Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751562AbdHBVp6 (ORCPT ); Wed, 2 Aug 2017 17:45:58 -0400 Received: from mail-oi0-f45.google.com ([209.85.218.45]:34882 "EHLO mail-oi0-f45.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751121AbdHBVp5 (ORCPT ); Wed, 2 Aug 2017 17:45:57 -0400 MIME-Version: 1.0 In-Reply-To: <20170802105018.GA2529@dhcp22.suse.cz> References: <20170802105018.GA2529@dhcp22.suse.cz> From: Paul Moore Date: Wed, 2 Aug 2017 17:45:56 -0400 Message-ID: Subject: Re: suspicious __GFP_NOMEMALLOC in selinux To: Michal Hocko Cc: Jeff Vander Stoep , linux-mm@kvack.org, LKML , selinux@tycho.nsa.gov Content-Type: text/plain; charset="UTF-8" Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1435 Lines: 31 On Wed, Aug 2, 2017 at 6:50 AM, Michal Hocko wrote: > Hi, > while doing something completely unrelated to selinux I've noticed a > really strange __GFP_NOMEMALLOC usage pattern in selinux, especially > GFP_ATOMIC | __GFP_NOMEMALLOC doesn't make much sense to me. GFP_ATOMIC > on its own allows to access memory reserves while the later flag tells > we cannot use memory reserves at all. The primary usecase for > __GFP_NOMEMALLOC is to override a global PF_MEMALLOC should there be a > need. > > It all leads to fa1aa143ac4a ("selinux: extended permissions for > ioctls") which doesn't explain this aspect so let me ask. Why is the > flag used at all? Moreover shouldn't GFP_ATOMIC be actually GFP_NOWAIT. > What makes this path important to access memory reserves? [NOTE: added the SELinux list to the CC line, please include that list when asking SELinux questions] The GFP_ATOMIC|__GFP_NOMEMALLOC use in SELinux appears to be limited to security/selinux/avc.c, and digging a bit, I'm guessing commit fa1aa143ac4a copied the combination from 6290c2c43973 ("selinux: tag avc cache alloc as non-critical") and the avc_alloc_node() function. I can't say that I'm an expert at the vm subsystem and the variety of different GFP_* flags, but your suggestion of moving to GFP_NOWAIT in security/selinux/avc.c seems reasonable and in keeping with the idea behind commit 6290c2c43973. -- paul moore security @ redhat