linux-kernel - Re: [PATCH v2] mm: BUG when __kmap_atomic_idx equals KM_TYPE

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Message-Id: <20140731154540.441ab79ff32ae5c10f64bcbd@linux-foundation.org>
Date:	Thu, 31 Jul 2014 15:45:40 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Chintan Pandya <cpandya@...eaurora.org>
Cc:	David Rientjes <rientjes@...gle.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [PATCH v2] mm: BUG when __kmap_atomic_idx equals KM_TYPE_NR

On Thu, 31 Jul 2014 14:58:58 +0530 Chintan Pandya <cpandya@...eaurora.org> wrote:

> >
> > I think Andrew's comment earlier was referring to the changelog only and
> > not the patch, which looked correct.
> 
> I think Andrew asked for a BUG case details also to justify the 
> overhead. But we have never encountered that BUG case. Present patch is 
> only logical fix to the code. However, in the fast path, if such 
> overhead is allowed, I can move BUG_ON out of any debug configs. 
> Otherwise, as per Andrew's suggestion, I will convert DEBUG_HIGHMEM into 
> DEBUG_VM which is used more frequently.

The v1 patch added a small amount of overhead to kmap_atomic() for what
is evidently a very small benefit.

Yes, I suggest we remove CONFIG_DEBUG_HIGHMEM from the kernel entirely
and switch all CONFIG_DEBUG_HIGHMEM sites to use CONFIG_DEBUG_VM.  That way
the BUG_ON which you believe is useful will be tested by more people
more often.
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