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 3BDB2C433EF for ; Wed, 24 Nov 2021 03:17:10 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S240811AbhKXDUR (ORCPT ); Tue, 23 Nov 2021 22:20:17 -0500 Received: from smtp-out1.suse.de ([195.135.220.28]:44772 "EHLO smtp-out1.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S240802AbhKXDUO (ORCPT ); Tue, 23 Nov 2021 22:20:14 -0500 Received: from imap2.suse-dmz.suse.de (imap2.suse-dmz.suse.de [192.168.254.74]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (P-521) server-digest SHA512) (No client certificate requested) by smtp-out1.suse.de (Postfix) with ESMTPS id 3E32F2195A; Wed, 24 Nov 2021 03:17:04 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.de; s=susede2_rsa; t=1637723824; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=vDPWm0p/mlhakN1e7xXVNW/moviPVK82GwF6kdGOEy0=; b=cZhmrdPKXdn+H/oudJqT/Oq1j2f75Hf43o5wUhZQpWL7W33LNbBHnGZbHFBb+ylJ35Ph4O mmln8qZQF1JySsR/L6TDKHGmwbOzU5szZ2iBYO/A+szLLJw7pVH8sEPMrAMklTRcjUJdFz WKQ0lxuSx/XGHpVIMVW4dwX1XtZzLRI= DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=suse.de; s=susede2_ed25519; t=1637723824; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=vDPWm0p/mlhakN1e7xXVNW/moviPVK82GwF6kdGOEy0=; b=YALzMeWScEmPJ5lOZdpgGHSa67p/G3HP9wdC2hxeW/q5VhiCJtnvkRSCDqv4PEBo6O+4qh lKCnrt1wMtZXy1DQ== Received: from imap2.suse-dmz.suse.de (imap2.suse-dmz.suse.de [192.168.254.74]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (P-521) server-digest SHA512) (No client certificate requested) by imap2.suse-dmz.suse.de (Postfix) with ESMTPS id DC5D913EB8; Wed, 24 Nov 2021 03:17:00 +0000 (UTC) Received: from dovecot-director2.suse.de ([192.168.254.65]) by imap2.suse-dmz.suse.de with ESMTPSA id z1hAJayunWGNRQAAMHmgww (envelope-from ); Wed, 24 Nov 2021 03:17:00 +0000 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit MIME-Version: 1.0 From: "NeilBrown" To: "Andrew Morton" Cc: "Uladzislau Rezki" , "Michal Hocko" , "Dave Chinner" , "Christoph Hellwig" , linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, "LKML" , "Ilya Dryomov" , "Jeff Layton" , "Michal Hocko" Subject: Re: [PATCH v2 2/4] mm/vmalloc: add support for __GFP_NOFAIL In-reply-to: <20211123170238.f0f780ddb800f1316397f97c@linux-foundation.org> References: <20211122153233.9924-1-mhocko@kernel.org>, <20211122153233.9924-3-mhocko@kernel.org>, , <20211123170238.f0f780ddb800f1316397f97c@linux-foundation.org> Date: Wed, 24 Nov 2021 14:16:56 +1100 Message-id: <163772381628.1891.9102201563412921921@noble.neil.brown.name> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, 24 Nov 2021, Andrew Morton wrote: > > I added GFP_NOFAIL back in the mesozoic era because quite a lot of > sites were doing open-coded try-forever loops. I thought "hey, they > shouldn't be doing that in the first place, but let's at least > centralize the concept to reduce code size, code duplication and so > it's something we can now grep for". But longer term, all GFP_NOFAIL > sites should be reworked to no longer need to do the retry-forever > thing. In retrospect, this bright idea of mine seems to have added > license for more sites to use retry-forever. Sigh. One of the costs of not having GFP_NOFAIL (or similar) is lots of untested failure-path code. When does an allocation that is allowed to retry and reclaim ever fail anyway? I think the answer is "only when it has been killed by the oom killer". That of course cannot happen to kernel threads, so maybe kernel threads should never need GFP_NOFAIL?? I'm not sure the above is 100%, but I do think that is the sort of semantic that we want. We want to know what kmalloc failure *means*. We also need well defined and documented strategies to handle it. mempools are one such strategy, but not always suitable. preallocating can also be useful but can be clumsy to implement. Maybe we should support a process preallocating a bunch of pages which can only be used by the process - and are auto-freed when the process returns to user-space. That might allow the "error paths" to be simple and early, and subsequent allocations that were GFP_USEPREALLOC would be safe. i.e. we need a plan for how to rework all those no-fail call-sites. NeilBrown