From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from cn.fujitsu.com ([59.151.112.132]:65463 "EHLO heian.cn.fujitsu.com" rhost-flags-OK-FAIL-OK-FAIL) by vger.kernel.org with ESMTP id S1751335AbaDOBQS convert rfc822-to-8bit (ORCPT ); Mon, 14 Apr 2014 21:16:18 -0400 Message-ID: <534C888D.1080509@cn.fujitsu.com> Date: Tue, 15 Apr 2014 09:17:01 +0800 From: Qu Wenruo MIME-Version: 1.0 To: , Subject: Re: [PATCH] btrfs-progs: Force 'btrfs dev scan' not using old libblkid cache References: <1397115021-30426-1-git-send-email-quwenruo@cn.fujitsu.com> <20140414162919.GO29256@twin.jikos.cz> In-Reply-To: <20140414162919.GO29256@twin.jikos.cz> Content-Type: text/plain; charset="UTF-8"; format=flowed Sender: linux-btrfs-owner@vger.kernel.org List-ID: -------- Original Message -------- Subject: Re: [PATCH] btrfs-progs: Force 'btrfs dev scan' not using old libblkid cache From: David Sterba To: Qu Wenruo Date: 2014年04月15日 00:29 > On Thu, Apr 10, 2014 at 03:30:21PM +0800, Qu Wenruo wrote: >> 'btrfs scan' uses libblkid to scan devices by default, and libblkid uses >> cache to reduce the probe. >> >> But if operations below is done in less than 2 seconds(BLKID_PROBE_MIN), >> 'btrfs scan' will still use the uncorrect cache and scan on the deleted device. >> 0. /dev/sda[1-4] mounted on /mnt using single data/metadata >> 1. btrfs dev scan >> 2. btrfs dev del /dev/sda3 /mnt >> 3. btrfs dev scan >> >> Since the cache made by step 1 is still validated, step 3 will use the >> cache and consider /dev/sda3 as a btrfs filesystem and try to scan it. >> But the superblock(at least the first one) is wiped and failed to scan, >> a error message, which can be avoided and is unneeded, is output. >> >> This patch will force scan_for_btrfs() not to use cache to avoid the >> problem. >> --- a/utils.c >> +++ b/utils.c >> @@ -2057,7 +2057,8 @@ int btrfs_scan_lblkid(int update_kernel) >> blkid_cache cache = NULL; >> char path[PATH_MAX]; >> >> - if (blkid_get_cache(&cache, 0) < 0) { >> + /* No to use libblkid cache to avoid old data */ >> + if (blkid_get_cache(&cache, "/dev/null") < 0) { > This effectively avoid the blkid cache for all devices but the point of > blkid was to use it so repeated probes are not done. > > I think it's not right to skip the whole cache because one entry may be > stale, more that we know which one and when. I totally agree with that. > > We should rather explicitly invalidate the removed device after delete, > I don't what's the right way to do that. Maybe blkid_do_probe() or > blkid_gc_cache() But the problem is, libblkid does not provide such API to invalidate one entry of cache. blkid_gc_cache() will only remove non-exsist device cache, and blkid_do_probe() seems not using/update the cache, so I used "/dev/null" as cache to avoid the cache completly. I'll try to dig the libblkid codes deeper to find a method to invalidate an entry. Thanks, Qu.