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Re: [PATCH] Fix off by one error in page_region_mask()

To: lachlan@xxxxxxx
Subject: Re: [PATCH] Fix off by one error in page_region_mask()
From: Timothy Shimmin <tes@xxxxxxx>
Date: Mon, 08 Dec 2008 15:42:04 +1100
Cc: xfs-oss <xfs@xxxxxxxxxxx>
In-reply-to: <49378B60.1060603@xxxxxxx>
References: <49378B60.1060603@xxxxxxx>
User-agent: Thunderbird 2.0.0.17 (Macintosh/20080914)
Lachlan McIlroy wrote:
> final is calculated to be the last bit to set (ie inclusive) but when we
> do the mask shifting final really needs to be first bit not to set.
> 
> For example if first and final are both bit 0 (ie only first bit to be set)
> then mask is completely shifted and becomes all zeroes.
> 
> Or if first is 0 and final is 63 then the mask is shifted one bit when it
> shouldn't be shifted at all.
> 
> --- xfs-fix.orig/fs/xfs/linux-2.6/xfs_buf.c
> +++ xfs-fix/fs/xfs/linux-2.6/xfs_buf.c
> @@ -129,15 +129,17 @@ page_region_mask(
>       int             first, final;
> 
>       first = BTOPR(offset);
> -     final = BTOPRT(offset + length - 1);
> -     first = min(first, final);
> +     final = BTOPRT(offset + length);
> +
> +     if (first >= final)
> +             return 0UL;
> 
>       mask = ~0UL;
>       mask <<= BITS_PER_LONG - (final - first);
>       mask >>= BITS_PER_LONG - (final);
> 
>       ASSERT(offset + length <= PAGE_CACHE_SIZE);
> -     ASSERT((final - first) < BITS_PER_LONG && (final - first) >= 0);
> +     ASSERT((final - first) <= BITS_PER_LONG && (final - first) > 0);
> 
>       return mask;
>   }
> 
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Gee, I always find these tricky to get right.
I tend to like a userspace version and input various ranges,
such as extremes like you did, and verify it is working.

I kind of like first and final to be inclusive of the range to be set
(not so keen on making final to be first bit not to set -
name doesn't seem so good then).
And if one needs to know the size of the range to use (final - first + 1)
and for 0..final => size = final-0+1 = final+1.

And the thing about min(first, final) and (first >= final),
is interesting - I would have thought final would be expected to be >=
to the first ??

Okay, the other thing that interests me is the use of both BTOPR and BTOPRT
for given offsets.
BTOPR is the typical howmany() implementation, where if you go over
a multiple-boundaries worth then you need another multiple.
I would expect it to have values starting from 1.
So I was thinking typically of BTOPR for sizes and BTOPRT for 0-based offsets.
e.g. given multiple, 512
BTOPR  1..512 => 1, 513..1024 => 2
BTOPRT 0..511 => 0, 512..1023 => 1

So I find the code a bit hard to follow then.

--Tim

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