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

[Adrien Mazarguil <adrien.mazarguil@6wind.com>]

On Mon, May 25, 2015 at 06:20:03PM +0000, Ananyev, Konstantin wrote:
> Hi Adrien,

Hi Konstantin,

> > -----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.

Well, I misunderstood the logic here, to me pgn was the number of pages
necessary for the current object on top of the number of pages used so
far. Assuming a single object uses at least one page (assuming 4K pages),
pgn wasn't supposed to be zero.

> 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?

This and also when (end - start) < page size.

> 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 {
> ...

That does not seem to be enough to solve the issue in my scenario, I get
weird results (j never reaches pg_num).

I'll come up with a new patch that takes your comment into account,
hopefully covering all cases.

Thanks,

-- 
Adrien Mazarguil
6WIND