Commit 50f5aa8a authored Apr 28, 2014 by Linus Torvalds

mm: don't pointlessly use BUG_ON() for sanity check



BUG_ON() is a big hammer, and should be used _only_ if there is some
major corruption that you cannot possibly recover from, making it
imperative that the current process (and possibly the whole machine) be
terminated with extreme prejudice.

The trivial sanity check in the vmacache code is *not* such a fatal
error.  Recovering from it is absolutely trivial, and using BUG_ON()
just makes it harder to debug for no actual advantage.

To make matters worse, the placement of the BUG_ON() (only if the range
check matched) actually makes it harder to hit the sanity check to begin
with, so _if_ there is a bug (and we just got a report from Srivatsa
Bhat that this can indeed trigger), it is harder to debug not just
because the machine is possibly dead, but because we don't have better
coverage.

BUG_ON() must *die*.  Maybe we should add a checkpatch warning for it,
because it is simply just about the worst thing you can ever do if you
hit some "this cannot happen" situation.

Reported-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
Cc: Davidlohr Bueso <davidlohr@hp.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent d1db0eea

mm/vmacache.c

+5 −3

Original line number	Diff line number	Diff line
		@@ -81,11 +81,13 @@ struct vm_area_struct vmacache_find(struct mm_struct mm, unsigned long addr)
		for (i = 0; i < VMACACHE_SIZE; i++) {
		struct vm_area_struct *vma = current->vmacache[i];

		if (vma && vma->vm_start <= addr && vma->vm_end > addr) {
		BUG_ON(vma->vm_mm != mm);
		if (!vma)
		continue;
		if (WARN_ON_ONCE(vma->vm_mm != mm))
		break;
		if (vma->vm_start <= addr && vma->vm_end > addr)
		return vma;
		}
		}

		return NULL;
		}

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