mm/hmm/test: destroy xa_array instead of looping

The test driver uses an xa_array to store virtual to physical address translations for a simulated hardware device. The MMU notifier invalidation callback is used to keep the table consistent with the CPU page table and is frequently called only for a page or two. However, if the test process exits unexpectedly or is killed, the range can be [0..ULONG_MAX] in which case calling xa_erase() for every possible PFN results in CPU timeouts. Munmap() can result in a large range being invalidated but in that case, the xa_array is likely to contain entries that need to be invalidated.
Check for [0..ULONG_MAX] explicitly and just destroy the whole table.

Signed-off-by Ralph Campbell <[email protected]>

This patch is based on Jason Gunthorpe's hmm tree and should be folded into the ("mm/hmm/test: add selftest driver for HMM") patch once this patch is reviewed, etc.

 lib/test_hmm.c | 6 ++++++
 1 file changed, 6 insertions(+)

lib/test_hmm.c +1 -0

index 8b36c26b717b..b89852ec3c29 100644
+++ b/lib/test_hmm.c

@@ -201,7 +201,13 @@ static void dmirror_do_update(struct dmirror *dmirror, unsigned long start,
 	 * The XArray doesn't hold references to pages since it relies on
 	 * the mmu notifier to clear page pointers when they become stale.
 	 * Therefore, it is OK to just clear the entry.

Quoted 27 lines · Ralph Campbell

The test driver uses an xa_array to store virtual to physical address translations for a simulated hardware device. The MMU notifier invalidation callback is used to keep the table consistent with the CPU page table and is frequently called only for a page or two. However, if the test process exits unexpectedly or is killed, the range can be [0..ULONG_MAX] in which case calling xa_erase() for every possible PFN results in CPU timeouts. Munmap() can result in a large range being invalidated but in that case, the xa_array is likely to contain entries that need to be invalidated.
Check for [0..ULONG_MAX] explicitly and just destroy the whole table.

Signed-off-by Ralph Campbell <[email protected]>

This patch is based on Jason Gunthorpe's hmm tree and should be folded into the ("mm/hmm/test: add selftest driver for HMM") patch once this patch is reviewed, etc.

  lib/test_hmm.c | 6 ++++++
  1 file changed, 6 insertions(+)

lib/test_hmm.c +1 -0

index 8b36c26b717b..b89852ec3c29 100644
+++ b/lib/test_hmm.c

@@ -201,7 +201,13 @@ static void dmirror_do_update(struct dmirror *dmirror, unsigned long start,
  	 * The XArray doesn't hold references to pages since it relies on
  	 * the mmu notifier to clear page pointers when they become stale.
  	 * Therefore, it is OK to just clear the entry.

lib/test_hmm.c static void dmirror_do_update(struct dmirror *dmirror, unsigned long start, · line 213

Quoted 10 lines

lib/test_hmm.c +6 -0

+	 * However, if the entire address space is being invalidated, it
+	 * takes too long to clear them one at a time so destroy the array.
  	 */
+	if (start == 0 && end == ULONG_MAX) {
+		xa_destroy(&dmirror->pt);
+		return;
+	}
  	for (pfn = start >> PAGE_SHIFT; pfn < (end >> PAGE_SHIFT); pfn++)
  		xa_erase(&dmirror->pt, pfn);
  }

Just use xa_for_each_range() instead of the naive loop, it already optimizes against membership and avoids the need for the xa_destroy hack

Jason