On Thu, Feb 22, 2018 at 08:49:30AM +0200, Nikolay Borisov wrote: > > > On 22.02.2018 00:38, Liu Bo wrote: > > On Wed, Feb 21, 2018 at 07:05:13PM +0000, Filipe Manana wrote: > >> On Wed, Feb 21, 2018 at 6:28 PM, Liu Bo <bo.li....@oracle.com> wrote: > >>> On Wed, Feb 21, 2018 at 02:42:08PM +0000, Filipe Manana wrote: > >>>> On Wed, Feb 21, 2018 at 2:15 PM, Nikolay Borisov <nbori...@suse.com> > >>>> wrote: > >>>>> > >>>>> > >>>>> On 21.02.2018 15:51, Filipe Manana wrote: > >>>>>> On Wed, Feb 21, 2018 at 11:41 AM, Nikolay Borisov <nbori...@suse.com> > >>>>>> wrote: > >>>>>>> Currently the DIO read cases uses a botched idea from ext4 to ensure > >>>>>>> that DIO reads don't race with truncate. The idea is that if we have a > >>>>>>> pending truncate we set BTRFS_INODE_READDIO_NEED_LOCK which in turn > >>>>>>> forces the dio read case to fallback to inode_locking to prevent > >>>>>>> read/truncate races. Unfortunately this is subtly broken for at least > >>>>>>> 2 reasons: > >>>>>>> > >>>>>>> 1. inode_dio_begin in btrfs_direct_IO is called outside of inode_lock > >>>>>>> (for the read case). This means that there is no ordering guarantee > >>>>>>> between the invocation of inode_dio_wait and the increment of > >>>>>>> i_dio_count in btrfs_direct_IO in the tread case. > >>>>>> > >>>>>> Also, looking at this changelog, the diff and the code, why is it a > >>>>>> problem not calling inode_dio_begin without the inode lock in the dio > >>>>>> read path? > >>>>>> The truncate path calls inode_dio_wait after setting the bit > >>>>>> BTRFS_INODE_READDIO_NEED_LOCK and before clearing it. > >>>>>> Assuming the functions to set and clear that bit are correct, I don't > >>>>>> see what problem this brings. > >>>>> > >>>>> Assume you have a truncate and a dio READ in parallel. So the following > >>>>> execution is possible: > >>>>> > >>>>> T1: T2: > >>>>> btrfs_setattr > >>>>> set_bit(BTRFS_INODE_READDIO_NEED_LOCK) > >>>>> inode_dio_wait (reads i_dio_count) > >>>>> btrfs_direct_IO > >>>>> clear_bit(BTRFS_INODE_READDIO_NEED_LOCK) > >>>>> inode_dio_begin (inc's i_dio_count) > >>>>> > >>>>> Since we have no ordering between beginning a dio and waiting for it > >>>>> then > >>>>> truncate can assume there isn't any pending dio. At the same time > >>>>> btrfs_direct_IO will increment i_dio_count but won't see > >>>>> BTRFS_INODE_READDIO_NEED_LOCK > >>>>> ever being set and so will proceed servicing the read. > >>>> > >>>> So what you are saying, is that you are concerned with a dio read > >>>> starting after clearing the BTRFS_INODE_READDIO_NEED_LOCK. > >>>> I don't think that is a problem, because the truncate path has already > >>>> started a transaction before, which means blocks/extents deallocated > >>>> by the truncation can not be reused and allocated to other inodes or > >>>> the same inode (only after the transaction is committed). > >>>> > >>>> And considering that, commit 2e60a51e62185cce48758e596ae7cb2da673b58f > >>>> ("Btrfs: serialize unlocked dio reads with truncate"), which > >>>> introduced all this protection logic, is completely bogus. Looking at > >>>> its changelog: > >>>> > >>>> Btrfs: serialize unlocked dio reads with truncate > >>>> > >>>> Currently, we can do unlocked dio reads, but the following race > >>>> is possible: > >>>> > >>>> dio_read_task truncate_task > >>>> ->btrfs_setattr() > >>>> ->btrfs_direct_IO > >>>> ->__blockdev_direct_IO > >>>> ->btrfs_get_block > >>>> ->btrfs_truncate() > >>>> #alloc truncated blocks > >>>> #to other inode > >>>> ->submit_io() > >>>> #INFORMATION LEAK > >>>> > >>>> In order to avoid this problem, we must serialize unlocked dio reads > >>>> with > >>>> truncate. There are two approaches: > >>>> - use extent lock to protect the extent that we truncate > >>>> - use inode_dio_wait() to make sure the truncating task will wait for > >>>> the read DIO. > >>>> > >>>> If we use the 1st one, we will meet the endless truncation problem > >>>> due to > >>>> the nonlocked read DIO after we implement the nonlocked write DIO. > >>>> It is > >>>> because we still need invoke inode_dio_wait() avoid the race between > >>>> write > >>>> DIO and truncation. By that time, we have to introduce > >>>> > >>>> btrfs_inode_{block, resume}_nolock_dio() > >>>> > >>>> again. That is we have to implement this patch again, so I choose > >>>> the 2nd > >>>> way to fix the problem. > >>>> > >>>> It's concerned with extents deallocated during the truncate operation > >>>> being leaked through concurrent reads from other inodes that got that > >>>> those extents allocated to them in the meanwhile (and the dio reads > >>>> complete after the re-allocations and before the extents get written > >>>> with new data) - but that can't happen because truncate is holding a > >>>> transaction open. Further all that code that it introduced, can only > >>>> prevent concurrent reads from the same inode, not from other inodes. > >>>> So I think that commit does absolutely nothing and we should revert > >>>> it. > >>>> > >>> > >>> Well...make sense, but still dio read can read stale data past isize > >>> if this inode_dio_wait() is removed. > >> > >> Yes, the inode_dio_wait() would remain, to prevent a dio read from > >> submitting the bio before truncate drops an extent and the bio finish > >> after the transaction from truncate commits (at which point the pinned > >> extents could have been allocated for someone else and be partially, > >> fully rewritten or discarded). All that stuff with the bit > >> BTRFS_INODE_READDIO_NEED_LOCK would go away. > > > > The commit description doesn't point it out but the code has the > > necessary comment, BTRFS_INODE_READDIO_NEED_LOCK is used to prevent a > > livelock if there are enough agreesive dio readers rushing in. > > > >> If the transaction commits after the dio read, then everything is fine > >> as for the cases where it reads data past the isize set by truncate, > >> that data is preserved since the dropped extents are pinned, there's > >> no chance for the application to read partial contents or garbage from > >> the dropped extents. > > > > Not even that far, isize is truncated before calling inode_dio_wait() > > and a memory barrier is set to ensure the correct order, so dio read > > would simply return if it's reading past isize. > > Please, describe concretely which meory barriers pairs with chich in > order to ensure proper visibility. Because I'm of the opinion there are > insufficient memoru barriers to ensure setting READDIO_LOCK is properly > visible in btrfs_direct_IO. Since the latter has no barriers whatsoever.
smp_mb() is supposed to be paired, so there is one missing, I agree. Thanks, -liubo -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html