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Date: Mon, 8 Nov 2021 10:11:05 +0800 From: yebin <yebin10@...wei.com> To: <tytso@....edu>, <adilger.kernel@...ger.ca>, <linux-ext4@...r.kernel.org> CC: <linux-kernel@...r.kernel.org>, <jack@...e.cz> Subject: Re: [PATCH -next v5 0/3] Fix some issues about mmp On 2021/10/20 11:17, Ye Bin wrote: > I test mmp function as follow steps: > 1. Inject delay 5s in ext4_multi_mount_protect function after > "skip:" label. > 2. Share HostA block device(sda) with HostB(nbd0) by NBD. > 3. Enable mmp feature when mkfs.ext4 sda. > 4. Mount sda and nbd0 at the same time. > > I found kmmpd never trigger detect multi-mount. Reason is as follow: > 1. Kmmpd init seq with 0, if two host have same nodename, will lead to > detect confliction very slow, even never detect confliction. > 2. When detect confliction in kmmpd, we get 'check_bh' is same with 'bh'. > so we compare data with itself. > 3. We only trigger detect when ”diff > mmp_check_interval * HZ“, > 'mmp_check_interval' is double of 'mmp_update_interval', 'diff' is > about 'mmp_update_interval'. So 'diff' is little than 'mmp_check_interval * HZ' > normaly. As Jan Kara explain as follows: > "I think the check is there only for the case where write_mmp_block() + > sleep took longer than mmp_check_interval. I agree that should rarely > happen but on a really busy system it is possible and in that case we would > miss updating mmp block for too long and so another node could have started > using the filesystem. " > > v1->v2: > Fix 'last_check_time' not initialized before checking. > > v2->v3: > 1. drop commit "ext4: introduce last_check_time record previous check time" > As Ted explain as follows: > "I'd like Andreas to comment here. My understanding is that MMP > originally intended as a safety mechanism which would be used as part > of a primary/backup high availability system, but not as the *primary* > system where you might try to have two servers simultaneously try to > mount the file system and use MMP as the "election" mechanism to > decide which server is going to be the primary system, and which would > be the backup system. > > The cost of being able to handle this particular race is it would slow > down the mounts of cleanly unmounted systems. > > There *are* better systems to implement leader elections[1] than using > MMP. Most of these more efficient leader elections assume that you > have a working IP network, and so if you have a separate storage > network (including a shared SCSI bus) from your standard IP network, > then MMP is a useful failsafe in the face of a network partition of > your IP network. The question is whether MMP should be useful for > more than that. And if it isn't, then we should probably document > what MMP is and isn't good for, and give advice in the form of an > application note for how MMP should be used in the context of a larger > system." > 2. drop commit "ext4: fix possible store wrong check interval value in disk when umount" > 3. simplify read_mmp_block fucntion to avoid UAF > > v3->v4: > 1. drop commit "ext4: init 'seq' with the value which set in 'ext4_multi_mount_protect'" > 2. merge "ext4: get buffer head before read_mmp_block" to > "ext4: simplify read_mmp_block fucntion" > 3. rename "ext4: avoid to re-read mmp check data get from page cache" to > "ext4: remove useless bh_check variable" > 4. reorder "ext4: remove useless bh_check variable" and > "ext4: simplify read_mmp_block fucntion" > > v4->v5: > 1. Fix follow warning: >>> fs/ext4/mmp.c:124:15: warning: variable 'mmp_block' set but not used [-Wunused-but-set-variable] > ext4_fsblk_t mmp_block; > 2. Fix incorrect judgement in 'ext4_multi_mount_protect'. > > Ye Bin (3): > ext4: compare to local seq and nodename when check conflict > ext4: remove useless bh_check variable > ext4: simplify read_mmp_block fucntion > > fs/ext4/ext4.h | 5 +++- > fs/ext4/mmp.c | 66 +++++++++++++++++++++----------------------------- > 2 files changed, 31 insertions(+), 40 deletions(-) > ping...
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