Re: [PATCH v6 7/7] xfs: add error injection for per-AG resv failure when shrinkfs

[Brian Foster <bfoster@redhat.com>]

On Wed, Feb 03, 2021 at 11:01:32PM +0800, Gao Xiang wrote:
> Hi Brian,
> 
> On Wed, Feb 03, 2021 at 09:23:59AM -0500, Brian Foster wrote:
> > On Tue, Jan 26, 2021 at 08:56:21PM +0800, Gao Xiang wrote:
> > > per-AG resv failure after fixing up freespace is hard to test in an
> > > effective way, so directly add an error injection path to observe
> > > such error handling path works as expected.
> > > 
> > > Reviewed-by: Darrick J. Wong <djwong@kernel.org>
> > > Signed-off-by: Gao Xiang <hsiangkao@redhat.com>
> > > ---
> > >  fs/xfs/libxfs/xfs_ag.c       | 5 +++++
> > >  fs/xfs/libxfs/xfs_errortag.h | 4 +++-
> > >  fs/xfs/xfs_error.c           | 2 ++
> > >  3 files changed, 10 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/fs/xfs/libxfs/xfs_ag.c b/fs/xfs/libxfs/xfs_ag.c
> > > index c6e68e265269..5076913c153f 100644
> > > --- a/fs/xfs/libxfs/xfs_ag.c
> > > +++ b/fs/xfs/libxfs/xfs_ag.c
> > > @@ -23,6 +23,7 @@
> > >  #include "xfs_ag_resv.h"
> > >  #include "xfs_health.h"
> > >  #include "xfs_error.h"
> > > +#include "xfs_errortag.h"
> > >  #include "xfs_bmap.h"
> > >  #include "xfs_defer.h"
> > >  #include "xfs_log_format.h"
> > > @@ -559,6 +560,10 @@ xfs_ag_shrink_space(
> > >  	be32_add_cpu(&agf->agf_length, -len);
> > >  
> > >  	err2 = xfs_ag_resv_init(agibp->b_pag, *tpp);
> > > +
> > > +	if (XFS_TEST_ERROR(false, mp, XFS_ERRTAG_SHRINKFS_AG_RESV_FAIL))
> > > +		err2 = -ENOSPC;
> > > +
> > 
> > Seems reasonable, but I feel like this could be broadened to serve as a
> > generic perag reservation error tag. I suppose we might not be able to
> > use it on a clean mount, but perhaps it could be reused for growfs and
> > remount. Hm?
> 
> I think it could be done in that way, yet currently the logic is just to
> verify the shrink error handling case above rather than extend to actually
> error inject per-AG reservation for now... I could rename the errortag
> for later reuse (some better naming? I'm not good at this...) in advance
> yet real per-AG reservation error injection might be more complicated
> than just error out with -ENOSPC, and it's somewhat out of scope of this
> patchset for now...
> 

I don't think it needs to be any more complicated than the logic you
have here. Just bury it further down in in the perag res init code,
rename it to something like ERRTAG_AG_RESV_FAIL, and use it the exact
same way for shrink testing. For example, maybe drop it into
__xfs_ag_resv_init() near the xfs_mod_fdblocks() call so we can also
take advantage of the tracepoint that triggers on -ENOSPC for
informational purposes:

	error = xfs_mod_fdblocks(...);
	if (!error && XFS_TEST_ERROR(false, mp, XFS_ERRTAG_AG_RESV_FAIL))
		error = -ENOSPC;
	if (error) {
		...
	}

Brian

> Thanks,
> Gao Xiang
> 
> > 
> > Brian
>