Re: [PATCH RFC 1/1] dmaengine: ptdma: use SLAB_TYPESAFE_BY_RCU for the DMA descriptor slab

From: Eder Zulian
Date: Fri Apr 11 2025 - 17:24:36 EST


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@xxxxxxxxxx>
> > ---
> > 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