[dpdk-dev] [PATCH] eal: unmap unneed dpdk VA spaces for legacy mem

Burakov, Anatoly anatoly.burakov at intel.com
Fri Mar 8 10:37:50 CET 2019

On 08-Mar-19 5:38 AM, Lilijun wrote:
> Comparing dpdk VA spaces to dpdk 16.11, the dpdk app process's VA spaces increase to above 30G.
> Here we can unmap the unneed VA spaces in rte_memseg_list.
> Signed-off-by: Lilijun <jerry.lilijun at huawei.com>
> ---
>   lib/librte_eal/linuxapp/eal/eal_memory.c | 13 ++++++++++++-
>   1 file changed, 12 insertions(+), 1 deletion(-)
> diff --git a/lib/librte_eal/linuxapp/eal/eal_memory.c b/lib/librte_eal/linuxapp/eal/eal_memory.c
> index 32feb41..56abdd2 100644
> --- a/lib/librte_eal/linuxapp/eal/eal_memory.c
> +++ b/lib/librte_eal/linuxapp/eal/eal_memory.c
> @@ -1626,8 +1626,19 @@ void numa_error(char *where)
>   		if (msl->base_va == NULL)
>   			continue;
>   		/* skip lists where there is at least one page allocated */
> -		if (msl->memseg_arr.count > 0)
> +		if (msl->memseg_arr.count > 0) {
> +			if (internal_config.legacy_mem) {
> +				struct rte_fbarray *arr = &msl->memseg_arr;
> +				int idx = rte_fbarray_find_next_free(arr, 0);
> +
> +				while (idx >= 0) {
> +					void *va = (void*)((char*)msl->base_va + idx * msl->page_sz);
> +					munmap(va, msl->page_sz);
> +					idx = rte_fbarray_find_next_free(arr, idx + 1);
> +				}

I am not entirely convinced this change is safe to do. Technically, this 
space is marked as free, so correctly written code should not attempt to 
access it, however it is still potentially dangerous to have memory area 
that is supposed to be allocated (according to data structures' 
parameters), but isn't.

If you are deallocating the VA space, ideally you should also resize the 
memseg list (as in, change its length), because that leftover memory 
area is no longer valid. However, this then presents us with a mismatch 
between (va_start + len) and (va_start + page_sz * memseg_arr.len), 
which may break things further.

May i ask what is the purpose of this change? I mean, i understand the 
part about unused VA space sitting there, but what is the consequence of 
that? This isn't 32-bit codepath, and in 64-bit there's plenty of 
address space to go around, and this memory doesn't take up any system 
resources anyway because it is read-only anonymous memory, and is 
therefore backed by zero page instead of real pages. So, what's wrong 
with just leaving it there?

I don't see any advantage of this change, and i see plenty of 
disadvantages, so for now i'm inclined to NACK this particular patch.

_However_, i should note that if you feel this is very important feature 
to have and would still like to implement it, my advise would be to look 
at how 32-bit code works, and model the 64-bit implementation after 
that, because 32-bit codepath does exactly what you propose, and doesn't 
leave unused address space.

> +			} >   			continue;
> +		}
>   		/* this is an unused list, deallocate it */
>   		mem_sz = msl->len;
>   		munmap(msl->base_va, mem_sz);


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