xfs: don't assume perags are initialised when trimming AGs

xfs: don't assume perags are initialised when trimming AGs

When running fstrim immediately after mounting a V4 filesystem,
the fstrim fails to trim all the free space in the filesystem. It
only trims the first extent in the by-size free space tree in each
AG and then returns. If a second fstrim is then run, it runs
correctly and the entire free space in the filesystem is iterated
and discarded correctly.

The problem lies in the setup of the trim cursor - it assumes that
pag->pagf_longest is valid without either reading the AGF first or
checking if xfs_perag_initialised_agf(pag) is true or not.

As a result, when a filesystem is mounted without reading the AGF
(e.g. a clean mount on a v4 filesystem) and the first operation is a
fstrim call, pag->pagf_longest is zero and so the free extent search
starts at the wrong end of the by-size btree and exits after
discarding the first record in the tree.

Fix this by deferring the initialisation of tcur->count to after
we have locked the AGF and guaranteed that the perag is properly
initialised. We trigger this on tcur->count == 0 after locking the
AGF, as this will only occur on the first call to
xfs_trim_gather_extents() for each AG. If we need to iterate,
tcur->count will be set to the length of the record we need to
restart at, so we can use this to ensure we only sample a valid
pag->pagf_longest value for the iteration.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Bill O'Donnell <bodonnel@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Fixes: 89cfa899608f ("xfs: reduce AGF hold times during fstrim operations")
Cc: <stable@vger.kernel.org> # v6.6
Signed-off-by: Carlos Maiolino <cem@kernel.org>