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[209.85.208.173]) by smtp.gmail.com with ESMTPSA id v6sm47826ljk.17.2019.09.04.16.45.12 for (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 04 Sep 2019 16:45:14 -0700 (PDT) Received: by mail-lj1-f173.google.com with SMTP id 7so442005ljw.7 for ; Wed, 04 Sep 2019 16:45:12 -0700 (PDT) X-Received: by 2002:a2e:3a0e:: with SMTP id h14mr127121lja.180.1567640710863; Wed, 04 Sep 2019 16:45:10 -0700 (PDT) MIME-Version: 1.0 References: <20190904201933.10736-1-cyphar@cyphar.com> <20190904201933.10736-11-cyphar@cyphar.com> <20190904214856.vnvom7h5xontvngq@yavin.dot.cyphar.com> <20592.1567636276@warthog.procyon.org.uk> <20190904232911.GN1131@ZenIV.linux.org.uk> In-Reply-To: <20190904232911.GN1131@ZenIV.linux.org.uk> From: Linus Torvalds Date: Wed, 4 Sep 2019 16:44:54 -0700 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [PATCH v12 10/12] namei: aggressively check for nd->root escape on ".." resolution To: Al Viro Cc: David Howells , Aleksa Sarai , Jeff Layton , "J. Bruce Fields" , Arnd Bergmann , Shuah Khan , Shuah Khan , Ingo Molnar , Peter Zijlstra , Christian Brauner , Jann Horn , Kees Cook , Eric Biederman , Andy Lutomirski , Andrew Morton , Alexei Starovoitov , Tycho Andersen , David Drysdale , Chanho Min , Oleg Nesterov , Rasmus Villemoes , Alexander Shishkin , Jiri Olsa , Namhyung Kim , Aleksa Sarai , Linux Containers , alpha , Linux API , linux-arch , Linux ARM , linux-fsdevel , linux-ia64@vger.kernel.org, Linux List Kernel Mailing , "open list:KERNEL SELFTEST FRAMEWORK" , linux-m68k , linux-mips@vger.kernel.org, linux-parisc@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-s390 , Linux-sh list , linux-xtensa@linux-xtensa.org, sparclinux@vger.kernel.org Content-Type: text/plain; charset="UTF-8" Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Sep 4, 2019 at 4:29 PM Al Viro wrote: > > On Wed, Sep 04, 2019 at 03:38:20PM -0700, Linus Torvalds wrote: > > On Wed, Sep 4, 2019 at 3:31 PM David Howells wrote: > > > > > > It ought to be reasonably easy to make them per-sb at least, I think. We > > > don't allow cross-super rename, right? > > > > Right now the sequence count handling very much depends on it being a > > global entity on the reader side, at least. > > > > And while the rename sequence count could (and probably should) be > > per-sb, the same is very much not true of the mount one. > > Huh? That will cost us having to have a per-superblock dentry > hash table; recall that lockless lockup can give false negatives > if something gets moved from chain to chain, and rename_lock is > first and foremost used to catch those and retry. If we split > it on per-superblock basis, we can't have dentries from different > superblocks in the same chain anymore... That's exactly the "very much depends on it being a global entity on the reader side" thing. I'm not convinced that's the _only_ way to handle things. Maybe a combination of (wild handwaving) per-hashqueue sequence count and some clever scheme for pathname handling could work. I've not personally seen a load where the global rename lock has been a problem (very few things really do a lot of renames), but system-wide locks do make me nervous. We have other (and worse) ones. tasklist_lock comes to mind. Linus