Re: [PATCH v2 05/15] dma-debug: track cache clean flag in entries

Petr Tesarik
2026-01-05T09:54:37Z

On Mon, 5 Jan 2026 03:23:10 -0500
"Michael S. Tsirkin" <mst@redhat.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@redhat.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@suse.com>

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,