Re: [dpdk-dev] [PATCH v4 2/4] mempool: add non-IO flag

[Dmitry Kozlyuk <dkozlyuk@oss.nvidia.com>]

> -----Original Message-----
> From: Olivier Matz <olivier.matz@6wind.com>
> Sent: 15 октября 2021 г. 12:43
> To: Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>
> Cc: Dmitry Kozlyuk <dkozlyuk@nvidia.com>; dev@dpdk.org; Matan Azrad
> <matan@nvidia.com>
> Subject: Re: [PATCH v4 2/4] mempool: add non-IO flag
> 
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> 
> On Fri, Oct 15, 2021 at 12:33:31PM +0300, Andrew Rybchenko wrote:
> > On 10/15/21 12:18 PM, Dmitry Kozlyuk wrote:
> > >> -----Original Message-----
> > >> From: Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru> [...]
> > >>> diff --git a/lib/mempool/rte_mempool.c b/lib/mempool/rte_mempool.c
> > >>> index 51c0ba2931..2204f140b3 100644
> > >>> --- a/lib/mempool/rte_mempool.c
> > >>> +++ b/lib/mempool/rte_mempool.c
> > >>> @@ -371,6 +371,8 @@ rte_mempool_populate_iova(struct rte_mempool
> > >>> *mp, char *vaddr,
> > >>>
> > >>>       STAILQ_INSERT_TAIL(&mp->mem_list, memhdr, next);
> > >>>       mp->nb_mem_chunks++;
> > >>> +     if (iova == RTE_BAD_IOVA)
> > >>> +             mp->flags |= MEMPOOL_F_NON_IO;
> > >>
> > >> As I understand rte_mempool_populate_iova() may be called few times
> > >> for one mempool. The flag must be set if all invocations are done
> > >> with RTE_BAD_IOVA. So, it should be set by default and just removed
> > >> when iova != RTE_BAD_IOVA happens.
> > >
> > > I don't agree at all. If any object of the pool is unsuitable for
> > > IO, the pool cannot be considered suitable for IO. So if there's a
> > > single invocation with RTE_BAD_IOVA, the flag must be set forever.
> >
> > If so, some objects may be used for IO, some cannot be used.
> > What should happen if an application allocates an object which is
> > suitable for IO and try to use it this way?
> 
> If the application can predict if the allocated object is usable for IO
> before allocating it, I would be surprised to have it used for IO. I agree
> with Dmitry here.

The flag hints to components, PMDs before all,
that objects from this mempool will never be used for IO,
so that the component can save some memory mapping or DMA configuration.
If the flag is set when even a single object may be used for IO,
the consumer of the flag will not be ready for that.
Whatever a corner case it is, Andrew is correct.
There is a subtle difference between "pool is not usable"
(as described now) and "objects from this mempool will never be used"
(as stated above), I'll highlight it in the flag description.