Re: [PATCH 1/1] sg: SG_IO 64k page fix

[Brian King <brking@us.ibm.com>]

Douglas Gilbert wrote:
> Brian King wrote:
>> The problem appears to be in the definition of SG_SCATTER_SZ. The comment
>> states that it must be a power of 2, must be less than PAGE_SIZE * 32,
>> and must be at least as large as PAGE_SIZE. For 64k pages, this is not
>> true. Increasing SG_SCATTER_SZ to 64k fixed the oops in my configuration.
>>
>> Signed-off-by: Brian King <brking@us.ibm.com>
>> ---
>>
>>  linux-2.6-bjking1/include/scsi/sg.h |    2 +-
>>  1 files changed, 1 insertion(+), 1 deletion(-)
>>
>> diff -puN include/scsi/sg.h~sg_large_page_support include/scsi/sg.h
>> --- linux-2.6/include/scsi/sg.h~sg_large_page_support	2006-09-19 09:24:06.000000000 -0500
>> +++ linux-2.6-bjking1/include/scsi/sg.h	2006-09-19 09:27:00.000000000 -0500
>> @@ -238,7 +238,7 @@ typedef struct sg_req_info { /* used by 
>>  #define SG_GET_ACCESS_COUNT 0x2289  
>>  
>>  
>> -#define SG_SCATTER_SZ (8 * 4096)  /* PAGE_SIZE not available to user */
>> +#define SG_SCATTER_SZ (16 * 4096)  /* PAGE_SIZE not available to user */
>>  /* Largest size (in bytes) a single scatter-gather list element can have.
>>     The value must be a power of 2 and <= (PAGE_SIZE * 32) [131072 bytes on
>>     i386]. The minimum value is PAGE_SIZE. If scatter-gather not supported
>> _
> 
> Brian,
> Yes that obviously needs to be fixed. However the patch
> is a bit fragile, for example if some architecture decides
> to have an option for 128 KB page size or larger.

I agree. 

> I've got a patch to add a sysfs/boottime attribute for
> setting max_scatter_elem_sz. If that logic made an additional
> check, adjusting max_scatter_elem_sz up to PAGE_SIZE if
> necessary. The SG_SCATTER_SZ define could stay as is,
> apart from an additional note that it will be forced to
> PAGE_SIZE if necessary.

You forgot to attach the patch ;) Is this something you plan
to be submitting soon?

Thanks,

Brian

-- 
Brian King
eServer Storage I/O
IBM Linux Technology Center