[dpdk-dev] Zeroing out memory on free

[Dmitry Kozlyuk <dkozlyuk@nvidia.com>]

Hello,

Me and Xueming are wondering why DPDK clears the memory on free
and not only when it's explicitly requested (rte_zmalloc).

It's been so for a while:

commit ea0bddbd14e68fb42d9774bc3543e51b510e48d3
Author: Sergio Gonzalez Monroy <sergio.gonzalez.monroy@intel.com>
Date:   Tue Jul 5 12:01:15 2016 +0100

    mem: zero out memory on free
    
    Since commit fafcc11985a2, memzones are not guaranteed to be zeroed out.
    This could potentially cause issues as applications might have been
    relying on the allocated memory being zeroed out.
    
    On init all allocated memory is zeroed by the kernel, so by zeroing out
    memory on free, all available dpdk memory is always zeroed.
    
    Fixes: fafcc11985a2 ("mem: rework memzone to be allocated by malloc")
    
    Signed-off-by: Sergio Gonzalez Monroy <sergio.gonzalez.monroy@intel.com>

At present this explanation doesn't look satisfying:
1. Memzone API does not promise they will be zeroed out.
   Memzones are mostly used for DMA, so their content will likely be overwritten.
2. If application assumptions are wrong, DPDK should not try to fix it.
   Memory manager poisons memory in debug mode to detect such errors.

Zeroing memory is quite slow, but in many cases unneeded.
It looks attractive to only do it in rte_zmalloc().
AFAIK what prohibits moving memset() there unconditionally
is that the kernel already gives us cleared pages,
so the first allocation of each piece of memory would do unnecessary work.
This can be solved by giving the user a choice in EAL options
or with more elaborate tracking of memory dirtiness in MM.
Are there any other reasons why clearing is done the current way?