Re: [PATCH net 2/2] page_pool: fix IOMMU crash when driver has already unbound

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With all the caching in the network stack, some pages may be
held in the network stack without returning to the page_pool
soon enough, and with VF disable causing the driver unbound,
the page_pool does not stop the driver from doing it's
unbounding work, instead page_pool uses workqueue to check
if there is some pages coming back from the network stack
periodically, if there is any, it will do the dma unmmapping
related cleanup work.

As mentioned in [1], attempting DMA unmaps after the driver
has already unbound may leak resources or at worst corrupt
memory. Fundamentally, the page pool code cannot allow DMA
mappings to outlive the driver they belong to.

Currently it seems there are at least two cases that the page
is not released fast enough causing dma unmmapping done after
driver has already unbound:
1. ipv4 packet defragmentation timeout: this seems to cause
   delay up to 30 secs:

2. skb_defer_free_flush(): this may cause infinite delay if
   there is no triggering for net_rx_action().

In order not to do the dma unmmapping after driver has already
unbound and stall the unloading of the networking driver, add
the pool->items array to record all the pages including the ones
which are handed over to network stack, so the page_pool can
do the dma unmmapping for those pages when page_pool_destroy()
is called.

So, I was thinking of a very similar idea. But what do you mean by
"all"? The pages that are still in caches (slow or fast) of the pool
will be unmapped during page_pool_destroy().

Yes, it includes the one in pool->alloc and pool->ring.

It worths mentioning that there is a semantics changing here:
Before this patch, there can be almost unlimited inflight pages used by
driver and network stack, as page_pool doesn't really track those pages.
After this patch, as we use a fixed-size pool->items array to track the
inflight pages, the inflight pages is limited by the pool->items, currently
the size of pool->items array is calculated as below in this patch:

+#define PAGE_POOL_MIN_ITEM_CNT	512
+	unsigned int item_cnt = (params->pool_size ? : 1024) +
+				PP_ALLOC_CACHE_SIZE + PAGE_POOL_MIN_ITEM_CNT;

Personally I would consider it is an advantage to limit how many pages which
are used by the driver and network stack, the problem seems to how to decide
the limited number of page used by network stack so that performance is not
impacted.

In theory, with respect to the specific problem at hand, you only have
a limit on the number of mapped pages inflight. Once you reach this
limit you can unmap these old pages, forget about them and remember
new ones.