From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wr0-f198.google.com (mail-wr0-f198.google.com [209.85.128.198]) by kanga.kvack.org (Postfix) with ESMTP id 31AF66B0673 for ; Thu, 3 Aug 2017 04:11:55 -0400 (EDT) Received: by mail-wr0-f198.google.com with SMTP id z48so986883wrc.4 for ; Thu, 03 Aug 2017 01:11:55 -0700 (PDT) Received: from mx1.suse.de (mx2.suse.de. [195.135.220.15]) by mx.google.com with ESMTPS id 75si1100700wrb.356.2017.08.03.01.11.53 for (version=TLS1 cipher=AES128-SHA bits=128/128); Thu, 03 Aug 2017 01:11:53 -0700 (PDT) Date: Thu, 3 Aug 2017 10:11:52 +0200 From: Michal Hocko Subject: Re: suspicious __GFP_NOMEMALLOC in selinux Message-ID: <20170803081152.GC12521@dhcp22.suse.cz> References: <20170802105018.GA2529@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: owner-linux-mm@kvack.org List-ID: To: Paul Moore Cc: Jeff Vander Stoep , linux-mm@kvack.org, LKML , selinux@tycho.nsa.gov, Mel Gorman [CC Mel] On Wed 02-08-17 17:45:56, Paul Moore wrote: > 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] Sorry about that. Will keep it in mind for next posts > 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. Thanks for the pointer. That makes much more sense now. Back in 2012 we really didn't have a good way to distinguish non sleeping and atomic with reserves allocations. > 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. What do you think about the following? I haven't tested it but it should be rather straightforward. ---