On Mon, May 16, 2016 at 10:24:01AM -0700, Liu Bo wrote:
> On Mon, May 16, 2016 at 10:44:38AM +0200, David Sterba wrote:
> > On Fri, May 13, 2016 at 05:07:00PM -0700, Liu Bo wrote:
> > > We have two BUG_ON in merge_bio, but since it can gracefully return errors
> > > to callers, use WARN_ONCE to give error information and don't leave a
> > > possible panic.
> > >
> > > Signed-off-by: Liu Bo <bo.li....@oracle.com>
> > > ---
> > > fs/btrfs/extent_io.c | 1 -
> > > fs/btrfs/inode.c | 6 ++++--
> > > 2 files changed, 4 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
> > > index e601e0f..99286d1 100644
> > > --- a/fs/btrfs/extent_io.c
> > > +++ b/fs/btrfs/extent_io.c
> > > @@ -2746,7 +2746,6 @@ static int merge_bio(int rw, struct extent_io_tree
> > > *tree, struct page *page,
> > > if (tree->ops && tree->ops->merge_bio_hook)
> > > ret = tree->ops->merge_bio_hook(rw, page, offset, size, bio,
> > > bio_flags);
> > > - BUG_ON(ret < 0);
> > > return ret;
> > >
> > > }
> > > diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
> > > index 5874562..3a989e3 100644
> > > --- a/fs/btrfs/inode.c
> > > +++ b/fs/btrfs/inode.c
> > > @@ -1827,8 +1827,10 @@ int btrfs_merge_bio_hook(int rw, struct page
> > > *page, unsigned long offset,
> > > map_length = length;
> > > ret = btrfs_map_block(root->fs_info, rw, logical,
> > > &map_length, NULL, 0);
> > > - /* Will always return 0 with map_multi == NULL */
> > > - BUG_ON(ret < 0);
> > > + if (ret < 0) {
> > > + WARN_ONCE(ret < 0, KERN_ERR "ret = %d\n", ret);
> >
> > btrfs_map_block is quite verbose about the errors so it's not needed to
> > print it here.
>
> OK.
>
> >
> > Otherwise I'm not sure if all paths that go through the merge hook
> > handle errors, eg. in btrfs_submit_compressed_read or _write. Some code
> > is skipped if merge hook returns nonzero. But, the code expects either 0
> > or 1, and when the BUG_ON(ret < 0) is removed suddenly the 'ret < 0' can
> > be returned. Unexpected.
>
> Right now btrfs_merge_bio_hook() only returns 1 or 0 and marks (ret < 0) as
> BUG_ON.
IOW, "ret < 0" never leaves btrfs_merge_bio_hook in current code.
> But compress code is ready to handle the error,
>
> btrfs_submit_compressed_read/write() {
> ...
> ret = merge_bio_hook()
> if (ret || bio_add_page()) {
This code expects 0 or 1, < 0 is not expected here and has to be
considered after the change in merge_bio_hook.
> ret = btrfs_map_bio();
> BUG_ON(ret);
> ...
> }
> ...
> }
>
> So ret < 0 is handled, if there's any errors from merge_bio_hook(), it submits
> the current bio, so the way is sane to me.
Well, that's what I don't call 'handled'. 'ret < 0' will get to the 'if'
but the question is whether the code really expects to see < 0 there and
if it does the right thing then.
> The other consumer of merge_bio is submit_extent_page(), where it's OK
> to return errors and callers are ready to handle them.
Same argument.
'if (... || merge_bio() || ...)'
returns 1 "we can merge, continue with submit_bio"
returns <0 "there was an error, we cannot continue" ... yet will still
try to do submit_bio.
> > It's IMO better to push up the BUG_ON error handling only one caller at
> > a time. That way it's easier to review the callgraph and call paths.
>
> merge_bio() is a wrapper for tree->ops->merge_bio_hook(), which is the
> same thing with btrfs_merge_bio_hook(). It makes no sense if we just
> look at BUG_ON in btrfs_merge_bio_hook but keep BUG_ON() in merge_bio().
So, looking at the code again: the BUG_ON in btrfs_merge_bio_hook can be
replaced by return ret (no warning needed IMO).
If merge_bio gets rid of the BUG_ON, the calles must explicitly handle
'ret < 0' unless it's provably not a problem.
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