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[209.85.208.174]) by smtp.gmail.com with ESMTPSA id y16sm1155798ljm.19.2020.06.05.13.18.38 for (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 05 Jun 2020 13:18:38 -0700 (PDT) Received: by mail-lj1-f174.google.com with SMTP id i27so2327653ljb.12 for ; Fri, 05 Jun 2020 13:18:38 -0700 (PDT) X-Received: by 2002:a2e:b5d9:: with SMTP id g25mr5890987ljn.285.1591388317824; Fri, 05 Jun 2020 13:18:37 -0700 (PDT) MIME-Version: 1.0 References: <20200605142300.14591-1-linux@rasmusvillemoes.dk> In-Reply-To: <20200605142300.14591-1-linux@rasmusvillemoes.dk> From: Linus Torvalds Date: Fri, 5 Jun 2020 13:18:21 -0700 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [PATCH resend] fs/namei.c: micro-optimize acl_permission_check To: Rasmus Villemoes Cc: Alexander Viro , linux-fsdevel , Linux Kernel Mailing List Content-Type: multipart/mixed; boundary="0000000000006294c705a75bf797" Sender: linux-fsdevel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org --0000000000006294c705a75bf797 Content-Type: text/plain; charset="UTF-8" On Fri, Jun 5, 2020 at 7:23 AM Rasmus Villemoes wrote: > > + /* > + * If the "group" and "other" permissions are the same, > + * there's no point calling in_group_p() to decide which > + * set to use. > + */ > + if ((((mode >> 3) ^ mode) & 7) && in_group_p(inode->i_gid)) > mode >>= 3; Ugh. Not only is this ugly, but it's not even the best optimization. We don't care that group and other match exactly. We only care that they match in the low 3 bits of the "mask" bits. So if we want this optimization - and it sounds worth it - I think we should do it right. But I also think it should be written more legibly. And the "& 7" is the same "& (MAY_READ | MAY_WRITE | MAY_EXEC)" we do later. In other words, if we do this, I'd like it to be done even more aggressively, but I'd also like the end result to be a lot more readable and have more comments about why we do that odd thing. Something like this *UNTESTED* patch, perhaps? I might have gotten something wrong, so this would need double-checking, but if it's right, I find it a _lot_ more easy to understand than making one expression that is pretty complicated and opaque. Hmm? 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