I noticed a bug related to xfs realtime device fsmap:
[root@fedora ~]# xfs_io -c 'fsmap -vvvv -r' /mnt
EXT: DEV BLOCK-RANGE OWNER FILE-OFFSET AG AG-OFFSET TOTAL
0: 253:48 [0..7]: unknown 8
1: 253:48 [8..1048575]: free space 1048568
2: 253:48 [1048576..1050623]: unknown 2048
3: 253:48 [1050624..2097151]: free space 1046528
Bug:
[root@fedora ~]# xfs_io -c 'fsmap -vvvv -r 1050621 1050621' /mnt
EXT: DEV BLOCK-RANGE OWNER FILE-OFFSET AG AG-OFFSET TOTAL
0: 253:48 [1050621..1050623]: unknown 3
1: 253:48 [1050624..1050631]: free space 8
Normally, we should not get any results, but we do get two queries.
The root cause of this problem lies in the calculation of "end_rtb" in
xfs_getfsmap_rtdev_rtbitmap(), which uses XFS_BB_TO_FSB method (round up).
However, in the subsequent call to xfs_rtalloc_query_range(), "high_rec"
calculated based on "end_rtb" has a semantic meaning of being reachable
within the loop. The first round of the loop in xfs_rtalloc_query_range()
doesn't find any free extents. But after incrementing "rtstart" by 1, start
still does not exceed "high_key", and the second round of the loop entered.
It finds free extent and obtains the first unknown extent by subtracting it
from "info->next_daddr". Even though we can accurately handle it through
"info->end_daddr", two incorrect extents has already been returned before
the last query. The main call stack is as follows:
xfs_getfsmap_rtdev_rtbitmap
// rounded up
end_rtb = XFS_BB_TO_FSB(..., keys[1].fmr_physical)
ahigh.ar_startext = xfs_rtb_to_rtxup(mp, end_rtb)
xfs_rtalloc_query_range
// high_key is calculated based on end_rtb
high_key = min(high_rec->ar_startext, ...)
while (rtstart <= high_key)
// First loop, doesn't find free extent
xfs_rtcheck_range
rtstart = rtend + 1
// Second loop, the free extent outside the query interval is found
xfs_getfsmap_rtdev_rtbitmap_helper
// unknown and free were printed out together in the second round
xfs_getfsmap_helper
The issue is resolved by adjusting the relevant calculations. Both the loop
exit condition in the xfs_rtalloc_query_range() and the length calculation
condition (high_key - start + 1) in the xfs_rtfind_forw() reflect the open
interval semantics of "high_key". Therefore, when calculating "end_rtb",
XFS_BB_TO_FSBT is used. In addition, in order to satisfy the close interval
semantics, "key[1].fmr_physical" needs to be decremented by 1. For the
non-eofs case, there is no need to worry about over-counting because we can
accurately count the block number through "info->end_daddr".
After applying this patch, the above problem have been solved:
[root@fedora ~]# xfs_io -c 'fsmap -vvvv -r 1050621 1050621' /mnt
[root@fedora ~]#
Fixes: 4c934c7dd60c ("xfs: report realtime space information via the rtbitmap")
Signed-off-by: Zizhi Wo <wozizhi@xxxxxxxxxx>
---
fs/xfs/libxfs/xfs_rtbitmap.c | 4 +---
fs/xfs/xfs_fsmap.c | 20 +++++++++++++++++---
2 files changed, 18 insertions(+), 6 deletions(-)
diff --git a/fs/xfs/libxfs/xfs_rtbitmap.c b/fs/xfs/libxfs/xfs_rtbitmap.c
index 386b672c5058..7af4e7afda7d 100644
--- a/fs/xfs/libxfs/xfs_rtbitmap.c
+++ b/fs/xfs/libxfs/xfs_rtbitmap.c
@@ -1034,8 +1034,7 @@ xfs_rtalloc_query_range(
if (low_rec->ar_startext > high_rec->ar_startext)
return -EINVAL;
- if (low_rec->ar_startext >= mp->m_sb.sb_rextents ||
- low_rec->ar_startext == high_rec->ar_startext)
+ if (low_rec->ar_startext >= mp->m_sb.sb_rextents)
return 0;
high_key = min(high_rec->ar_startext, mp->m_sb.sb_rextents - 1);
@@ -1057,7 +1056,6 @@ xfs_rtalloc_query_range(
if (is_free) {
rec.ar_startext = rtstart;
rec.ar_extcount = rtend - rtstart + 1;
-
error = fn(mp, tp, &rec, priv);
if (error)
break;
diff --git a/fs/xfs/xfs_fsmap.c b/fs/xfs/xfs_fsmap.c
index 8a2dfe96dae7..42c4b94b0493 100644
--- a/fs/xfs/xfs_fsmap.c
+++ b/fs/xfs/xfs_fsmap.c
@@ -515,11 +515,20 @@ xfs_getfsmap_rtdev_rtbitmap(
int error;
eofs = XFS_FSB_TO_BB(mp, xfs_rtx_to_rtb(mp, mp->m_sb.sb_rextents));
- if (keys[0].fmr_physical >= eofs)
+ if (keys[0].fmr_physical >= eofs ||
+ keys[0].fmr_physical == keys[1].fmr_physical)
return 0;
start_rtb = XFS_BB_TO_FSBT(mp,
keys[0].fmr_physical + keys[0].fmr_length);
- end_rtb = XFS_BB_TO_FSB(mp, min(eofs - 1, keys[1].fmr_physical));
+ /*
+ * The passed keys[1] is an unreachable value, while "end_rtb" is used
+ * to calculate "ahigh.ar_startext", serving as an input parameter for
+ * xfs_rtalloc_query_range(), which is a value that can be reached.
+ * Therefore, it is necessary to use "keys[1].fmr_physical - 1" here.
+ * And because of the semantics of "end_rtb", it needs to be
+ * supplemented by 1 in the last calculation.
+ */
+ end_rtb = XFS_BB_TO_FSBT(mp, min(eofs - 1, keys[1].fmr_physical - 1));
info->missing_owner = XFS_FMR_OWN_UNKNOWN;
@@ -549,9 +558,14 @@ xfs_getfsmap_rtdev_rtbitmap(
/*
* Report any gaps at the end of the rtbitmap by simulating a null
* rmap starting at the block after the end of the query range.
+ * For the boundary case of eofs, we need to increment the count
+ * by 1 to prevent omission in block statistics.
+ * For the boundary case of non-eofs, even if incrementing by 1
+ * may lead to over-counting, it doesn't matter because it is
+ * handled by "info->end_daddr" in this situation, not "ahigh".
*/
info->last = true;
- ahigh.ar_startext = min(mp->m_sb.sb_rextents, ahigh.ar_startext);
+ ahigh.ar_startext = min(mp->m_sb.sb_rextents, ahigh.ar_startext + 1);
error = xfs_getfsmap_rtdev_rtbitmap_helper(mp, tp, &ahigh, info);
if (error)
--
2.39.2
