On Tue 10-01-12 21:38:29, Surbhi Palande wrote: > On second thoughts, I fail to see why there is still a race window > after this patch. > > Here are the reasons why i fail to see how the data can be dirtied > when all the operations involve a journal: > > ---------- > So here is the problem that we see > CPU1 CPU2 > Task1 (write operation) Task2 > --------------------------------------------------------------------------------------- > t1 ext4_journal_start() > t2 ext4_journal_start_sb() > t3 vfs_check_frozen sb->frozen=SB_FREEZE_WRITE > t4 jbd2_journal_start() /* hence forth all processes calling > vfs_check_frozen will wait */ Note that we call vfs_check_frozen(sb, SB_FREEZE_TRANS) in ext4_journal_start_sb(). Thus we start blocking only when s_frozen == SB_FREEZE_TRANS and we just ignore s_frozen == SB_FREEZE_WRITE. > Now, our aim is to stop Task1 from dirtying the page cache ie in > starting this transaction. However if it is successful in starting > this transaction, then we want to make sure that this transaction is > flushed out. > Correct? Not quite. Flushing a journal will flush dirty metadata but we will still have dirty pages (dirty data is not part of any transaction). So in the scenarion I describe in http://marc.info/?l=linux-fsdevel&m=132585911925796&w=2 all metadata changes will be flushed inside ->freeze_fs (at least for journalling filesystems) but pages will be left dirty. Is it clearer now? But your comment makes me realize that the situation is simpler than I thought by the fact that we only have to protect paths that create dirty data as dirty metadata can be handled by flushing a journal. And there are only a few places creating dirty data. So a reasonably clean solution shouldn't be that complicated after all. I'll tweak my patch and try it in a moment. Honza -- Jan Kara <jack@xxxxxxx> SUSE Labs, CR -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
