在 2024/8/19 13:21, Darrick J. Wong 写道:
On Mon, Aug 19, 2024 at 08:53:20AM +0800, Zizhi Wo wrote:
In the fsmap query of xfs, there is an interval missing problem:
[root@fedora ~]# xfs_io -c 'fsmap -vvvv' /mnt
EXT: DEV BLOCK-RANGE OWNER FILE-OFFSET AG AG-OFFSET TOTAL
0: 253:16 [0..7]: static fs metadata 0 (0..7) 8
1: 253:16 [8..23]: per-AG metadata 0 (8..23) 16
2: 253:16 [24..39]: inode btree 0 (24..39) 16
3: 253:16 [40..47]: per-AG metadata 0 (40..47) 8
4: 253:16 [48..55]: refcount btree 0 (48..55) 8
5: 253:16 [56..103]: per-AG metadata 0 (56..103) 48
6: 253:16 [104..127]: free space 0 (104..127) 24
......
BUG:
[root@fedora ~]# xfs_io -c 'fsmap -vvvv -d 104 107' /mnt
[root@fedora ~]#
Normally, we should be able to get [104, 107), but we got nothing.
The problem is caused by shifting. The query for the problem-triggered
scenario is for the missing_owner interval (e.g. freespace in rmapbt/
unknown space in bnobt), which is obtained by subtraction (gap). For this
scenario, the interval is obtained by info->last. However, rec_daddr is
calculated based on the start_block recorded in key[1], which is converted
by calling XFS_BB_TO_FSBT. Then if rec_daddr does not exceed
info->next_daddr, which means keys[1].fmr_physical >> (mp)->m_blkbb_log
<= info->next_daddr, no records will be displayed. In the above example,
104 >> (mp)->m_blkbb_log = 12 and 107 >> (mp)->m_blkbb_log = 12, so the two
are reduced to 0 and the gap is ignored:
before calculate ----------------> after shifting
104(st) 107(ed) 12(st/ed)
|---------| |
sector size block size
Resolve this issue by introducing the "end_daddr" field in
xfs_getfsmap_info. This records key[1].fmr_physical at the granularity of
sector. If the current query is the last, the rec_daddr is end_daddr to
prevent missing interval problems caused by shifting. We only need to focus
on the last query, because xfs disks are internally aligned with disk
blocksize that are powers of two and minimum 512, so there is no problem
with shifting in previous queries.
After applying this patch, the above problem have been solved:
[root@fedora ~]# xfs_io -c 'fsmap -vvvv -d 104 107' /mnt
EXT: DEV BLOCK-RANGE OWNER FILE-OFFSET AG AG-OFFSET TOTAL
0: 253:16 [104..106]: free space 0 (104..106) 3
Fixes: e89c041338ed ("xfs: implement the GETFSMAP ioctl")
Signed-off-by: Zizhi Wo <wozizhi@xxxxxxxxxx>
---
fs/xfs/xfs_fsmap.c | 19 ++++++++++++++++++-
1 file changed, 18 insertions(+), 1 deletion(-)
diff --git a/fs/xfs/xfs_fsmap.c b/fs/xfs/xfs_fsmap.c
index 3a30b36779db..4734f8d6303c 100644
--- a/fs/xfs/xfs_fsmap.c
+++ b/fs/xfs/xfs_fsmap.c
@@ -162,6 +162,7 @@ struct xfs_getfsmap_info {
xfs_daddr_t next_daddr; /* next daddr we expect */
/* daddr of low fsmap key when we're using the rtbitmap */
xfs_daddr_t low_daddr;
+ xfs_daddr_t end_daddr; /* daddr of high fsmap key */
u64 missing_owner; /* owner of holes */
u32 dev; /* device id */
/*
@@ -294,6 +295,19 @@ xfs_getfsmap_helper(
return 0;
}
+ /*
+ * For an info->last query, we're looking for a gap between the
+ * last mapping emitted and the high key specified by userspace.
+ * If the user's query spans less than 1 fsblock, then
+ * info->high and info->low will have the same rm_startblock,
+ * which causes rec_daddr and next_daddr to be the same.
+ * Therefore, use the end_daddr that we calculated from
+ * userspace's high key to synthesize the record. Note that if
+ * the btree query found a mapping, there won't be a gap.
+ */
+ if (info->last && info->end_daddr != LLONG_MAX)
XFS_BUF_DADDR_NULL (and yes, I know the rest of the file is wildly
inconsistent, I'll send a patch to fix that too...)
--D
From what I understand, you mean that info->end_daddr is initialized to
XFS_BUF_DADDR_NULL, correct?
Then, regarding this specific issue, are you going to propose a fix
patch to address it, or is the fix patch you mentioned intended to fix
other file contiguity problems in fsmap? I'm a bit unclear about that.
Thanks,
Zizhi Wo
+ rec_daddr = info->end_daddr;
+
/* Are we just counting mappings? */
if (info->head->fmh_count == 0) {
if (info->head->fmh_entries == UINT_MAX)
@@ -946,6 +960,7 @@ xfs_getfsmap(
info.next_daddr = head->fmh_keys[0].fmr_physical +
head->fmh_keys[0].fmr_length;
+ info.end_daddr = LLONG_MAX;
info.fsmap_recs = fsmap_recs;
info.head = head;
@@ -966,8 +981,10 @@ xfs_getfsmap(
* low key, zero out the low key so that we get
* everything from the beginning.
*/
- if (handlers[i].dev == head->fmh_keys[1].fmr_device)
+ if (handlers[i].dev == head->fmh_keys[1].fmr_device) {
dkeys[1] = head->fmh_keys[1];
+ info.end_daddr = dkeys[1].fmr_physical;
+ }
if (handlers[i].dev > head->fmh_keys[0].fmr_device)
memset(&dkeys[0], 0, sizeof(struct xfs_fsmap));
--
2.39.2
