Re: [dpdk-dev] [PATCH 2/2] mempool: fix pages computation to determine number of objects

["Ananyev, Konstantin" <konstantin.ananyev@intel.com>]

Hi Adrien,

> -----Original Message-----
> From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Adrien Mazarguil
> Sent: Monday, May 25, 2015 5:28 PM
> To: dev@dpdk.org
> Subject: [dpdk-dev] [PATCH 2/2] mempool: fix pages computation to determine number of objects
> 
> In rte_mempool_obj_iter(), even when a single page is required per object,
> a loop checks that the the next page is contiguous and drops the first one
> otherwise. This commit checks subsequent pages only when several are
> required per object.
> 
> Also a minor fix for the amount of remaining space that prevents using the
> entire region.
> 
> Fixes: 148f963fb532 ("xen: core library changes")
> 
> Signed-off-by: Adrien Mazarguil <adrien.mazarguil@6wind.com>
> ---
>  lib/librte_mempool/rte_mempool.c | 11 ++++++++---
>  1 file changed, 8 insertions(+), 3 deletions(-)
> 
> diff --git a/lib/librte_mempool/rte_mempool.c b/lib/librte_mempool/rte_mempool.c
> index d1a02a2..3c1efec 100644
> --- a/lib/librte_mempool/rte_mempool.c
> +++ b/lib/librte_mempool/rte_mempool.c
> @@ -175,12 +175,17 @@ rte_mempool_obj_iter(void *vaddr, uint32_t elt_num, size_t elt_sz, size_t align,
>  		pgn += j;
> 
>  		/* do we have enough space left for the next element. */
> -		if (pgn >= pg_num)
> +		if (pgn > pg_num)
>  			break;

Hmm, that doesn't look right.
Suppose:
start==0; end==5120; pg_shift==12; pg_num == 1;
So:
pgn = 1; // (5120>>12)-(0>>12)

And we end-up accessing element that is beyond  allocated memory.

> 
> -		for (k = j;
> +		/*
> +		 * Compute k so that (k - j) is the number of contiguous
> +		 * pages starting from index j. Note that there is at least
> +		 * one page.
> +		 */
> +		for (k = j + 1;
>  				k != pgn &&
> -				paddr[k] + pg_sz == paddr[k + 1];
> +				paddr[k - 1] + pg_sz == paddr[k];
>  				k++)
>  			;


Again, suppose:
j==0; start==0; end==2048; pg_shift==12; pg_num == 2;
So:
pgn = 0;
k = 1;
and the loop goes beyond paddr[] boundaries.

The problem here, I think that you treat pgn as number of pages, while it is index of last page to be used.
As I understand, what you are trying to fix here, is a situation when end is a multiply of page size (end == N * pg_sz),  right?
Then, probably something simple like that would do:

- pgn = (end >> pg_shift) - (start >> pg_shift);
+ pgn = (end - 1 >> pg_shift) - (start >> pg_shift);
+ pg_next = (end >> pg_shift) - (start >> pg_shift);
...
  if (k == pgn) {
                        if (obj_iter != NULL)
                                obj_iter(obj_iter_arg, (void *)start,
                                        (void *)end, i);
                        va = end;
 -                      j = pgn;
+                      j = pg_next;
                        i++;
                } else {
...


Konstantin


> 
> --
> 2.1.0