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Tue, 6 Nov 2018 14:47:03 -0500 Received: from mx2.suse.de ([195.135.220.15]:50722 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S2387480AbeKFTrC (ORCPT ); Tue, 6 Nov 2018 14:47:02 -0500 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay1.suse.de (unknown [195.135.220.254]) by mx1.suse.de (Postfix) with ESMTP id 33CEEAE06; Tue, 6 Nov 2018 10:22:29 +0000 (UTC) Subject: Re: [PATCH] slab.h: Avoid using & for logical and of booleans To: David Laight , 'Bart Van Assche' , Andrew Morton Cc: "linux-kernel@vger.kernel.org" , Mel Gorman , Christoph Lameter , Roman Gushchin References: <20181105204000.129023-1-bvanassche@acm.org> <62188a351f2249188ce654ee03c894b1@AcuMS.aculab.com> From: Vlastimil Babka Openpgp: preference=signencrypt Autocrypt: addr=vbabka@suse.cz; prefer-encrypt=mutual; keydata= xsFNBFZdmxYBEADsw/SiUSjB0dM+vSh95UkgcHjzEVBlby/Fg+g42O7LAEkCYXi/vvq31JTB KxRWDHX0R2tgpFDXHnzZcQywawu8eSq0LxzxFNYMvtB7sV1pxYwej2qx9B75qW2plBs+7+YB 87tMFA+u+L4Z5xAzIimfLD5EKC56kJ1CsXlM8S/LHcmdD9Ctkn3trYDNnat0eoAcfPIP2OZ+ 9oe9IF/R28zmh0ifLXyJQQz5ofdj4bPf8ecEW0rhcqHfTD8k4yK0xxt3xW+6Exqp9n9bydiy tcSAw/TahjW6yrA+6JhSBv1v2tIm+itQc073zjSX8OFL51qQVzRFr7H2UQG33lw2QrvHRXqD Ot7ViKam7v0Ho9wEWiQOOZlHItOOXFphWb2yq3nzrKe45oWoSgkxKb97MVsQ+q2SYjJRBBH4 8qKhphADYxkIP6yut/eaj9ImvRUZZRi0DTc8xfnvHGTjKbJzC2xpFcY0DQbZzuwsIZ8OPJCc LM4S7mT25NE5kUTG/TKQCk922vRdGVMoLA7dIQrgXnRXtyT61sg8PG4wcfOnuWf8577aXP1x 6mzw3/jh3F+oSBHb/GcLC7mvWreJifUL2gEdssGfXhGWBo6zLS3qhgtwjay0Jl+kza1lo+Cv BB2T79D4WGdDuVa4eOrQ02TxqGN7G0Biz5ZLRSFzQSQwLn8fbwARAQABzSFWbGFzdGltaWwg QmFia2EgPHZiYWJrYUBzdXNlLmNvbT7CwZcEEwEKAEECGwMFCwkIBwMFFQoJCAsFFgIDAQAC HgECF4ACGQEWIQSpQNQ0mSwujpkQPVAiT6fnzIKmZAUCWi/zTwUJBbOLuQAKCRAiT6fnzIKm ZIpED/4jRN/6LKZZIT4R2xoou0nJkBGVA3nfb+mUMgi3uwn/zC+o6jjc3ShmP0LQ0cdeuSt/ t2ytstnuARTFVqZT4/IYzZgBsLM8ODFY5vGfPw00tsZMIfFuVPQX3xs0XgLEHw7/1ZCVyJVr mTzYmV3JruwhMdUvIzwoZ/LXjPiEx1MRdUQYHAWwUfsl8lUZeu2QShL3KubR1eH6lUWN2M7t