Commit 926150db authored May 14, 2019 by Vineet Gupta

ARC: mm: do_page_fault refactor #8: release mmap_sem sooner



In case of successful page fault handling, this patch releases mmap_sem
before updating the perf stat event for major/minor faults. So even
though the contention reduction is NOT super high, it is still an
improvement.

There's an additional code size improvement as we only have 2 up_read()
calls now.

Note to myself:
--------------

1. Given the way it is done, we are forced to move @bad_area label earlier
   causing the various "goto bad_area" cases to hit perf stat code.

 - PERF_COUNT_SW_PAGE_FAULTS is NOW updated for access errors which is what
   arm/arm64 seem to be doing as well (with slightly different code)
 - PERF_COUNT_SW_PAGE_FAULTS_{MAJ,MIN} must NOT be updated for the
   error case which is guarded by now setting @fault initial value
   to VM_FAULT_ERROR which serves both cases when handle_mm_fault()
   returns error or is not called at all.

2. arm/arm64 use two homebrew fault flags VM_FAULT_BAD{MAP,MAPACCESS}
   which I was inclined to add too but seems not needed for ARC

 - given that we have everything is 1 function we can still use goto
 - we setup si_code at the right place (arm* do that in the end)
 - we init fault already to error value which guards entry into perf
   stats event update

Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>

parent 5e91bf5c

arch/arc/mm/fault.c

+3 −4

Original line number	Diff line number	Diff line
		@@ -152,6 +152,9 @@ retry:
		}
		}

		bad_area:
		up_read(&mm->mmap_sem);

		/*
		* Major/minor page fault accounting
		* (in case of retry we only land here once)
		@@ -170,13 +173,9 @@ retry:
		}

		/* Normal return path: fault Handled Gracefully */
		up_read(&mm->mmap_sem);
		return;
		}

		bad_area:
		up_read(&mm->mmap_sem);

		if (!user_mode(regs))
		goto no_context;

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