xsk: always clear DMA mapping information when unmapping the pool

When the umem is shared, the DMA mapping is also shared between the xsk
pools, therefore it should stay valid as long as at least 1 user remains.
However, the pool also keeps the copies of DMA-related information that are
initialized in the same way in xp_init_dma_info(), but cleared by
xp_dma_unmap() only for the last remaining pool, this causes the problems
below.

The first one is that the commit adbf5a4 ("ice: remove af_xdp_zc_qps
bitmap") relies on pool->dev to determine the presence of a ZC pool on a
given queue, avoiding internal bookkeeping. This works perfectly fine if
the UMEM is not shared, but reliably fails otherwise as stated in the
linked report.

The second one is pool->dma_pages which is dynamically allocated and
only freed in xp_dma_unmap(), this leads to a small memory leak. kmemleak
does not catch it, but by printing the allocation results after terminating
the userspace program it is possible to see that all addresses except the
one belonging to the last detached pool are still accessible through the
kmemleak dump functionality.

Always clear the DMA mapping information from the pool and free
pool->dma_pages when unmapping the pool, so that the only difference
between results of the last remaining user's call and the ones before would
be the destruction of the DMA mapping.

Fixes: adbf5a4 ("ice: remove af_xdp_zc_qps bitmap")
Fixes: 921b686 ("xsk: Enable sharing of dma mappings")
Reported-by: Alasdair McWilliam <[email protected]>
Closes: https://lore.kernel.org/PA4P194MB10056F208AF221D043F57A3D86512@PA4P194MB1005.EURP194.PROD.OUTLOOK.COM
Acked-by: Maciej Fijalkowski <[email protected]>
Signed-off-by: Larysa Zaremba <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Alexei Starovoitov <[email protected]>

Author: walking-machine

net/xdp/xsk_buff_pool.c

@@ -387,10 +387,9 @@ void xp_dma_unmap(struct xsk_buff_pool *pool, unsigned long attrs)
 		return;
 	}
 
-	if (!refcount_dec_and_test(&dma_map->users))
-		return;
+	if (refcount_dec_and_test(&dma_map->users))
+		__xp_dma_unmap(dma_map, attrs);
 
-	__xp_dma_unmap(dma_map, attrs);
 	kvfree(pool->dma_pages);
 	pool->dma_pages = NULL;
 	pool->dma_pages_cnt = 0;