From mboxrd@z Thu Jan 1 00:00:00 1970 From: Eric Biggers Subject: Re: [PATCH] fscrypt: cache decrypted symlink target in ->i_link Date: Tue, 9 Apr 2019 21:16:23 -0700 Message-ID: <20190410041623.GD7140@sol.localdomain> References: <20190409233544.156665-1-ebiggers@kernel.org> <20190410003346.GT2217@ZenIV.linux.org.uk> <20190410004553.GA2454@sol.localdomain> <20190410010425.GU2217@ZenIV.linux.org.uk> <20190410012247.GB2454@sol.localdomain> <20190410013934.GV2217@ZenIV.linux.org.uk> <20190410025808.GA7140@sol.localdomain> <20190410034414.GW2217@ZenIV.linux.org.uk> <20190410040413.GC7140@sol.localdomain> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Received: from [172.30.20.202] (helo=mx.sourceforge.net) by sfs-ml-1.v29.lw.sourceforge.com with esmtps (TLSv1.2:ECDHE-RSA-AES256-GCM-SHA384:256) (Exim 4.90_1) (envelope-from ) id 1hE4f3-0008V6-TF for linux-f2fs-devel@lists.sourceforge.net; Wed, 10 Apr 2019 04:16:33 +0000 Received: from mail.kernel.org ([198.145.29.99]) by sfi-mx-1.v28.lw.sourceforge.com with esmtps (TLSv1.2:ECDHE-RSA-AES256-GCM-SHA384:256) (Exim 4.90_1) id 1hE4f2-00AzUw-LI for linux-f2fs-devel@lists.sourceforge.net; Wed, 10 Apr 2019 04:16:33 +0000 Content-Disposition: inline In-Reply-To: <20190410040413.GC7140@sol.localdomain> List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: linux-f2fs-devel-bounces@lists.sourceforge.net To: Al Viro Cc: linux-fsdevel@vger.kernel.org, linux-fscrypt@vger.kernel.org, linux-ext4@vger.kernel.org, linux-f2fs-devel@lists.sourceforge.net On Tue, Apr 09, 2019 at 09:04:15PM -0700, Eric Biggers wrote: > On Wed, Apr 10, 2019 at 04:44:14AM +0100, Al Viro wrote: > > On Tue, Apr 09, 2019 at 07:58:08PM -0700, Eric Biggers wrote: > > > > > It could check a flag IOP_GET_LINK in ->i_opflags instead, so it would be the > > > same number of checks. See patch below. > > > > With that patch ->i_link is completely unused if ->get_link() is non-NULL, > > so you get a method call on each traversal... > > > > .get_link would be left NULL in all inode_operations that currently use > simple_get_link, then simple_get_link() would be removed. My example patch just > changed it in ext4 as an example. > > > > Benefits are that we get code that isn't actively misleading (via > > > simple_get_link() existing but actually never being called), and filesystems can > > > cache a symlink target in ->i_link if it becomes available later, i.e. if it's > > > not immediately available at iget() time. Otherwise a filesystem-private field > > > has to be used instead. (For fscrypt, I'd probably use fscrypt_info::ci_link.) > > > > What's to stop you from doing just that right now? You'd need to take > > care with barriers, but you'd need that anyway... As soon as ->i_link is set > > you'll get no more ->get_link() on that sucker, using the cached value > > from that point on. IDGI... > > 1.) The VFS won't know to drop of RCU-walk mode, so waiting an RCU grace period > before freeing the symlink target becomes mandatory. (Which I'd like to do > for fscrypt anyway, but doing it sanely appears to require implementing > .destroy_inode() for ext4, f2fs, and ubifs. I hoped I could do non-RCU mode > as a simpler first step.) > > 2.) The VFS won't know to use a read memory barrier when loading i_link. > The VFS could issue one unconditionally, but it would be unnecessary for > regular fast symlinks. > > - Eric Okay, actually all three filesystems have .destroy_inode() anyway. Not sure how I missed that. So it should be possible to free the decrypted symlink target from {ext4,f2fs,ubifs}_i_callback(). - Eric