Re: [PATCH -next v2 1/6] ext4: init seq with random value in kmmpd
From: Jan Kara <jack@suse.cz>
Date: 2021-10-07 12:26:11
Also in:
lkml
On Sat 11-09-21 17:00:54, Ye Bin wrote:
If two host has the same nodename, and seq start from 0, May cause the detection mechanism to fail. So init seq with random value to accelerate conflict detection. Signed-off-by: Ye Bin <redacted>
Thanks for the patch. I agree the code in kmmpd looks suspicious. The sequence number is initialized to a random number in ext4_multi_mount_protect() which is called before kmmpd is started. I think kmmpd() should initialize its 'seq' to a number fetched from the mmp block, instead of 0 as is currently in the code. I don't think generating a new random number as you do is really needed... Honza
quoted hunk ↗ jump to hunk
--- fs/ext4/mmp.c | 32 ++++++++++++++++---------------- 1 file changed, 16 insertions(+), 16 deletions(-)diff --git a/fs/ext4/mmp.c b/fs/ext4/mmp.c index cebea4270817..12af6dc8457b 100644 --- a/fs/ext4/mmp.c +++ b/fs/ext4/mmp.c@@ -122,6 +122,21 @@ void __dump_mmp_msg(struct super_block *sb, struct mmp_struct *mmp, (int)sizeof(mmp->mmp_bdevname), mmp->mmp_bdevname); } +/* + * Get a random new sequence number but make sure it is not greater than + * EXT4_MMP_SEQ_MAX. + */ +static unsigned int mmp_new_seq(void) +{ + u32 new_seq; + + do { + new_seq = prandom_u32(); + } while (new_seq > EXT4_MMP_SEQ_MAX); + + return new_seq; +} + /* * kmmpd will update the MMP sequence every s_mmp_update_interval seconds */@@ -132,7 +147,7 @@ static int kmmpd(void *data) struct buffer_head *bh = EXT4_SB(sb)->s_mmp_bh; struct mmp_struct *mmp; ext4_fsblk_t mmp_block; - u32 seq = 0; + u32 seq = mmp_new_seq(); unsigned long failed_writes = 0; int mmp_update_interval = le16_to_cpu(es->s_mmp_update_interval); unsigned mmp_check_interval;@@ -258,21 +273,6 @@ void ext4_stop_mmpd(struct ext4_sb_info *sbi) } } -/* - * Get a random new sequence number but make sure it is not greater than - * EXT4_MMP_SEQ_MAX. - */ -static unsigned int mmp_new_seq(void) -{ - u32 new_seq; - - do { - new_seq = prandom_u32(); - } while (new_seq > EXT4_MMP_SEQ_MAX); - - return new_seq; -} - /* * Protect the filesystem from being mounted more than once. */-- 2.31.1
-- Jan Kara [off-list ref] SUSE Labs, CR