Re: [PATCH 2/3] xfs: set firstfsb to NULLFSBLOCK before feeding it to _bmapi_write

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Wed, Jul 19, 2017 at 09:20:22AM -0400, Brian Foster wrote:
> On Tue, Jul 18, 2017 at 11:24:22AM -0700, Darrick J. Wong wrote:
> > From: Darrick J. Wong <darrick.wong@oracle.com>
> > 
> > We must initialize the firstfsb parameter to _bmapi_write so that it
> > doesn't incorrectly treat stack garbage as a restriction on which AGs
> > it can search for free space.
> > 
> > Fixes-coverity-id: 1402025
> > Fixes-coverity-id: 1415167
> > Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
> > ---
> >  fs/xfs/libxfs/xfs_bmap.c |    2 +-
> >  fs/xfs/xfs_reflink.c     |    2 +-
> >  2 files changed, 2 insertions(+), 2 deletions(-)
> > 
> > 
> > diff --git a/fs/xfs/libxfs/xfs_bmap.c b/fs/xfs/libxfs/xfs_bmap.c
> > index 935adde..8c4ee60 100644
> > --- a/fs/xfs/libxfs/xfs_bmap.c
> > +++ b/fs/xfs/libxfs/xfs_bmap.c
> > @@ -6508,7 +6508,7 @@ xfs_bmap_finish_one(
> >  	xfs_filblks_t			*blockcount,
> >  	xfs_exntst_t			state)
> >  {
> > -	xfs_fsblock_t			firstfsb;
> > +	xfs_fsblock_t			firstfsb = NULLFSBLOCK;
> >  	int				error = 0;
> >  
> 
> Shouldn't firstfsb be tied to the transaction lifetime in (at least)
> this case? It is used in the allocator to control things like AG locking
> order.

That is correct...

> Here, it looks like we could potentially do multiple unmaps in a
> single transaction without carrying the firstfsb state across..?

...but keep in mind that we call xfs_bmap_finish_one once per
transaction and roll transactions until the work is done.  I'll add a
comment here to that effect:

/*
 * firstfsb is tied to the transaction lifetime and is used to ensure
 * correct AG locking order and schedule work item continuations.
 * XFS_BUI_MAX_FAST_EXTENTS (== 1) restricts us to only making one bmap
 * call per transaction, so it should be safe as a local variable.
 */

> FWIW this patch still looks fine either way:
> 
> Reviewed-by: Brian Foster <bfoster@redhat.com>

Thank you for the review!

--D
> 
> >  	trace_xfs_bmap_deferred(tp->t_mountp,
> > diff --git a/fs/xfs/xfs_reflink.c b/fs/xfs/xfs_reflink.c
> > index ab2270a..d9b3d57 100644
> > --- a/fs/xfs/xfs_reflink.c
> > +++ b/fs/xfs/xfs_reflink.c
> > @@ -329,7 +329,7 @@ xfs_reflink_convert_cow_extent(
> >  	xfs_filblks_t			count_fsb,
> >  	struct xfs_defer_ops		*dfops)
> >  {
> > -	xfs_fsblock_t			first_block;
> > +	xfs_fsblock_t			first_block = NULLFSBLOCK;
> >  	int				nimaps = 1;
> >  
> >  	if (imap->br_state == XFS_EXT_NORM)
> > 
> > --
> > To unsubscribe from this list: send the line "unsubscribe linux-xfs" in
> > the body of a message to majordomo@vger.kernel.org
> > More majordomo info at  http://vger.kernel.org/majordomo-info.html
> --
> To unsubscribe from this list: send the line "unsubscribe linux-xfs" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html