Messages in this thread | | | Subject | Re: [PATCH] slab.h: Avoid using & for logical and of booleans | From | Vlastimil Babka <> | Date | Tue, 6 Nov 2018 11:22:27 +0100 |
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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 <vbabka@suse.cz> >> Cc: Mel Gorman <mgorman@techsingularity.net> >> Cc: Christoph Lameter <cl@linux.com> >> Cc: Roman Gushchin <guro@fb.com> >> Signed-off-by: Bart Van Assche <bvanassche@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 > > - > Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK > Registration No: 1397386 (Wales) >
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