On Thu, May 24, 2018 at 11:49:04AM +0100, Filipe Manana wrote:
> On Wed, May 23, 2018 at 4:58 PM, Josef Bacik <josef@xxxxxxxxxxxxxx> wrote:
> > From: Josef Bacik <jbacik@xxxxxx>
> >
> > There's a priority inversion that exists currently with btrfs fsync. In
> > some cases we will collect outstanding ordered extents onto a list and
> > only wait on them at the very last second. However this "very last
> > second" falls inside of a transaction handle, so if we are in a lower
> > priority cgroup we can end up holding the transaction open for longer
> > than needed, so if a high priority cgroup is also trying to fsync()
> > it'll see latency.
> >
> > Signed-off-by: Josef Bacik <jbacik@xxxxxx>
> > ---
> > fs/btrfs/file.c | 56 ++++----------------------------------------------------
> > 1 file changed, 4 insertions(+), 52 deletions(-)
> >
> > diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
> > index 5772f0cbedef..2b1c36612384 100644
> > --- a/fs/btrfs/file.c
> > +++ b/fs/btrfs/file.c
> > @@ -2069,53 +2069,12 @@ int btrfs_sync_file(struct file *file, loff_t start, loff_t end, int datasync)
> > atomic_inc(&root->log_batch);
> > full_sync = test_bit(BTRFS_INODE_NEEDS_FULL_SYNC,
> > &BTRFS_I(inode)->runtime_flags);
> > +
> > /*
> > - * We might have have had more pages made dirty after calling
> > - * start_ordered_ops and before acquiring the inode's i_mutex.
> > + * We have to do this here to avoid the priority inversion of waiting on
> > + * IO of a lower priority task while holding a transaciton open.
> > */
> > - if (full_sync) {
> > - /*
> > - * For a full sync, we need to make sure any ordered operations
> > - * start and finish before we start logging the inode, so that
> > - * all extents are persisted and the respective file extent
> > - * items are in the fs/subvol btree.
> > - */
> > - ret = btrfs_wait_ordered_range(inode, start, len);
> > - } else {
> > - /*
> > - * Start any new ordered operations before starting to log the
> > - * inode. We will wait for them to finish in btrfs_sync_log().
> > - *
> > - * Right before acquiring the inode's mutex, we might have new
> > - * writes dirtying pages, which won't immediately start the
> > - * respective ordered operations - that is done through the
> > - * fill_delalloc callbacks invoked from the writepage and
> > - * writepages address space operations. So make sure we start
> > - * all ordered operations before starting to log our inode. Not
> > - * doing this means that while logging the inode, writeback
> > - * could start and invoke writepage/writepages, which would call
> > - * the fill_delalloc callbacks (cow_file_range,
> > - * submit_compressed_extents). These callbacks add first an
> > - * extent map to the modified list of extents and then create
> > - * the respective ordered operation, which means in
> > - * tree-log.c:btrfs_log_inode() we might capture all existing
> > - * ordered operations (with btrfs_get_logged_extents()) before
> > - * the fill_delalloc callback adds its ordered operation, and by
> > - * the time we visit the modified list of extent maps (with
> > - * btrfs_log_changed_extents()), we see and process the extent
> > - * map they created. We then use the extent map to construct a
> > - * file extent item for logging without waiting for the
> > - * respective ordered operation to finish - this file extent
> > - * item points to a disk location that might not have yet been
> > - * written to, containing random data - so after a crash a log
> > - * replay will make our inode have file extent items that point
> > - * to disk locations containing invalid data, as we returned
> > - * success to userspace without waiting for the respective
> > - * ordered operation to finish, because it wasn't captured by
> > - * btrfs_get_logged_extents().
> > - */
> > - ret = start_ordered_ops(inode, start, end);
> > - }
> > + ret = btrfs_wait_ordered_range(inode, start, len);
> > if (ret) {
> > inode_unlock(inode);
> > goto out;
> > @@ -2240,13 +2199,6 @@ int btrfs_sync_file(struct file *file, loff_t start, loff_t end, int datasync)
> > goto out;
> > }
> > }
> > - if (!full_sync) {
> > - ret = btrfs_wait_ordered_range(inode, start, len);
> > - if (ret) {
> > - btrfs_end_transaction(trans);
> > - goto out;
> > - }
> > - }
> > ret = btrfs_commit_transaction(trans);
> > } else {
> > ret = btrfs_end_transaction(trans);
>
> There's more code in this function that can go away after this.
> The logic to check if the inode is already in the log can now be
> simplified since we always for the ordered extents to complete before
> deciding whether the inode needs to be blogged. The big commet about
> it can go away too:
>
> https://friendpaste.com/5MHqvkBmdIQgrySryhhjMy
The paste is still alive, the change looks good, can you please send is
as a proper patch? The other patches are in misc-next now. Thanks.
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