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

[Lachlan McIlroy]

Eric Sandeen wrote:
> 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.
> 
> Lachlan, what's the end result of this bug?  What's the broken behavior?

There was no observed bug - well nothing I can tie directly to this code.
I found this by inspection while investigating the page bitmap stuff.
We have a problem with ia64 going to 64K page size with filesystems that
use a filesystem sector size of 512 bytes - we don't have the granularity
we need in the bitmap.

I suppose it is possible this bug could indicate a page region is not up
to date when it actually is and we might re-read something from disk and
overwrite the more up to date in-memory version.

> 
> Thanks,
> -Eric
> 
>> --- 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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> 
>