Re: [PATCH 7/7] Documentation for Pmalloc

[J Freyensee]

[...]

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:
> >
> > .
> > .
> > +ÂÂÂ pmalloc_pool_set_protection(pool, false);
> > +ÂÂÂ gen_pool_for_each_chunk(pool, pmalloc_chunk_free, NULL);
> > +ÂÂÂ gen_pool_destroy(pool);
> > +ÂÂÂ 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.
>
> >  Â 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