Re: [PATCH v8 7/9] memarea: support backup memory mechanism

[Dmitry Kozlyuk <dmitry.kozliuk@gmail.com>]

2022-10-11 12:17 (UTC+0000), Chengwen Feng:
> This patch adds a memarea backup mechanism, where an allocation request
> which cannot be met by the current memarea is deferred to its backup
> memarea.

This is a controversial feature.

1. It violates memarea property of freeing all allocated objects
   at once when the memarea itself is destroyed. Objects allocated
   in the backup memarea through the destroyed one will remain.

2. If there was an API to check that the object belongs to a memarea
   (the check from rte_memarea_update_refcnt() in this patch),
   it would be trivial to implement this feature on top of memarea API.

Nit: "Deferred" is about time -> "forwarded", "delegated", or "handled over".

A general note about this series.
IMO, libraries should have limited scope and allow composition
rather than accumulate features and control their use via options.
The core idea of memarea is an allocator within a memory region,
a fast one and with low overhead, usable to free all objects at once.

This is orthogonal to the question from where the memory comes from.
HEAP and LIBC sources could be built on top of USER source,
which means that the concept of source is less relevant.
Backup mechanism could instead be a way to add memory to the area,
in which case HEAP and LIBC memarea would also be expandable.
Memarea API could be defined as a structure with callbacks,
and different types of memarea could be combined,
for example, interlocked memarea on top of expandable memarea on top of
memarea with a particular memory management algorithm.

I'm not saying we should immediately build all this complexity.
On the contrary, I would merge the basic things first,
then try to _stack_ new features on top,
then look if interfaces emerge that can be used for composition.