From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pg0-f70.google.com (mail-pg0-f70.google.com [74.125.83.70]) by kanga.kvack.org (Postfix) with ESMTP id BE73C6B0038 for ; Fri, 16 Dec 2016 19:28:26 -0500 (EST) Received: by mail-pg0-f70.google.com with SMTP id y71so224240194pgd.0 for ; Fri, 16 Dec 2016 16:28:26 -0800 (PST) Received: from mail-pg0-x242.google.com (mail-pg0-x242.google.com. [2607:f8b0:400e:c05::242]) by mx.google.com with ESMTPS id n19si9942125pfk.284.2016.12.16.16.28.25 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Fri, 16 Dec 2016 16:28:25 -0800 (PST) Received: by mail-pg0-x242.google.com with SMTP id p66so11190286pga.2 for ; Fri, 16 Dec 2016 16:28:25 -0800 (PST) Date: Fri, 16 Dec 2016 16:28:21 -0800 From: Alexei Starovoitov Subject: Re: [PATCH 1/2] bpf: do not use KMALLOC_SHIFT_MAX Message-ID: <20161217002820.GB5359@ast-mbp.thefacebook.com> References: <20161215164722.21586-1-mhocko@kernel.org> <20161215164722.21586-2-mhocko@kernel.org> <20161216180209.GA77597@ast-mbp.thefacebook.com> <20161216220235.GD7645@dhcp22.suse.cz> <20161216232340.GA99159@ast-mbp.thefacebook.com> <20161216233917.GB23392@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20161216233917.GB23392@dhcp22.suse.cz> Sender: owner-linux-mm@kvack.org List-ID: To: Michal Hocko Cc: linux-mm@kvack.org, Cristopher Lameter , Andrew Morton , Alexei Starovoitov , netdev@vger.kernel.org, Daniel Borkmann On Sat, Dec 17, 2016 at 12:39:17AM +0100, Michal Hocko wrote: > On Fri 16-12-16 15:23:42, Alexei Starovoitov wrote: > > On Fri, Dec 16, 2016 at 11:02:35PM +0100, Michal Hocko wrote: > > > On Fri 16-12-16 10:02:10, Alexei Starovoitov wrote: > > > > On Thu, Dec 15, 2016 at 05:47:21PM +0100, Michal Hocko wrote: > > > > > From: Michal Hocko > > > > > > > > > > 01b3f52157ff ("bpf: fix allocation warnings in bpf maps and integer > > > > > overflow") has added checks for the maximum allocateable size. It > > > > > (ab)used KMALLOC_SHIFT_MAX for that purpose. While this is not incorrect > > > > > it is not very clean because we already have KMALLOC_MAX_SIZE for this > > > > > very reason so let's change both checks to use KMALLOC_MAX_SIZE instead. > > > > > > > > > > Cc: Alexei Starovoitov > > > > > Signed-off-by: Michal Hocko > > > > > > > > Nack until the patches 1 and 2 are reversed. > > > > > > I do not insist on ordering. The thing is that it shouldn't matter all > > > that much. Or are you worried about bisectability? > > > > This patch 1 strongly depends on patch 2 ! > > Therefore order matters. > > The patch 1 by itself is broken. > > The commit log is saying > > '(ab)used KMALLOC_SHIFT_MAX for that purpose .. use KMALLOC_MAX_SIZE instead' > > that is also incorrect. We cannot do that until KMALLOC_MAX_SIZE is fixed. > > So please change the order > > Yes, I agree that using KMALLOC_MAX_SIZE could lead to a warning with > the current ordering. Why that matters all that much is less clear to > me. The allocation would simply fail and you would return ENOMEM rather > than E2BIG. Does this really matter? > > Anyway, as I've said, I do not really insist on the current ordering and > the will ask Andrew to reorder them. I am just really wondering about > such a strong pushback about something that barely matters. Or maybe I > am just missing your point and checking KMALLOC_MAX_SIZE without an > update would lead to a wrong behavior, user space breakage, crash or > anything similar. if admin set ulimit for locked memory high enough for the particular user, that non-root user will be able to trigger warn_on_once in __alloc_pages_slowpath which is not acceptable. Also see the comment in hashtab.c if (htab->map.value_size >= (1 << (KMALLOC_SHIFT_MAX - 1)) - MAX_BPF_STACK - sizeof(struct htab_elem)) /* if value_size is bigger, the user space won't be able to * access the elements via bpf syscall. This check also makes * sure that the elem_size doesn't overflow and it's * kmalloc-able later in htab_map_update_elem() */ goto free_htab; > > and fix the commit log to say that KMALLOC_MAX_SIZE > > is actually valid limit now. > > KMALLOC_MAX_SIZE has always been the right limit. It's value has been > incorrect but that is to be fixed now. Using KMALLOC_SHIFT_MAX is simply > abusing an internal constant. So I am not sure what should be fixed in > the changelog. that's exactly my problem with this patch and the commit log. You think it's abusing KMALLOC_SHIFT_MAX whereas it's doing so for reasons stated above. That piece of code cannot use KMALLOC_MAX_SIZE until it's fixed. So commit log should say something like: "now since KMALLOC_MAX_SIZE is fixed and size < KMALLOC_MAX_SIZE condition guarantees warn free allocation in kmalloc(value_size, GFP_USER | __GFP_NOWARN); we can safely use KMALLOC_MAX_SIZE instead of KMALLOC_SHIFT_MAX" -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org