linux-kernel - Re: [PATCH RFC 1/1] dmaengine: ptdma: use SLAB_TYPESAFE_BY

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Message-ID: <Z_mIRn6G1wBH5jfP@f39>
Date: Fri, 11 Apr 2025 23:23:18 +0200
From: Eder Zulian <ezulian@...hat.com>
To: Dave Jiang <dave.jiang@...el.com>
Cc: Basavaraj.Natikar@....com, vkoul@...nel.org, dmaengine@...r.kernel.org,
	linux-kernel@...r.kernel.org, jsnitsel@...hat.com,
	ddutile@...hat.com
Subject: Re: [PATCH RFC 1/1] dmaengine: ptdma: use SLAB_TYPESAFE_BY_RCU for
 the DMA descriptor slab

Hello Dave,

On Fri, Apr 11, 2025 at 01:34:28PM -0700, Dave Jiang wrote:
> 
> 
> On 4/11/25 12:41 PM, Eder Zulian wrote:
> > The SLAB_TYPESAFE_BY_RCU flag prevents a change of type for objects
> > allocated from the slab cache (although the memory may be reallocated to
> > a completetly different object of the same type.) Moreover, when the
> > last reference to an object is dropped the finalization code must not
> > run until all __rcu pointers referencing the object have been updated,
> > and then a grace period has passed.
> 
> I would pull some of the explanation on why and how from your cover. Also, a fixes tag may be needed?

Sure. Thanks for your suggestion. I will add both the explanation and a
fixes tag. Right now I think the fixes tag would point to the initial
commit, but let me double check that on Monday. Maybe I should wait a bit
more and integrate feedback from others as well in a v2.

> 
> DJ
>  
> > 
> > Signed-off-by: Eder Zulian <ezulian@...hat.com>
> > ---
> >  drivers/dma/amd/ptdma/ptdma-dmaengine.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/dma/amd/ptdma/ptdma-dmaengine.c b/drivers/dma/amd/ptdma/ptdma-dmaengine.c
> > index 715ac3ae067b..b70dd1b0b9fb 100644
> > --- a/drivers/dma/amd/ptdma/ptdma-dmaengine.c
> > +++ b/drivers/dma/amd/ptdma/ptdma-dmaengine.c
> > @@ -597,7 +597,8 @@ int pt_dmaengine_register(struct pt_device *pt)
> >  
> >  	pt->dma_desc_cache = kmem_cache_create(desc_cache_name,
> >  					       sizeof(struct pt_dma_desc), 0,
> > -					       SLAB_HWCACHE_ALIGN, NULL);
> > +					       SLAB_HWCACHE_ALIGN |
> > +					       SLAB_TYPESAFE_BY_RCU, NULL);
> >  	if (!pt->dma_desc_cache) {
> >  		ret = -ENOMEM;
> >  		goto err_cache;
> 

Thank you,
Eder