[PATCH for rc] mm/shmem: Ensure proper fallback if page faults

[ira.weiny@intel.com]

From: Ira Weiny <ira.weiny@intel.com>

The kernel test robot flagged a recursive lock as a result of a
conversion from kmap_atomic() to kmap_local_folio()[Link]

The cause was due to the code depending on the kmap_atomic() side effect
of disabling page faults.  In that case the code expects the fault to
fail and take the fallback case.

git archaeology implied that the recursion may not be an actual bug.[1]
However, the mmap_lock needed in the fault may be the one held.[2]

Add an explicit pagefault_disable() and a big comment to explain this
for future souls looking at this code.

[1] https://lore.kernel.org/all/Y1MymJ%2FINb45AdaY@iweiny-desk3/
[2] https://lore.kernel.org/all/Y1M2p9OtBGnKwGUE@x1n/

Fixes: 7a7256d5f512 ("shmem: convert shmem_mfill_atomic_pte() to use a folio")
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Peter Xu <peterx@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Reported-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reported-by: kernel test robot <yujie.liu@intel.com>
Link: https://lore.kernel.org/r/202210211215.9dc6efb5-yujie.liu@intel.com
Signed-off-by: Ira Weiny <ira.weiny@intel.com>

---
Thanks to Matt and Andrew for initial diagnosis.
Thanks to Randy for pointing out C code needs ';'  :-D
Thanks to Andrew for suggesting an elaborate comment
Thanks to Peter for pointing out that the mm's may be the same.
---
 mm/shmem.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/mm/shmem.c b/mm/shmem.c
index 8280a5cb48df..c1bca31cd485 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2424,9 +2424,16 @@ int shmem_mfill_atomic_pte(struct mm_struct *dst_mm,
 
 		if (!zeropage) {	/* COPY */
 			page_kaddr = kmap_local_folio(folio, 0);
+			/*
+			 * The mmap_lock is held here.  Disable page faults to
+			 * prevent deadlock should copy_from_user() fault.  The
+			 * copy will be retried outside the mmap_lock.
+			 */
+			pagefault_disable();
 			ret = copy_from_user(page_kaddr,
 					     (const void __user *)src_addr,
 					     PAGE_SIZE);
+			pagefault_enable();
 			kunmap_local(page_kaddr);
 
 			/* fallback to copy_from_user outside mmap_lock */
-- 
2.37.2