From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id F295DC7EE23 for ; Tue, 9 May 2023 21:54:35 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S236098AbjEIVye (ORCPT ); Tue, 9 May 2023 17:54:34 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:46524 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229664AbjEIVyd (ORCPT ); Tue, 9 May 2023 17:54:33 -0400 Received: from out-57.mta1.migadu.com (out-57.mta1.migadu.com [95.215.58.57]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 234D930CB for ; Tue, 9 May 2023 14:54:31 -0700 (PDT) Date: Tue, 9 May 2023 17:54:26 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1683669269; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=1n3YMxW+Zb7xSLAr/eeZIRD4PtBRt8sxu+5EqSQhpSY=; b=DnM8qOmeFjElK/7kmR9lrk8seVkOltc4T81yq9nzxwJ0tWrpzuZjphK0O0rFxUZuREoLjH 4BGrtcWLzxP0pNXP/Rrv20VFYXzZePCsv+ToELAp1vj4npjoYANBuaQ9qfSjDvpQsbPq+u I+IxdbTG3un3iPQvVU9g3Lw60JsigXg= X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Kent Overstreet To: "Darrick J. Wong" Cc: Lorenzo Stoakes , Christoph Hellwig , linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-bcachefs@vger.kernel.org, Kent Overstreet , Andrew Morton , Uladzislau Rezki , linux-mm@kvack.org Subject: Re: [PATCH 07/32] mm: Bring back vmalloc_exec Message-ID: References: <20230509165657.1735798-1-kent.overstreet@linux.dev> <20230509165657.1735798-8-kent.overstreet@linux.dev> <20230509214319.GA858791@frogsfrogsfrogs> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20230509214319.GA858791@frogsfrogsfrogs> X-Migadu-Flow: FLOW_OUT Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, May 09, 2023 at 02:43:19PM -0700, Darrick J. Wong wrote: > On Tue, May 09, 2023 at 02:12:41PM -0700, Lorenzo Stoakes wrote: > > On Tue, May 09, 2023 at 01:46:09PM -0700, Christoph Hellwig wrote: > > > On Tue, May 09, 2023 at 12:56:32PM -0400, Kent Overstreet wrote: > > > > From: Kent Overstreet > > > > > > > > This is needed for bcachefs, which dynamically generates per-btree node > > > > unpack functions. > > > > > > No, we will never add back a way for random code allocating executable > > > memory in kernel space. > > > > Yeah I think I glossed over this aspect a bit as it looks ostensibly like simply > > reinstating a helper function because the code is now used in more than one > > place (at lsf/mm so a little distracted :) > > > > But it being exported is a problem. Perhaps there's another way of acheving the > > same aim without having to do so? > > I already trolled Kent with this on IRC, but for the parts of bcachefs > that want better assembly code than whatever gcc generates from the C > source, could you compile code to BPF and then let the BPF JIT engines > turn that into machine code for you? It's an intriguing idea, but it'd be a _lot_ of work and this is old code that's never had a single bug - I'm not in a hurry to rewrite it. And there would still be the issue that we've still got lots of little unpack functions that go with other tables; we can't just burn a full page per unpack function, that would waste way too much memory, and if we put them together then we're stuck writing a whole nother allocator - nope, and then we're also mucking with the memory layout of the data structures used in the very hottest paths in the filesystem - I'm very wary of introducing performance regressions there. I think it'd be much more practical to find some way of making vmalloc_exec() more palatable. What are the exact concerns?