
> Quite the opposite.
> I thought it was clear, but obviously it isn't, I'll reword this.
Sounds good, thank you,
Jay
>
> -igor
>
>
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