On 11/6/18 11:08 AM, David Laight wrote:
> From: Bart Van Assche
>> Sent: 05 November 2018 20:40
>>
>> This patch suppresses the following sparse warning:
>>
>> ./include/linux/slab.h:332:43: warning: dubious: x & !y
BTW, I wonder why the warnings appeared only now, after maybe months in
linux-next. Don't the various automated testing bots run sparse also on
linux-next?
>>
>> Fixes: 1291523f2c1d ("mm, slab/slub: introduce kmalloc-reclaimable caches")
>> Cc: Vlastimil Babka <vba...@suse.cz>
>> Cc: Mel Gorman <mgor...@techsingularity.net>
>> Cc: Christoph Lameter <c...@linux.com>
>> Cc: Roman Gushchin <g...@fb.com>
>> Signed-off-by: Bart Van Assche <bvanass...@acm.org>
>> ---
>> include/linux/slab.h | 2 +-
>> 1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/include/linux/slab.h b/include/linux/slab.h
>> index 918f374e7156..97d0599ddb7b 100644
>> --- a/include/linux/slab.h
>> +++ b/include/linux/slab.h
>> @@ -329,7 +329,7 @@ static __always_inline enum kmalloc_cache_type
>> kmalloc_type(gfp_t flags)
>> * If an allocation is both __GFP_DMA and __GFP_RECLAIMABLE, return
>> * KMALLOC_DMA and effectively ignore __GFP_RECLAIMABLE
>> */
>> - return type_dma + (is_reclaimable & !is_dma) * KMALLOC_RECLAIM;
>> + return type_dma + is_reclaimable * !is_dma * KMALLOC_RECLAIM;
>
> ISTM that changing is_dma and is_reclaimable from int to bool will stop the
> bleating.
>
> It is also strange that this code is trying so hard here to avoid conditional
> instructions
I primarily wanted to avoid branches in a hot path, not cmovs. Note
those are also not "free" (latency-wise) if the result of cmov is
immediately used for further computation.
> and then uses several to generate the boolean values in the first place.
I'm not sure where exactly?
> OTOH I'd probably write:
> int gfp_dma = 0;
>
> #ifdef CONFIG_ZONE_DMA
> gfp_dma = __GFP_DMA;
> #endif
>
> return flags & gfp_dma ? KMALLOC_DMA : flags & __GFP_RECLAIMABLE ?
> KMALLOC_RECLAIM : 0;
I'm not opposed to this. Christoph might :)
>
> That might generate cmovs, but is may be better to put unlikely() around both
> conditional expressions. Or redo as:
>
> return !unlikely(flags & (dfp_dma | __GFP_RECLAIMABLE)) ? 0 : flags &
> gfp_dma ? KMALLOC_DMA : KMALLOC_RECLAIM;
I guess it should be structured so that the fast path is for gfp without
both __GFP_DMA and __GFP_RECLAIMABLE, with a single test+branch. IIRC
that's what Christoph originally requested.
> David
>
> -
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