contrib: add negdentdelete.py script

Signed-off-by: Stephen Brennan <stephen.s.brennan@oracle.com>

contrib/negdentdelete.py

@@ -0,0 +1,129 @@
+# Copyright (c) 2024, Oracle and/or its affiliates.
+"""
+negdentdelete - remove negative dentries from a directory
+
+Normally, there aren't many good ways to get rid of negative dentries. You
+could:
+
+1. Delete the entire directory containing them. This works, but if there are
+   real files in it, then it's a lot of work.
+2. Use drop_caches. This option is unfortunately global and so you can't target
+   specific files.
+3. Wait until your memory fills up and the LRU starts to handle it...
+
+This script can help you target a specific directory and remove all negative
+dentries within it. Note that while the script is reasonably safe, it can't be
+100% reliable: iterating over a frequently changing linked list in the kernel
+may go awry. The script also creates and unlinks files within the target
+directory, so the user must have permission to do so. If the script runs as
+root, then you should ensure that root-owned files inside the directory won't
+cause any problems.
+"""
+import argparse
+import os
+import time
+from typing import Iterator, List
+
+from drgn import Object, Program
+from drgn.helpers.linux.fs import path_lookup
+from drgn.helpers.linux.list import hlist_for_each_entry, list_for_each_entry
+
+
+def for_each_child_dentry(dentry: Object) -> Iterator[Object]:
+    try:
+        # da549bdd15c29 ("dentry: switch the lists of children to hlist")
+        return hlist_for_each_entry(
+            "struct dentry",
+            dentry.d_children.address_of_(),
+            "d_sib",
+        )
+    except LookupError:
+        return list_for_each_entry(
+            "struct dentry", dentry.d_subdirs.address_of_(), "d_child"
+        )
+
+
+def yield_negative_dentries(
+    prog: Program, dir_: str, chunk_size: int = 10000
+) -> Iterator[List[bytes]]:
+    parent = path_lookup(prog, dir_).dentry
+    negdent_names = []
+
+    for child in for_each_child_dentry(parent):
+        # Do this at the top of the loop so that there's less of a chance of the
+        # current child being freed out from under us. It's sort of like
+        # list_for_each_entry_safe(). Of course, there's no guarantee that this
+        # dentry will still be here when we come back (we're not holding a
+        # reference, after all). But at least we're not actively freeing the
+        # dentry we're currently looking at.
+        if len(negdent_names) >= chunk_size:
+            yield negdent_names
+            negdent_names = []
+
+        if not child.d_inode:
+            negdent_names.append(child.d_name.name.string_())
+    if negdent_names:
+        yield negdent_names
+
+
+def remove_negative_dentries(
+    dir_: str, names: List[bytes], verbose: bool = False
+) -> None:
+    dir_fd = os.open(dir_, os.O_PATH)
+    try:
+        for name in names:
+            # When a file is open and it is unlinked, its associated dentry cannot
+            # remain a part of the dentry cache (since a new file of the same name
+            # could be created). So, it is removed from the dentry hash table so it
+            # can no longer be looked up (see d_delete() in fs/dcache.c).
+            #
+            # When the file is closed, dput() will find that the dentry is
+            # unhashed, and so it will immediately free it. Thus, creating a file
+            # with the same name as a negative dentry, unlinking, and then closing
+            # it, is a sneaky way of removing the cached negative dentry. While
+            # this isn't ideal (creating the file does result in some I/O), it is
+            # still remarkably quick.
+            fd = os.open(name, os.O_RDONLY | os.O_CREAT, dir_fd=dir_fd)
+            os.unlink(name, dir_fd=dir_fd)
+            os.close(fd)
+            if verbose:
+                print(name.decode(), fd)
+    finally:
+        os.close(dir_fd)
+
+
+def main(prog: Program):
+    parser = argparse.ArgumentParser(
+        description="remove negative dentries from a directory"
+    )
+    parser.add_argument(
+        "directory",
+        help="directory to clear negative dentries from",
+    )
+    parser.add_argument(
+        "--verbose",
+        "-v",
+        action="store_true",
+        help="print each dentry we delete (much slower!)",
+    )
+    parser.add_argument(
+        "--chunk-size",
+        type=int,
+        default=10000,
+        help="number of negative dentries to read at a time",
+    )
+    args = parser.parse_args()
+    directory = os.path.abspath(args.directory)
+    removed = 0
+    start = time.time()
+    for batch in yield_negative_dentries(prog, directory, args.chunk_size):
+        remove_negative_dentries(directory, batch, verbose=args.verbose)
+        removed += len(batch)
+    total = time.time() - start
+    dps = removed / total
+    print(f"removed {removed} negative dentries in {total:.2f}s ({dps:.2f}/s)")
+
+
+if __name__ == "__main__":
+    prog: Program
+    main(prog)