VcokLsnGg4LTajZzZfq2NqCKEQMY3JkAmOu/ooPTrfHCJYMF/5dpi8YF1CkQF/PVbnYbPUuh dRM0m3NzPtn5DdyfFltJ7fobGR039+zoCo6dFF9fPltwcyLlt1gaItfX5yNbOjX3aJSHY2Vc A5T+XAVC2sCwj0lHvgGDz/dTsMM9Ob/6rRJANlJPRWGYk3WVWnbgW8UejCWtn1FkiY/L/4qJ UsqkId8NkkVdVAenCcHQmOGjRQYTpe6Cf4aQ4HGNDeWEm3H8Uq9vmHhXXcPLkxBLRbGDSHyq vUBVaK+dAwAsXn/5PlGxw1cWtur1ep7RDgG3vVQDhIOpAXAg6HULjcbWpBEFaoH720oyGmO5 kV+yHciYO3nPzz/CZJzP5Ki7Q1zqBb/U6gib2at5Ycvews+vTueYO+rOb9sfD8BFTK386LUK uce7E38owtgo/V2GV4LMWqVOy1xtCB6OAUfnGDU2EM7ATQRbGTU1AQgAn0H6UrFiWcovkh6E XVcl+SeqyO6JHOPm+e9Wu0Vw+VIUvXZVUVVQLa1PQDUi6j00ChlcR66g9/V0sPIcSutacPKf dKYOBvzd4rlhL8rfrdEsQw5ApZxrA8kYZVMhFmBRKAa6wos25moTlMKpCWzTH84+WO5+ziCT sTUZASAToz3RdunTD+vQcHj0GqNTPAHK63sfbAB2I0BslZkXkY1RLb/YhuA6E7JyEd2pilZO rIuBGl/5q2qSakgnAVFWFBR/DO27JuAksYnq+aH8vI0xGvwn75KqSk4UzAkDzWSmO4ZHuahK tQgZNsMYV+PGayRBX9b9zbldzopoLBdqHc4njQARAQABwsF8BBgBCgAmFiEEqUDUNJksLo6Z ED1QIk+n58yCpmQFAlsZNTUCGwwFCQPCZwAACgkQIk+n58yCpmQ83g/9Frg1sRMdGPn98zV+ O2eC3h0p5f/oxxQ8MhG5znwHoW4JDG2TuxfcQuz7X7Dd5JWscjlw4VFJ2DD+IrDAGLHwPhCr RyfKalnrbYokvbClM9EuU1oUuh7k+Sg5ECNXEsamW9AiWGCaKWNDdHre3Lf4xl+RJWxghOVW RiUdpLA/a3yDvJNVr6rxkDHQ1P24ZZz/VKDyP+6g8aty2aWEU0YFNjI+rqYZb2OppDx6fdma YnLDcIfDFnkVlDmpznnGCyEqLLyMS3GH52AH13zMT9L9QYgT303+r6QQpKBIxAwn8Jg8dAlV OLhgeHXKr+pOQdFf6iu2sXlUR4MkO/5KWM1K0jFR2ug8Pb3aKOhowVMBT64G0TXhQ/kX4tZ2 ZF0QZLUCHU3Cigvbu4AWWVMNDEOGD/4sn9OoHxm6J04jLUHFUpFKDcjab4NRNWoHLsuLGjve Gdbr2RKO2oJ5qZj81K7os0/5vTAA4qHDP2EETAQcunTn6aPlkUnJ8aw6I1Rwyg7/XsU7gQHF IM/cUMuWWm7OUUPtJeR8loxZiZciU7SMvN1/B9ycPMFs/A6EEzyG+2zKryWry8k7G/pcPrFx O2PkDPy3YmN1RfpIX2HEmnCEFTTCsKgYORangFu/qOcXvM83N+2viXxG4mjLAMiIml1o2lKV cqmP8roqufIAj+Ohhzs= Message-ID: Date: Tue, 6 Nov 2018 11:22:27 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.2.1 MIME-Version: 1.0 In-Reply-To: <62188a351f2249188ce654ee03c894b1@AcuMS.aculab.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 >> Cc: Mel Gorman >> Cc: Christoph Lameter >> Cc: Roman Gushchin >> Signed-off-by: Bart Van Assche >> --- >> 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) >