arm64: readahead: fault retry breaks mmap file read random detection

This is the arm64 portion of commit 45cac65b0fcd ("readahead: fault
retry breaks mmap file read random detection"), which was absent from
the initial port and has since gone unnoticed. The original commit says:

> .fault now can retry.  The retry can break state machine of .fault.  In
> filemap_fault, if page is miss, ra->mmap_miss is increased.  In the second
> try, since the page is in page cache now, ra->mmap_miss is decreased.  And
> these are done in one fault, so we can't detect random mmap file access.
>
> Add a new flag to indicate .fault is tried once.  In the second try, skip
> ra->mmap_miss decreasing.  The filemap_fault state machine is ok with it.

With this change, Mark reports that:

> Random read improves by 250%, sequential read improves by 40%, and
> random write by 400% to an eMMC device with dm crypto wrapped around it.

Cc: Shaohua Li <shli@kernel.org>
Cc: Rik van Riel <riel@redhat.com>
Cc: Wu Fengguang <fengguang.wu@intel.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Mark Salyzyn <salyzyn@android.com>
Signed-off-by: Riley Andrews <riandrews@android.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

arch/arm64/mm/fault.c

@@ -287,6 +287,7 @@ retry:
 			 * starvation.
 			 */
 			mm_flags &= ~FAULT_FLAG_ALLOW_RETRY;
+			mm_flags |= FAULT_FLAG_TRIED;
 			goto retry;
 		}
 	}