On Fri 03-02-12 21:03:20, Eric Sandeen wrote: > On 1/20/12 2:34 PM, Jan Kara wrote: > > vfs_check_frozen() tests are racy since the filesystem can be frozen just after > > the test is performed. Thus in write paths we can end up marking some pages or > > inodes dirty even though filesystem is already frozen. This creates problems > > with flusher thread hanging on frozen filesystem. > > > > Another problem is that exclusion between ->page_mkwrite() and filesystem > > freezing has been handled by setting page dirty and then verifying s_frozen. > > This guaranteed that either the freezing code sees the faulted page, writes it, > > and writeprotects it again or we see s_frozen set and bail out of page fault. > > This works to protect from page being marked writeable while filesystem > > freezing is running but has an unpleasant artefact of leaving dirty (although > > unmodified and writeprotected) pages on frozen filesystem resulting in similar > > problems with flusher thread as the first problem. > > > > This patch aims at providing exclusion between write paths and filesystem > > freezing. We implement a writer-freeze read-write semaphores in the superblock > > for each freezing level (currently there are two - SB_FREEZE_WRITE for data and > > SB_FREEZE_TRANS for metadata). Write paths which should block freezing on given > > level (e.g. ->block_page_mkwrite(), ->aio_write() for SB_FREEZE_WRITE level; > > transaction lifetime for SB_FREEZE_TRANS level) hold reader side of the > > semaphore. Code freezing the filesystem to a given level takes the writer side. > > > > Only that we don't really want to bounce cachelines of the semaphore between > > CPUs for each write happening. So we implement the reader side of the semaphore > > as a per-cpu counter and the writer side is implemented using s_frozen > > superblock field. > > > > Acked-by: "Theodore Ts'o" <tytso@xxxxxxx> > > Signed-off-by: Jan Kara <jack@xxxxxxx> > > ... > > > @@ -135,6 +157,11 @@ static struct super_block *alloc_super(struct file_system_type *type) > > #else > > INIT_LIST_HEAD(&s->s_files); > > #endif > > + if (init_sb_writers(s, SB_FREEZE_WRITE, "sb_writers_write")) > > + goto err_out; > > + if (init_sb_writers(s, SB_FREEZE_TRANS, "sb_writers_trans")) > > + goto err_out; > > + > > s->s_bdi = &default_backing_dev_info; > > INIT_LIST_HEAD(&s->s_instances); > > INIT_HLIST_BL_HEAD(&s->s_anon); > > @@ -186,6 +213,17 @@ static struct super_block *alloc_super(struct file_system_type *type) > > } > > out: > > return s; > > +err_out: > > + security_sb_free(s); > > +#ifdef CONFIG_SMP > > + if (s->s_files) > > + free_percpu(s->s_files); > > +#endif > > + destroy_sb_writers(s, SB_FREEZE_WRITE); > > + destroy_sb_writers(s, SB_FREEZE_TRANS); > > You probably ran into this already but the writer percpu vars need > to be torn down in destroy_super() as well. Actually not. Thanks for spotting this. 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
