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 X-Spam-Level: X-Spam-Status: No, score=-3.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id DE422C433E0 for ; Tue, 23 Mar 2021 23:24:07 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 8A780619CC for ; Tue, 23 Mar 2021 23:24:07 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S234240AbhCWXXd (ORCPT ); Tue, 23 Mar 2021 19:23:33 -0400 Received: from zeniv-ca.linux.org.uk ([142.44.231.140]:36376 "EHLO zeniv-ca.linux.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229574AbhCWXX2 (ORCPT ); Tue, 23 Mar 2021 19:23:28 -0400 Received: from viro by zeniv-ca.linux.org.uk with local (Exim 4.94 #2 (Red Hat Linux)) id 1lOqIe-008gcD-SL; Tue, 23 Mar 2021 23:19:01 +0000 Date: Tue, 23 Mar 2021 23:19:00 +0000 From: Al Viro To: =?iso-8859-1?Q?Andr=E9?= Almeida Cc: Hugh Dickins , Andrew Morton , krisman@collabora.com, smcv@collabora.com, kernel@collabora.com, linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, Daniel Rosenberg Subject: Re: [RFC PATCH 2/4] mm: shmem: Support case-insensitive file name lookups Message-ID: References: <20210323195941.69720-1-andrealmeid@collabora.com> <20210323195941.69720-3-andrealmeid@collabora.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20210323195941.69720-3-andrealmeid@collabora.com> Sender: Al Viro Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Mar 23, 2021 at 04:59:39PM -0300, André Almeida wrote: > * dcache handling: > > For a +F directory, tmpfs only stores the first equivalent name dentry > used in the dcache. This is done to prevent unintentional duplication of > dentries in the dcache, while also allowing the VFS code to quickly find > the right entry in the cache despite which equivalent string was used in > a previous lookup, without having to resort to ->lookup(). > > d_hash() of casefolded directories is implemented as the hash of the > casefolded string, such that we always have a well-known bucket for all > the equivalencies of the same string. d_compare() uses the > utf8_strncasecmp() infrastructure, which handles the comparison of > equivalent, same case, names as well. > > For now, negative lookups are not inserted in the dcache, since they > would need to be invalidated anyway, because we can't trust missing file > dentries. This is bad for performance but requires some leveraging of > the VFS layer to fix. We can live without that for now, and so does > everyone else. "For now"? Not a single practical suggestion has ever materialized. Pardon me, but by now I'm very sceptical about the odds of that ever changing. And no, I don't have any suggestions either. > The lookup() path at tmpfs creates negatives dentries, that are later > instantiated if the file is created. In that way, all files in tmpfs > have a dentry given that the filesystem exists exclusively in memory. > As explained above, we don't have negative dentries for casefold files, > so dentries are created at lookup() iff files aren't casefolded. Else, > the dentry is created just before being instantiated at create path. > At the remove path, dentries are invalidated for casefolded files. Umm... What happens to those assertions if previously sane directory gets case-buggered? You've got an ioctl for doing just that... Incidentally, that ioctl is obviously racy - result of that simple_empty() might have nothing to do with reality before it is returned to caller. And while we are at it, simple_empty() doesn't check a damn thing about negative dentries in there...