linux-kernel - Re: [PATCH 7/8] mm: use vm_unmapped

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Message-ID: <1357702376.4838.32.camel@pasglop>
Date:	Wed, 09 Jan 2013 14:32:56 +1100
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Michel Lespinasse <walken@...gle.com>
Cc:	Rik van Riel <riel@...hat.com>,
	"James E.J. Bottomley" <jejb@...isc-linux.org>,
	Matt Turner <mattst88@...il.com>,
	David Howells <dhowells@...hat.com>,
	Tony Luck <tony.luck@...el.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	linuxppc-dev@...ts.ozlabs.org, linux-parisc@...r.kernel.org,
	linux-alpha@...r.kernel.org, linux-ia64@...r.kernel.org
Subject: Re: [PATCH 7/8] mm: use vm_unmapped_area() on powerpc architecture

On Tue, 2013-01-08 at 18:38 -0800, Michel Lespinasse wrote:
> 
> Well no fair, the previous patch (for powerpc as well) has 22
> insertions and 93 deletions :)
> 
> The benefit is that the new code has lower algorithmic complexity, it
> replaces a per-vma loop with O(N) complexity with an outer loop that
> finds contiguous slice blocks and passes them to vm_unmapped_area()
> which is only O(log N) complexity. So the new code will be faster for
> workloads which use lots of vmas.
> 
> That said, I do agree that the code that looks for contiguous
> available slices looks kinda ugly - just not sure how to make it look
> nicer though.

Ok. I think at least you can move that construct:

+               if (addr < SLICE_LOW_TOP) {
+                       slice = GET_LOW_SLICE_INDEX(addr);
+                       addr = (slice + 1) << SLICE_LOW_SHIFT;
+                       if (!(available.low_slices & (1u << slice)))
+                               continue;
+               } else {
+                       slice = GET_HIGH_SLICE_INDEX(addr);
+                       addr = (slice + 1) << SLICE_HIGH_SHIFT;
+                       if (!(available.high_slices & (1u << slice)))
+                               continue;
+               }

Into some kind of helper. It will probably compile to the same thing but
at least it's more readable and it will avoid a fuckup in the future if
somebody changes the algorithm and forgets to update one of the
copies :-)

Cheers,
Ben.


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