Re: [patch] dma-pool: warn when coherent pool is depleted

[David Rientjes <rientjes@google.com>]

On Sun, 21 Jun 2020, Guenter Roeck wrote:

> > When a DMA coherent pool is depleted, allocation failures may or may not
> > get reported in the kernel log depending on the allocator.
> > 
> > The admin does have a workaround, however, by using coherent_pool= on the
> > kernel command line.
> > 
> > Provide some guidance on the failure and a recommended minimum size for
> > the pools (double the size).
> > 
> > Signed-off-by: David Rientjes <rientjes@google.com>
> 
> Tested-by: Guenter Roeck <linux@roeck-us.net>
> 
> Also confirmed that coherent_pool=256k works around the crash
> I had observed.
> 

Thanks Guenter.  Christoph, does it make sense to apply this patch since 
there may not be an artifact left behind in the kernel log on allocation 
failure by the caller?

> Guenter
> 
> > ---
> >  kernel/dma/pool.c | 6 +++++-
> >  1 file changed, 5 insertions(+), 1 deletion(-)
> > 
> > diff --git a/kernel/dma/pool.c b/kernel/dma/pool.c
> > --- a/kernel/dma/pool.c
> > +++ b/kernel/dma/pool.c
> > @@ -239,12 +239,16 @@ void *dma_alloc_from_pool(struct device *dev, size_t size,
> >  	}
> >  
> >  	val = gen_pool_alloc(pool, size);
> > -	if (val) {
> > +	if (likely(val)) {
> >  		phys_addr_t phys = gen_pool_virt_to_phys(pool, val);
> >  
> >  		*ret_page = pfn_to_page(__phys_to_pfn(phys));
> >  		ptr = (void *)val;
> >  		memset(ptr, 0, size);
> > +	} else {
> > +		WARN_ONCE(1, "DMA coherent pool depleted, increase size "
> > +			     "(recommended min coherent_pool=%zuK)\n",
> > +			  gen_pool_size(pool) >> 9);
> >  	}
> >  	if (gen_pool_avail(pool) < atomic_pool_size)
> >  		schedule_work(&atomic_pool_work);
>