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Subject[PATCH 9/9] ARC: mm: do_page_fault refactor #8: release mmap_sem sooner
Date
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 supe rhigh, 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 cabn stil 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>
---
arch/arc/mm/fault.c | 9 ++++-----
1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/arch/arc/mm/fault.c b/arch/arc/mm/fault.c
index 20402217d9da..e93ea06c214c 100644
--- a/arch/arc/mm/fault.c
+++ b/arch/arc/mm/fault.c
@@ -68,7 +68,7 @@ void do_page_fault(unsigned long address, struct pt_regs *regs)
struct mm_struct *mm = tsk->mm;
int sig, si_code = SEGV_MAPERR;
unsigned int write = 0, exec = 0, mask;
- vm_fault_t fault; /* handle_mm_fault() output */
+ vm_fault_t fault = VM_FAULT_ERROR; /* handle_mm_fault() output */
unsigned int flags; /* handle_mm_fault() input */

/*
@@ -155,6 +155,9 @@ void do_page_fault(unsigned long address, struct pt_regs *regs)
}
}

+bad_area:
+ up_read(&mm->mmap_sem);
+
/*
* Major/minor page fault accounting
* (in case of retry we only land here once)
@@ -173,13 +176,9 @@ void do_page_fault(unsigned long address, struct pt_regs *regs)
}

/* 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;

--
2.7.4
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