On Mon, 13 Jan 2014 16:48:47 -0800 Dan Williams <dan.j.willi...@intel.com>
wrote:
> Record actively mapped pages and provide an api for asserting a given
> page is dma inactive before execution proceeds. Placing
> debug_dma_assert_idle() in cow_user_page() flagged the violation of the
> dma-api in the NET_DMA implementation (see commit 77873803363c "net_dma:
> mark broken").
Some discussion of the overlap counter thing would be useful.
> --- a/include/linux/dma-debug.h
> +++ b/include/linux/dma-debug.h
>
> ...
>
> +static void __active_pfn_inc_overlap(struct dma_debug_entry *entry)
> +{
> + unsigned long pfn = entry->pfn;
> + int i;
> +
> + for (i = 0; i < RADIX_TREE_MAX_TAGS; i++)
> + if (radix_tree_tag_get(&dma_active_pfn, pfn, i) == 0) {
> + radix_tree_tag_set(&dma_active_pfn, pfn, i);
> + return;
> + }
> + pr_debug("DMA-API: max overlap count (%d) reached for pfn 0x%lx\n",
> + RADIX_TREE_MAX_TAGS, pfn);
> +}
> +
> +static void __active_pfn_dec_overlap(struct dma_debug_entry *entry)
> +{
> + unsigned long pfn = entry->pfn;
> + int i;
> +
> + for (i = RADIX_TREE_MAX_TAGS - 1; i >= 0; i--)
> + if (radix_tree_tag_get(&dma_active_pfn, pfn, i)) {
> + radix_tree_tag_clear(&dma_active_pfn, pfn, i);
> + return;
> + }
> + radix_tree_delete(&dma_active_pfn, pfn);
> +}
> +
> +static int active_pfn_insert(struct dma_debug_entry *entry)
> +{
> + unsigned long flags;
> + int rc;
> +
> + spin_lock_irqsave(&radix_lock, flags);
> + rc = radix_tree_insert(&dma_active_pfn, entry->pfn, entry);
> + if (rc == -EEXIST)
> + __active_pfn_inc_overlap(entry);
> + spin_unlock_irqrestore(&radix_lock, flags);
> +
> + return rc;
> +}
> +
> +static void active_pfn_remove(struct dma_debug_entry *entry)
> +{
> + unsigned long flags;
> +
> + spin_lock_irqsave(&radix_lock, flags);
> + __active_pfn_dec_overlap(entry);
> + spin_unlock_irqrestore(&radix_lock, flags);
> +}
OK, I think I see what's happening. The tags thing acts as a crude
counter and if the map/unmap count ends up imbalanced, we deliberately
leak an entry in the radix-tree so it can later be reported via undescribed
means. Thoughts:
- RADIX_TREE_MAX_TAGS=3 so the code could count to 7, with a bit of
futzing around.
- from a style/readability point of view it is unexpected that
__active_pfn_dec_overlap() actually removes radix-tree items. It
would be better to do:
spin_lock_irqsave(&radix_lock, flags);
if (__active_pfn_dec_overlap(entry) == something) {
/*
* Nice comment goes here
*/
radix_tree_delete(...);
}
spin_unlock_irqrestore(&radix_lock, flags);
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