On Thu 05-05-11 09:06:29, Surbhi Palande wrote: > On 05/05/2011 01:48 AM, Jan Kara wrote: > >On Thu 05-05-11 00:34:51, Surbhi Palande wrote: > >>On 05/04/2011 10:19 PM, Jan Kara wrote: > >>>On Wed 04-05-11 15:09:37, Surbhi Palande wrote: > >>>>On 05/03/2011 06:19 PM, Jan Kara wrote: > >>>>>On Tue 03-05-11 14:01:50, Surbhi Palande wrote: > >>>>>>What happens in the case as follows: > >>>>>> > >>>>>>Task 1: Mmapped writes > >>>>>>t1)ext4_page_mkwrite() > >>>>>> t2) ext4_write_begin() (FS is thawed so we proceed) > >>>>>> t3) ext4_write_end() (journal is stopped now) > >>>>>>-----Pre-empted----- > >>>>>> > >>>>>> > >>>>>>Task 2: Freeze Task > >>>>>>t4) freezes the super block... > >>>>>>...(continues).... > >>>>>>tn) the page cache is clean and the F.S is frozen. Freeze has > >>>>>>completed execution. > >>>>>> > >>>>>>Task 1: Mmapped writes > >>>>>>tn+1) ext4_page_mkwrite() returns 0. > >>>>>>tn+2) __do_fault() gets control, code gets executed. > >>>>>>tn+3) _do_fault() marks the page dirty if the intent is to write to > >>>>>>a file based page which faulted. > >>>>>> > >>>>>>So you end up dirtying the page cache when the F.S is frozen? No? > >>>>> You are right ext4_page_mkrite() as currently implemented has problems. > >>>>>You have to return the page locked (and check for frozen fs with page lock > >>>>>held) to avoid races. > >>>>> > >>>>>If you check for frozen fs with page lock held, you are guaranteed that > >>>>>freezing code must wait for the page to get unlocked before proceeding. And > >>>>>before the page is unlocked, it is marked dirty by the pagefault code which > >>>>>makes freezing code write the page and writeprotect it again. So everything > >>>>>will be safe. > >>>>For the locked page to be a part of the freeze initiated sync, > >>>>should its owner inode not be dirtied? The page fault handler > >>>>dirties the page, but who ensures that the inode is dirtied at this > >>>>point? > >>Well, I mean it as follows: > >> > >>Doesn't the writeback code (invoked via sync_filesystem(sb)) write > >>all the dirty pages of all the _dirty_ inodes of a superblock? > >> > >>So in the window from the point where ext4_page_mkwrite returns to > >>__do_fault() _till_ you mark the inode dirty (in > >>__mark_inode_dirty()), you can have a race with freeze i.e if freeze > >>happens meanwhile, then the sync initiated by freeze will not > >>consider this locked page as the owner inode is _clean_ (or not > >>dirtied yet) at that point? > > Ah, I see. That's actually a good point! Thanks for persistence. So we > >should also dirty the page before checking for frozen fs. > > Should we not also dirty the inode? IMHO, marking an inode will be > racy as well! Marking the page dirty marks the inode dirty as well as I've explained in my previous emails. So I'm missing what you are concerned about... 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
