Message-ID: <20260105144031.2520c81b@mordecai>
Date: Mon, 5 Jan 2026 14:40:31 +0100
From: Petr Tesarik <ptesarik@...e.com>
To: "Michael S. Tsirkin" <mst@...hat.com>
Cc: linux-kernel@...r.kernel.org, Cong Wang <xiyou.wangcong@...il.com>,
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Subject: Re: [PATCH v2 05/15] dma-debug: track cache clean flag in entries

On Mon, 5 Jan 2026 07:37:31 -0500
"Michael S. Tsirkin" <mst@...hat.com> wrote:

> On Mon, Jan 05, 2026 at 10:54:33AM +0100, Petr Tesarik wrote:
> > On Mon, 5 Jan 2026 03:23:10 -0500
> > "Michael S. Tsirkin" <mst@...hat.com> wrote:
> >   
> > > If a driver is buggy and has 2 overlapping mappings but only
> > > sets cache clean flag on the 1st one of them, we warn.
> > > But if it only does it for the 2nd one, we don't.
> > > 
> > > Fix by tracking cache clean flag in the entry.
> > > 
> > > Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
> > > ---
> > >  kernel/dma/debug.c | 27 ++++++++++++++++++++++-----
> > >  1 file changed, 22 insertions(+), 5 deletions(-)
> > > 
> > > diff --git a/kernel/dma/debug.c b/kernel/dma/debug.c
> > > index 7e66d863d573..43d6a996d7a7 100644
> > > --- a/kernel/dma/debug.c
> > > +++ b/kernel/dma/debug.c
> > > @@ -63,6 +63,7 @@ enum map_err_types {
> > >   * @sg_mapped_ents: 'mapped_ents' from dma_map_sg
> > >   * @paddr: physical start address of the mapping
> > >   * @map_err_type: track whether dma_mapping_error() was checked
> > > + * @is_cache_clean: driver promises not to write to buffer while mapped
> > >   * @stack_len: number of backtrace entries in @stack_entries
> > >   * @stack_entries: stack of backtrace history
> > >   */
> > > @@ -76,7 +77,8 @@ struct dma_debug_entry {
> > >  	int		 sg_call_ents;
> > >  	int		 sg_mapped_ents;
> > >  	phys_addr_t	 paddr;
> > > -	enum map_err_types  map_err_type;
> > > +	enum map_err_types map_err_type;  
> > 
> > *nitpick* unnecessary change in white space (breaks git-blame).
> > 
> > Other than that, LGTM. I'm not formally a reviewer, but FWIW:
> > 
> > Reviewed-by: Petr Tesarik <ptesarik@...e.com>
> > 
> > Petr T  
> 
> 
> I mean, yes it's not really required here, but the padding we had before
> was broken (two spaces not aligning to anything).

Oh, you're right! Yes, then let's fix it now, because you touch the
neighbouring line.

Sorry for the noise.

Petr T

> > > +	bool		 is_cache_clean;
> > >  #ifdef CONFIG_STACKTRACE
> > >  	unsigned int	stack_len;
> > >  	unsigned long	stack_entries[DMA_DEBUG_STACKTRACE_ENTRIES];
> > > @@ -472,12 +474,15 @@ static int active_cacheline_dec_overlap(phys_addr_t cln)
> > >  	return active_cacheline_set_overlap(cln, --overlap);
> > >  }
> > >  
> > > -static int active_cacheline_insert(struct dma_debug_entry *entry)
> > > +static int active_cacheline_insert(struct dma_debug_entry *entry,
> > > +				   bool *overlap_cache_clean)
> > >  {
> > >  	phys_addr_t cln = to_cacheline_number(entry);
> > >  	unsigned long flags;
> > >  	int rc;
> > >  
> > > +	*overlap_cache_clean = false;
> > > +
> > >  	/* If the device is not writing memory then we don't have any
> > >  	 * concerns about the cpu consuming stale data.  This mitigates
> > >  	 * legitimate usages of overlapping mappings.
> > > @@ -487,8 +492,16 @@ static int active_cacheline_insert(struct dma_debug_entry *entry)
> > >  
> > >  	spin_lock_irqsave(&radix_lock, flags);
> > >  	rc = radix_tree_insert(&dma_active_cacheline, cln, entry);
> > > -	if (rc == -EEXIST)
> > > +	if (rc == -EEXIST) {
> > > +		struct dma_debug_entry *existing;
> > > +
> > >  		active_cacheline_inc_overlap(cln);
> > > +		existing = radix_tree_lookup(&dma_active_cacheline, cln);
> > > +		/* A lookup failure here after we got -EEXIST is unexpected. */
> > > +		WARN_ON(!existing);
> > > +		if (existing)
> > > +			*overlap_cache_clean = existing->is_cache_clean;
> > > +	}
> > >  	spin_unlock_irqrestore(&radix_lock, flags);
> > >  
> > >  	return rc;
> > > @@ -583,20 +596,24 @@ DEFINE_SHOW_ATTRIBUTE(dump);
> > >   */
> > >  static void add_dma_entry(struct dma_debug_entry *entry, unsigned long attrs)
> > >  {
> > > +	bool overlap_cache_clean;
> > >  	struct hash_bucket *bucket;
> > >  	unsigned long flags;
> > >  	int rc;
> > >  
> > > +	entry->is_cache_clean = !!(attrs & DMA_ATTR_CPU_CACHE_CLEAN);
> > > +
> > >  	bucket = get_hash_bucket(entry, &flags);
> > >  	hash_bucket_add(bucket, entry);
> > >  	put_hash_bucket(bucket, flags);
> > >  
> > > -	rc = active_cacheline_insert(entry);
> > > +	rc = active_cacheline_insert(entry, &overlap_cache_clean);
> > >  	if (rc == -ENOMEM) {
> > >  		pr_err_once("cacheline tracking ENOMEM, dma-debug disabled\n");
> > >  		global_disable = true;
> > >  	} else if (rc == -EEXIST &&
> > > -		   !(attrs & (DMA_ATTR_SKIP_CPU_SYNC | DMA_ATTR_CPU_CACHE_CLEAN)) &&
> > > +		   !(attrs & DMA_ATTR_SKIP_CPU_SYNC) &&
> > > +		   !(entry->is_cache_clean && overlap_cache_clean) &&
> > >  		   !(IS_ENABLED(CONFIG_DMA_BOUNCE_UNALIGNED_KMALLOC) &&
> > >  		     is_swiotlb_active(entry->dev))) {
> > >  		err_printk(entry->dev, entry,  
> 

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