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ceph: do not cache negative dentries for snapped directories

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When a LOOKUP/LOOKUPSNAP in a snapped directory returns ENOENT
without a trace, ceph_finish_lookup() creates a negative dentry
via d_add(dentry, NULL).  For live directories this is fine — the
dentry naturally expires.  But for snapped directories,
ceph_d_revalidate() unconditionally trusts all cached dentries
(valid = 1), so a negative dentry created by a transient error
persists forever, hiding entries that genuinely exist in the
snapshot.

Only cache negative dentries for live (non-snapshotted) parent
directories.  For snapped parents, skip the negative dentry so
that VFS retries the lookup on the next access.  Since the
conditions that trigger a negative dentry (MDS transient error,
local ENOENT shortcut, or MDS null dentry lease) are all rare in
snapped directories, the performance impact of this change is
negligible.

Link: https://tracker.ceph.com/issues/78529
Reported-by: Andras Pataki <apataki@flatironinstitute.org>
Signed-off-by: Xiubo Li <xiubo.li@clyso.com>
Reviewed-by: Viacheslav Dubeyko <slava@dubeyko.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
This commit is contained in:
Xiubo Li
2026-07-23 14:28:19 +08:00
committed by Ilya Dryomov
parent 699411a353
commit af59562a5b
2 changed files with 10 additions and 3 deletions
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10
fs/ceph/dir.c
View File

@@ -774,8 +774,13 @@ struct dentry *ceph_finish_lookup(struct ceph_mds_request *req,
d_drop(dentry);
err = -ENOENT;
} else {
if (d_unhashed(dentry))
d_add(dentry, NULL);
if (d_unhashed(dentry)) {
struct inode *parent =
d_inode(dentry->d_parent);
if (!parent ||
ceph_snap(parent) == CEPH_NOSNAP)
d_add(dentry, NULL);
}
}
}
}
@@ -840,6 +845,7 @@ static struct dentry *ceph_lookup(struct inode *dir, struct dentry *dentry,
dentry->d_name.len) &&
!is_root_ceph_dentry(dir, dentry) &&
ceph_test_mount_opt(fsc, DCACHE) &&
ceph_snap(dir) == CEPH_NOSNAP &&
__ceph_dir_is_complete(ci) &&
__ceph_caps_issued_mask_metric(ci, CEPH_CAP_FILE_SHARED, 1)) {
__ceph_touch_fmode(ci, mdsc, CEPH_FILE_MODE_RD);

3
fs/ceph/inode.c
View File

@@ -1814,7 +1814,8 @@ int ceph_fill_trace(struct super_block *sb, struct ceph_mds_request *req)
ceph_dir_clear_ordered(dir);
d_delete(dn);
} else if (have_lease) {
if (d_unhashed(dn))
if (d_unhashed(dn) &&
ceph_snap(dir) == CEPH_NOSNAP)
d_add(dn, NULL);
}