IB/qib: Convert qib_user_sdma_pin_pages() to use get_user_pages_fast()

From: Jan Kara <[email protected]>

commit 603e7729920e42b3c2f4dbfab9eef4878cb6e8fa upstream.

qib_user_sdma_queue_pkts() gets called with mmap_sem held for writing. Except for get_user_pages() deep down in qib_user_sdma_pin_pages() we don't seem to need mmap_sem at all. Even more interestingly the function qib_user_sdma_queue_pkts() (and also qib_user_sdma_coalesce() called somewhat later) call copy_from_user() which can hit a page fault and we deadlock on trying to get mmap_sem when handling that fault.

So just make qib_user_sdma_pin_pages() use get_user_pages_fast() and leave mmap_sem locking for mm.

This deadlock has actually been observed in the wild when the node is under memory pressure.

Patch for v3.4.

[Backported to 3.4: (Thank to Ben Hutchings)
- Adjust context
- Adjust indentation and nr_pages argument in qib_user_sdma_pin_pages()]

---
 drivers/infiniband/hw/qib/qib_user_sdma.c |    6 +-----
 1 file changed, 1 insertion(+), 5 deletions(-)

index 8244208..573b460 100644
@@ -284,8 +284,7 @@ static int qib_user_sdma_pin_pages(const struct qib_devdata *dd,
 	int j;
 	int ret;
 
-	ret = get_user_pages(current, current->mm, addr,
-			     npages, 0, 1, pages, NULL);
+	ret = get_user_pages_fast(addr, npages, 0, pages);
 
 	if (ret != npages) {
 		int i;

@@ -830,10 +829,7 @@ int qib_user_sdma_writev(struct qib_ctxtdata *rcd,
 	while (dim) {
 		const int mxp = 8;
 
-		down_write(&current->mm->mmap_sem);
 		ret = qib_user_sdma_queue_pkts(dd, pq, &list, iov, dim, mxp);
-		up_write(&current->mm->mmap_sem);
-
 		if (ret <= 0)
 			goto done_unlock;
 		else {

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