Re: [PATCH 1/4] raid5-ppl: use a single mempool for ppl_io_unit and header_page

[Shaohua Li <shli@kernel.org>]

On Tue, Apr 11, 2017 at 11:20:13AM +0200, Artur Paszkiewicz wrote:
> On 04/11/2017 10:28 AM, Artur Paszkiewicz wrote:
> > On 04/10/2017 09:09 PM, Shaohua Li wrote:
> >>> +static void *ppl_io_pool_alloc(gfp_t gfp_mask, void *pool_data)
> >>> +{
> >>> +	struct kmem_cache *kc = pool_data;
> >>> +	struct ppl_io_unit *io;
> >>> +
> >>> +	io = kmem_cache_alloc(kc, gfp_mask);
> >>> +	if (!io)
> >>> +		return NULL;
> >>> +
> >>> +	io->header_page = alloc_page(gfp_mask);
> >>> +	if (!io->header_page) {
> >>> +		kmem_cache_free(kc, io);
> >>> +		return NULL;
> >>> +	}
> >>> +
> >>> +	return io;
> >>
> >> Maybe directly use GFP_NOWAIT here, we don't use other gfp
> > 
> > Do you mean ignore the gfp_mask parameter? I don't think we should do
> > that. It is provided by the mempool implementation. We only use
> > GFP_NOWAIT for explicit allocations, but GFP_KERNEL is used for
> > allocating initial items in mempool_create().
> > 
> >>> +}
> >>> +
> >>> +static void ppl_io_pool_free(void *element, void *pool_data)
> >>> +{
> >>> +	struct kmem_cache *kc = pool_data;
> >>> +	struct ppl_io_unit *io = element;
> >>> +
> >>> +	__free_page(io->header_page);
> >>> +	kmem_cache_free(kc, io);
> >>> +}
> >>> +
> >>>  static struct ppl_io_unit *ppl_new_iounit(struct ppl_log *log,
> >>>  					  struct stripe_head *sh)
> >>>  {
> >>> @@ -202,18 +228,19 @@ static struct ppl_io_unit *ppl_new_iounit(struct ppl_log *log,
> >>>  	struct ppl_io_unit *io;
> >>>  	struct ppl_header *pplhdr;
> >>>  
> >>> -	io = mempool_alloc(ppl_conf->io_pool, GFP_ATOMIC);
> >>> +	io = mempool_alloc(ppl_conf->io_pool, GFP_NOWAIT);
> >>>  	if (!io)
> >>>  		return NULL;
> >>>  
> >>> -	memset(io, 0, sizeof(*io));
> >>>  	io->log = log;
> >>> +	io->entries_count = 0;
> >>> +	io->pp_size = 0;
> >>> +	io->submitted = false;
> >>
> >> I'd suggest moving the memset to ppl_io_pool_alloc. Don't think we need to
> >> optimize to avoid setting header_page. And doing memset is less error prone,
> >> for example adding new fields.
> > 
> > OK, I'll change this and resend.
> 
> Well, on second thought, actually I can't move the memset to
> ppl_io_pool_alloc. Mempool can use the preallocated elements, which are
> then returned back to the pool after calling mempool_free() and can be
> reused later. So a ppl_io_unit must be initialized after retrieving it
> from mempool_alloc(). I can leave the memset in ppl_new_iounit() but
> io->header_page pointer will have to be temporarily copied before
> zeroing the iounit and then set back. Is this ok?

Yes, that's preferred.

Thanks,
Shaohua